Resetting a Neck on a Bozo Podnuavac Part 2

Now that we have the neck off, and I’ve let it dry for at least a week, we can proceed to the reset.

This guitar will have a fret job as well so all the frets were pulled.The first step in the reset is to determine how much of the heel needs to be removed.  As I stated in the previous post the current neck angle left the straight edge about 1/8″ above the top or about 1/4″ too shallow.   Since the heel is about 4″ and the distance from the nut to the saddle is about 25 1/2″ it takes a fraction of amount taken off at the heel to make the 1/4″ needed to correct the angle.  My usual formula (not scientific by any standard) is to take about an 1/8th of the amount off the heel.  So in this case, I would take off a 1/32″ from the heel or an 1/8th of the 1/4″.  Some people use a file to do this but I prefer a very sharp chisel.  I cut to the line on a bevel, then back cut on a slight angle so that shoulders of the neck “bite” into the body. I may repeat this several times, until I get the angle right.  Make sure you take the same amount of material off both sides of the heel to maintain the correct trajectory to the bridge.

This gets me into the ball park, but I use another method to get the final fit.  For this step I use 100 or 120 grit self adhesive sandpaper with the backing left on.  I put the neck on the body and put the sandpaper between them, slick side toward body.This does the final fitting.  I’ve run across several bodies that are not completely flat, so this works very well for this.  Make sure when you are doing this, you do not pull the sandpaper up because you will take too much off the heel.  Again, care should be taken to ensure both sides are even.  When you are satisfied with the fit you must now adjust the dovetail geometry.

I you have taken more than an 1/8″ off the heel you will first need to take off material off the back of the dovetail to clear the back of the head block.  Once this is done you can now make the shim for the re-fit.In taking wood from the bottom of the heel you will have a lose fit because the dovetail is now smaller at the bottom.  The top should be relatively tight because no material should have been removed (in fact, if wood is taken away form the top of the dovetail, it will effect the intonation because the neck will be closer to the bridge).  I make two wedges, one for each side of the dovetail.  Using .032″ thick veneer 5/8″ wide and almost the length of the dovetail I taper the thickness from the full .032″ to nothing.  A trick for getting this done is to use a piece of double stick tape on one side of the shim so that I can secure it to my fingers while sanding. Carbon paper or lamp black used in between the neck and head block is a good way to test the fit.   I do this until I”m satisfied with the joint.  I have to say that the original dovetail was a very fine fit, a testament to Mr. Podnuavac’s work.

I then glue the neck back on using the same type of glue as the original.  In this case hide glue was used.  NEVER use epoxy, polyurathane or any other glue that can’t be reversed!!!   I touch up any finish issues, such as chipped lacquer around the body or neck.

The 15th fret will have to be replaced, making sure it is in line with the others and doesn’t cause any buzzing (in this case a re-fret was needed).  Finally a new saddle will have to be constructed.

Even if you never do this on your own guitar it will give you some understanding of why this kind of repair costs so much money.

Resetting a Neck on a Bozo Podnuavac Part 1

This is a pretty common but more advanced repair.  Steel string “flat top” guitars have over two hundred lbs. of force from the strings trying to collapse the neck into the body.  Unlike classical guitars where the neck is part of the head block (they have much lower tension), steel strings were made with a neck separate from the body so that they can be disassembled.  Most of the older instruments use a dovetail, either tapered or straight, to attach the neck to the body.  A lot of modern luthiers use bolts to anchor the neck.  There has been a lot debate about what’s better, but as someone who has built over 50 instruments, half bolt, half dovetail, I can’t tell the difference in tone.

If you lay a straight edge along the fret board on a guitar with a proper neck set, the straight edge will just come to the top of the bridge.  A saddle that is too short or too tall will affect the tone and playability.  A general rule is about 3/8″ above the top at the bridge position.

