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Replacement bridge screws smaller than original ones

Discussion in 'Hardware, Setup & Repair [BG]' started by cossie, Mar 30, 2009.

  1. cossie


    Apr 29, 2005

    I got a new bridge and was going to put it on over the weekend but the screws for the new bridge are fractionally smaller which means they spin around in the old holes. And the original bridge's screws are too big ( it's starting to sound like Goldilocks & the 3 Bears...) to be used with the new bridge.

    I'd be really grateful for any tips or advice on how best to fix the problem. I read a thread about "worn screw holes" but it didn't really cover bridge screws.

    I'm hoping to fit this new bridge myself without going to a luthier unless it's absolutely necessary.


  2. 202dy

    202dy Supporting Member

    Sep 26, 2006
    You have three choices.

    A. Plug the existing holes and bore new ones to the proper size for the new screws.
    B. Counter-bore the new bridge to accept the old screws.
    C. Put the original bridge back on and warehouse the new one for another project.

    Plugging the existing holes and boring new ones is the easiest solution if you want to use the new bridge.

    1. Measure outside thread dimension of the existing screws.
    2. Purchase dowel rod to fit.
    3. Measure depth of hole.
    4. Cut dowel to length.
    5. Coat dowel with white or yellow glue.
    6. Inject some of the glue into the hole.
    7. Insert dowels into holes.
    8. Wipe up glue the squeezes out with a damp cloth.
    9. Let cure for twenty four hours.

    At this point it's time to bore new holes. The important part of this procedure is to correctly measure the new screw's shaft. That is the diameter hole you want to drill. That will allow the threads to have purchase on the wall of the holes. Hold the screw up to the light while comparing the shaft of various sizes of twist drills to the shaft (not outside threads!) of the screw. When you have decided on the right size chuck it up in the drill.

    10. Center punch the doweled hole.
    11. Put a masking tape flag on the bit to act as a depth gauge so that you do not drill through the body.
    12. Bore the new hole using the divot to center the bit.
    13. Repeat for the rest of the holes.
    14. Install the new bridge.

    It is important to keep the drill bit perpendicular to the body. If you are not a regular tool user this can be difficult to do on your own. You can use a level standing on the body or you can ask a friend to stand back and watch. If they line the work up with a vertical line in the room such as a door or window frame it will be a lot easier for them to tell you what to do.

    Good luck.
  3. The heck with dowel rods and re-drilling. Take a couple of toothpicks, snip a few pieces to fit into each screw hole, dab in a drop of wood glue and then insert the toothpick pieces and screw in the smaller screw.

    Voila! Tight screws.
  4. S Sanders

    S Sanders

    Feb 15, 2009
    Tulsa, OK

    I considered posting this recommendation. I work in maintenance and have used this technique for many repairs. However, having never tried to repair an instrument, I wasn't sure this fix would be appropriate to the application.

    In fact, given string tension, I still have to wonder if such an approach would be advisable long term. Again, though, I have absolutely zero expeirence with instrument repair.
  5. HogieWan