A common method in the past was to shave the bridge (not a good idea) or to “slip the block.”  This is when the back was heated and pressure put on the neck to “slip” the head block, moving the neck angle. When the back cooled the glue would harden.  The problem is that the back was exposed and needed to be trimmed and rebound.

In this example, I’ll be resetting a neck on a guitar by Bozo Podnuavac (pronounced Bo-zho).  He gained fame in the 70’s for his flamboyant 6 and 12 string guitars played by people like Leo Kottke. 

This guitar had been pretty mistreated.  The head stock was broken and poorly repaired.  Numerous cracks have been haphazardly repaired and pressure from the neck had cracked the top on both sides of the fret board and moved the head block toward the body.

A straight edge determined that the neck angle left the height at the bridge about 1/4″ below what it should have been.

The first thing I need to do is separate the fret board from the body. This is done by heating the fret board and carefully working a thinned putty knife under the board.  I’m very careful about this and don’t force the knife.  When the glue is soft it will slide under the board fairly easily.  LMI and a few other companies make a silicone heating blanket just for this purpose.  Stew-Mac also makes a massive iron with space milled for the frets just for this purpose.  In this case I’ll be using the blanket

If the neck joins the body at the 14th fret I pull the 15th fret to make way for two holes I drill into the slot.  In a typical dovetail the male portion of the neck is about 5/8″ long and the corresponding female joint in the body is 3/4″ deep.  This leaves a gap in the head block and the 15th fret is about 5/8″.

Two tools that I think are essential are from Stew-Mac.  One is the guitar neck removal jig and the other is the neck steamer needle and hose.

The other necessity is a cheap coffee maker capable of making steam.  I set up the removal jig as in the picture and turn the cappuccino on steam and wait for the unit to heat the water.  As an aside always use distilled water or you will have a great deal of mineral buildup.  I put a slight amount of pressure on the bottom screw of the jig, just enough to ensure the jig is not moving.  The steam works very rapidly to soften both hide and yellow glues.  I usually only have to wait 5 or 6 minutes for the joint to loosen.  I keep putting slightly more pressure on the bottom screw until the joint separates.   I also elevate the bottom of the guitar so that all the water from the cooled steam stays relatively contained.  Tapered dovetails remove the easiest but in some instruments the dovetail is straight and they require constant pressure because of the friction.

Sometimes, however the mojo doesn’t work, and this was such a case.  After waiting over 10 min. the joint had not budged!  I was afraid that the joint may be epoxy.  Mr. Podnuavac is still building guitars at 82 so I called him in his current shop in Florida and he did indeed confirm that the joint was a dovetail and should come off.

Take two:  This time I put a little more pressure on the screw and after 10 more minutes it did break loose.  If you look at the truss rod you can see a grey bar on either side of the truss rod housing.  This appears to be epoxy that goes through the head block and under the top. This is what I think was causing the hangup.

Next week.  Refitting and re-gluing the neck.

Gibson Style O part 3

During work on the body I was also working on the neck.  As you might expect it was in the same kind of shape. A bad binding repair on the peghead, coupled with a piece of missing head stock overlay. The back was also very banged up.  When the fretboard was removed, big chunks of very brittle mahogany were torn out.  There was a v shaped maple insert in the center of the neck.  Only a third of the fretboard was left after I got the guitar.  It didn’t appear or smell to be rosewood, looking a lot like Macassar ebony.  The Fret House had already ordered a new “24 ¾” slotted ebony fretboard from Luthier’s Mercantile to replace the old one.  It was interesting to compare what I had left of the existing one and the new one to see that they were completely different.  Something I wouldn’t even have checked if I hadn’t read Dan Erlewine, Frank Ford and Tim Shaw’s article in American Lutherie Magazine on the different scales Gibson has used in the past.

The first thing I did on the neck was level it with sand paper stuck to the bottom of my joiner plane.  Then using a simple jig I cut the truss rod slot using a ¼” round bottom up-cut spiral bit.  I used a plunge router to make the cut because I could lift it up after the cut in the peghead, leaving wood in place for the screw in the cover.  The next step was to drill a ¼” hole to join the two slots.  The rod was a double action from Stewart MacDonald and the square ends of the rod were fit into the channel.

The fretboard was next.  I cut the board to shape to the 1 ¾” wide minus the thickness of the ivoroid binding, using the original as a guide.  After heating the binding in hot water I fitted it to the tight curves of the end of the board.  ¼” Holes were then cut for the mother of pearl position markers and the side dots.  With the glue dry a 16” radius was put on the fretboard and it was compression fretted with medium fret wire.

Since the neck had to be cut down and a piece of the peghead binding had been poorly repaired, I decided to replace it all.  By doing this and veneering the back with mahogany, the peghead almost looked new. I filled the missing piece of ebony on the front with dust and glue, taking care to fill the missing filler in the inlay, which thank God, was still intact.  The channels for the peghead binding were done with a router, with the exception of the two nearest the nut as the router bearing would have cut too much out.  Those channels were done with a Sloane purfling cutter. The peghead holes were then drilled to accommodate the new turners.

Before I put the fretboard on the neck, the angle had to be checked.  As you might have guessed 85 years had taken its toll and was at a much too shallow angle to be playable.  One of the things I did when I put the body together was to try and keep the original registration of the scrolls on the top and back aligned with the sides, but I realized that I’d have to slip the block a little.  I heated the back and head block with a silicone heating blanket and moved it as much as I dared, and took up the chisel to finish the job.  I took a full  3/16” off the heal of the neck to get the proper angle.  Luckily there was a lot of wood on the neck and I thought it actually made a better match to what would be a new svelte neck shaft.

Once I was satisfied with the neck I put the truss rod in, filled the gaps in the mahogany with dust and epoxy and glued the fretboard to the neck.  The neck was thicknessed with a Wagner safety plane and contoured with rasps and files.  The ebonized stripe in the center of the neck was lost and I thought it would very difficult to redo without a lot of head scratching so rather than spraying the stripe on with color, Jim and I decided to leave it out altogether.

After finishing the contouring, I sanded down to 320 grit and joined the neck to the body.  The neck angle had positioned the fretboard so that it didn’t contact the body by the sound hole, so a piece of spruce was fashioned to make up for the gap. After rounding the part of spruce that was under the exposed edge of the fretboard, it was barely visible.  When the glue was dry I re-contoured the body to blend in to the narrower board.  Jim had supplied mammoth ivory bridge pins, end pin, and Waverly’s with real ivory buttons, so that was a given.  I have one, yes just one, ivory nut that I’ve saved for about 30 years. And while I don’t put elephant ivory on my new guitars, nor would I put it on any modern guitar I repaired, I thought this is the instrument for it.

Next week:  Finish, set up and trouble shooting.

10 worst “do it yourself repairs”

Here’s my 10 WTF moments in my guitar repair/building experience.

Let me start out with my labor rates:

$75.00 an hour

$85.00 an hour if you watch

$95.00 an hour if you help

$125.00 an hour if you tried to fix it yourself first

Now I know that this is kind of a smart ass intro but it makes the point that you have to look at the job as a whole when making your estimate.

10.  A 1981 Martin D-28 with the binding top and back coming off.  Customer “fix” Scotch taping the binding on the body.  Result: tape UV cured to the finish.  Real fix:  New binding and refinish of the sides.

9.  Jack plug installed on the side of a Martin D-18.  Result: Jack pulls out and takes a good chunk of side with it.  Fix: Make football sized patch to repair side, refinish and install proper jack in end block.

8.  Customer refinish of Gibson Flying V using bar top epoxy as a finish.  Result: The epoxy is improperly mixed so a great deal of it never hardens.  All of the studs for the tail piece were left in and covered with epoxy.  Fix:  Physically removing the epoxy, installing new studs and refinishing.

7.  Customer installed Grover Rotomatics. Holes enlarged with conventional drill bit.  Result:  Cracked peg head when drill bit torqued and caught the wood.  Fix:  Re-gluing peg head, plugging holes, drilling new holes in the plugs and reaming out the ones that didn’t get drilled.  Finally touch up finish the cracked areas.

6.  I’ve said this before but think it needs to be said again… Customer using truss rod to “fix” action.  Result:  Broken truss rod.  Fix:  Well, the fix costs more than the instrument and the guitar was scrapped.  This one depends on where the break occurred and what kind of rod was used.  If it’s a conventional rod and the break is close to the adjusting nut, Stew-Mac has a tool to re-thread the rod and save it.  If the break is farther down the rod or double action you may have to remove the fret board and that my friend is major surgery.

5.  Customer installed strap button on heel of acoustic.  This was a simple job that went horribly wrong because a pilot hole wasn’t drilled.  Result:  Cracked heal.  Fix:  Careful application of cyanoacrylate glue and touch up refinish.  I’ve also seen strap button installations on guitars with bolt on necks where the pilot hole has hit the threaded insert in the heel.  Make sure you know where the insert is placed on that particular guitar before you drill.

4.  Cracked end block because customer used a drill bit meant for steel to enlarge hole for the jack used on an acoustic.  Fix:  This can be tricky.  First you have to assess the damage and that can be challenging.  Some of these miniature cameras work great.  I’ve had success using a point and shoot on a timer to take a snap.  If the crack is small you might be able to use cyano to repair it.  If the end block is cracked all the way through, the back may need to come off and the block replaced… Again, not something you’re going to do on a cheap guitar.   The proper way is to use a step reamer to get the correct sized hole.

3.  This one is not so much a customer fix as customer negligence.  A crack in the top of an acoustic that is not addressed right away.  Dirt and grime are allowed into the crack and oxidizes the surfaces so that what could have been fixed with a diamond patch on the inside has now become a fix that includes taking away the rotten wood and inserting a splint.  It can be very difficult to match the new wood with the old, and you better make sure the customer knows that his or her guitar will not look brand new.

2.  Cracked peg head.  Customer “fixed” with mystery glue and a wood screw.  Result:  Peg head and neck shaft not aligned. Fix: If the peg head can be re-broken you may be able to re-align the neck and re-glue (if it was glued with aliphatic glue you won’t be able to as the glue will not stick to itself).  The joint may have to be resurfaced and new wood may have to be inserted, possibly a spline as well.

1.  And now for my favorite customer fix… A re-glued bridge on an acoustic using Gorilla Glue and wood screws.  This is an epic failure on so many levels.  Wood screws should never be used to hold a bridge on (Gibson, take note) .  And anyone who has ever used Gorilla Glue knows it has no place in guitar construction/repair.  It’s a polyurethane glue that works very well in certain circumstances (water contact, etc.) but the foaming that occurs when the glue is curing can create a humongous mess.  Fix:  Take the bridge off and refinish the top.  What could have been a $85.00 repair is now over $400.00.

The last thing to remember is that repairing a cheap guitar takes just as long and just as expensive as a high end one (sometimes more).

Installing a pickup in an acoustic

First a bit about different pickups and how they perform.

Guitar pickups have come a long way since the DeArmond sound hole pickups of the 50’s.  I remember my uncle playing his Gibson J-45 with a DeArmond, a Fender Twin and an Electro-Voice microphone.  That DeArmond was about the only choice if you wanted to play live without two microphones.

Now there are so many options it’s staggering.  Fishman, D-Tar, Schertler, Dean Markley, LR Baggs, etc.  In fact there are so many pickups you may wonder where to start.  I have to say that there is no right or wrong.  All the pickups currently made are very high quality and will serve most people very well.  It’s more a question of what’s right for you.  Your guitar, how you play and what kind of setting you will be playing in are very important.  I feel the best way is to try them out at you local music store.  Try them out and see what happens

There are three basic types of pickups:

Electro-magnetic, such as Dean Markley Pro-Mag, Seymour Duncan Woody, Fishman Pro-Rep 102, the LR Baggs M1, as well as the aforementioned DeArmond to name a few.  These pickups are identical in structure to a pickup for an electric guitar.  They are however, voiced for an acoustic.  A magnet made of alnico (aluminum, nickel and copper), ceramic or rare earth metals is surrounded by wire, usually copper and when a ferrous string is struck the vibration creates an electric charge that is transmitted to an amplifier. These pickups vary a great deal in quality, starting with the Dean Markley to the limited run Sunrise.

Piezo ceramic, (the most common) are typically under the saddle consisting of a thin ribbon of piezo material.  The name piezo  comes from the Greek “to squeeze” or press. When the string (ferrous or non ferrous) is struck the vibration causes the piezo to compress and produce the electric charge.  Some piezos can be contact as well.  These are attached directly to the sound board and can work on many other instruments.  Be aware that the contact pickups will also pick up any surface noise from the top and can feed back more easily than the under saddle piezos.  This applies to some pickups such as LR Baggs i Beam that are attached to the bottom of the bridge plate.  The charge is minute and must be amplified by means of a pre-amp.  Most modern units include the pre-amp in the 1/4″ jack supplied with the pickup.  Almost all of them come with an option to use volume and tone controls inside the edge of the sound hole. With the right amplifier you can get something that approaches a natural guitar sound.

The third is a microphone, usually installed inside the instrument but sometimes, in the case of Schertler, under the saddle.  In most cases this is my least favorite kind of amplification.  The sound made inside the guitar is not the sound you hear from outside.  Having said that, you should check them out for yourself.  Some pickups like the B-Band make a pickup with a combination of piezo and microphone with a mixer.  I make an exception with the Schertler as I have recently installed this in a parlor guitar I made with stunning results.

I find that all of these are a compromise of one sort or the other.  None of these will produce as natural of a sound as a good microphone.  Sometimes that’s just not possible so the pickup is a must for most stage performers.  If someone from a music store tells you a certain brand of pickup is the best, walk out.  There is no best, try them all out in a guitar that is closest to what you have.

Now, enough procrastination… on with the install.

If you’re using a sound hole pickup, you can simply run the cable out the sound hole and be done.  The Dean Markley Pro-Mag is such a pickup.  Not all of the sound hole pickups are like this. The higher end LR Baggs and Sunrise use an end jack strap button.

For most pickups there are three steps in installation:  First, for the under saddle variety you must drill an appropriate hole for the pickup wire.  The piezo units come in two sizes, 3/32″ and 1/8″. Pick the one that fits your saddle slot.  Fishman’s have the lead at the end of the pckup,  Schertler’s have a rectangular lead in the middle and some like the DTar have a coaxial cable that has to have a hole drilled at a 45 degree angle so as not to kink the cable.  This brings me to the next step in fitting.  Some DTar’s (Timberline model) and the Highlander pickup have a round cable and the bottom of the saddle slot has to be altered to fit.  This means you will have to have the correct router bit and make or buy a jig to route the slot properly.  This should only be attempted by someone who knows what he or she is doing.  It’s pretty easy to muck up things.

If we simply put the saddle back in, the action will be to high so we must take away material from the saddle in the same amount as the thickness of the pickup.  Sand paper on a piece of glass is the simplest way to accomplish this.  Be sure you maintain a flat and true surface on the bottom.  If the saddle does not contact the pickup evenly your sound will suffer.

The next step is to install the end jack.  If you don’t have a strap button on your guitar it’s simply a matter of drilling the correct size hole.  This is usually 15/32″ or 1/2″ depending on the brand of pickup.  Make sure of your position and use a center punch to mark the location.  Then use a brad point drill to drill the hole, taking care to keep it straight and true.  If you have a strap button this becomes much more difficult as your drill bit has no starting place.  DO NOT under any circumstances try to free hand this operation.  You can crack the end block!  The simple solution is to buy a step reamer for your drill which will take the tapered hole and enlarge it to the right size.  These are expensive at about $70, so unless you’re doing a lot of them, you can take a wooden end pin and cut it off at the point it is even with the body.  Then you can safely drill the hole.

Some guitars such as late model Taylor’s have an end pin that is all ready for the end jack, just unscrew the end pin and install the jack.

If you’re using a DTar Load n Lock where the battery is in the end jack, the drill size is 7/8″ and this is the only method to use.  As always make sure your tools are sharp and do not use regular drill or spade bits as this will not end well.

You must now connect the pickup to the end jack.  Some models are a simple 1/8″ jack (DTar), others (Fishman) require you solder the connection.

Set the inside nut to fit the thickness of the end block.  Most end blocks are a pretty consistent 3/4″ but I have encountered some Martin’s from the late seventies and early eighties that have end blocks over an 1″ thick.  If this is the case it’s best left to a professional luthier.

The last thing is to set the battery box.  Usually it’s got a clip or bag that attaches to the back or head block with self sticking Velcro.  Clean the surface of the wood with alcohol or naptha to insure good adhesion.  Use the wire guides to do the same with the wires left dangling.

This should work for most set ups.  Good luck and see you next time.

Replacing tuning machines.

This is one of those projects that, on the face of it, looks simple but it’s one I’ve seen botched more than a few times.

If you’re replacing your tuners with ones that are exactly a match then it’s a simple matter of just taking the existing tuning machines off and dropping the new ones in.  This is rarely the case.  Usually upgrades are what you’re after and that can be challenging.

First lets get into what’s available.  There are two basic kinds (with all sorts of variations) open backed and sealed.  Open backs are most often seen on acoustic flat tops but also on older cheaper electrics.  Open back does not mean cheap.  Waverly tuners sold by Stewart MacDonald are extremely high quality and the cost starts at $140 and up past $300.  The posts are bronze and the gears are nylon bushed stainless steel.  They are copies of pre WWII Grovers and far surpass the originals in quality and materials.  They also are the same foot print as Grovers so it makes upgrading your pre-war Martin easy to take.  No extra screw holes and you can replace them without hurting the value of a vintage instrument. Grover Sta-Tite is the modern version of their vintage tuner.  They are about 25% of the cost of Waverlys and an excellent choice for replacing worn machines on a budget.  Several other companies that produce this kind of machine are Ping, Schaller, Gotoh, and Schertler.

I include the older style Klusons and their copies in the open back category because they are basically open backs with a cover.  These are not to be confused with the next group: sealed machines.

Sealed machines were an invention of Grover.  I’m guessing in the late ’50’s although I couldn’t find much information on their history.  These machines called Rotomatics had permanently lubricated gears with sealed backs.  This meant no more messy oil to contaminate the head stock.  Almost overnight this replaced the open back.  Martin, Gibson and later Fender all put sealed tuning machines on their guitars.  Grover had some quality control issues in the 70’s and Schaller, a German company, quickly became the tuning machine du jour. Gotoh, a Japanese company, makes what I consider the finest of the sealed tuners, the 510.  It has an 18 to 1 gear ratio and almost no backlash. It is well finished and in my opinion a better value than Schaller.  Other companies that make sealed tuners are Ping, Spertzel and Planet Waves.

A sub genre in both open back and sealed tuners are “locking” machines.  These are machines with an internal post that pins the string to the main post through the threading hole.  These are either adjusted by a knob on the bottom of the machine or on the top of the post itself.  While these work very well on guitars with tremelos, I don’t think they’re necessary on most guitars.  Most designs add excess weight and if the string is put on correctly with an ordinary turner you won’t get much slippage anyway.

While I have preferences in what I like to put on my guitars, with the exception of economy machines you can’t go wrong in your choice.  All will preform well and get the job done.

Now after that long winded intro, on to installation!

To start with, most of the open back use a 1/4″ post with a bushing that is between .328 and .393 inches in diameter.  While you can just drill the hole at the bushing diameter, it leaves the post only supported at the bushing.  The correct way would be to drill an initial hole at 1/4″ then use a bushing reamer or pilot bit to enlarge the hole or the bushing.  This reamer is a cutter of proper diameter with a solid 1/4″ “pilot” to guide the cutter.  These are sometimes called step drills.  Of course, if you are replacing tuners you will already have the holes so you won’t have to do anything but swap parts.

If you’re going from the open back to sealed machines you have two choices.  Most sealed machines have a base that is .390″ or 10mm.  DO NOT under any circumstances try to drill the hole with a standard or brad point drill bit.  You have no way of ensuring that the hole drilled will follow the proper course.  If you have no other choice, you can plug the hole with the proper sized dowel and drill with a brad point bit but his can still be tricky as you will be drilling into the end grain of the dowel and the bit can still wander off center. I’ve seen more than one head stock cracked because someone tried to use a standard drill bit and the torque from the drill got the better of them.

The easiest is to use a reamer like that used for the bushing on a open back.  Most of the threaded bushings on sealed tuners are .310″ so it’s the opposite of open back as the top of the peghead should be smaller than the back.  Pilot bits are also made for this but sometimes it can be very difficult.  I also feel that the tuner is much better supported as the bushing is threaded into the base so this is not as critical as the step in an open back tuner.

Going from a sealed tuner to a open back is a little easier as you can buy bushings that will fit the .390″ hole.  These are press fit and will look pretty close to original open back bushings. The shaft will only be supported at the bushing but in most cases that will not be a problem.

If you’re using open back tuners you should fit the bushing now.  They should be tight but be careful not to make the fit too tight, so as to not crack the peg head.  If you’re using the pilot bit the fit should be perfect.  I pad the back of the peg head so it won’t be marred and use a clamp to press the bushing in.  DO NOT use a hammer as the trauma of brute force can also crack the peg head, plus a misdirected blow can occur.  Neither of these is good for business!  If your machines are sealed use the appropriate wrench to tighten the threaded bushing.  Again go easy and don’t over tighten.  Some builders will countersink the washer so that when torque is applied to the bushing it doesn’t pucker the lacquer.

The next common problem is that hole position of the screws that hold the tuner in place are often in different positions than the replacement.  You can try to fill the holes but I find that this often looks worse than just leaving them.  If you’re worried about moisture getting in the hole just drop a bit of lacquer in to seal it.  Just be careful not to get any on the surrounding peg head.  The problem comes when the screws are put in without first pre-drilling.  At the least you can break the screw and worst, crack the peg head.  I’m repairing a guitar now with this issue.  You want to pick the bit size that is equal to the barrel of the screw (the solid part not including the threads).  Temporarily put the machines on the peg head and line them up where you want them.  Get them even on the peg head.  If they are off line your eye will pick that up and it will look like a very unprofessional job.  Take a piece of tape and after measuring the depth of the hole needed, put the tape around the drill bit.  Don’t ruin your day by drilling all the way through to the face of the peg head!  The next important step is to lubricate the screw with wax or a small amount of soap.  Don’t use auto wax as this can contain silicone which will be hard to remove if you want to refinish someday.  Make sure you use the proper sized screwdriver and carefully install the screw.  These screws are very soft and easily stripped, so go easy.

Some machines have a small post on the underside of the surface (Spertzel). This post is used to stop the tuner from turning.  To install these, put the machine in the proper position and lightly tighten the threaded bushing just enough to imprint the peg head, then drill the hole to the proper dimension.

I hope this helps dispel any misconceptions and happy lutherie!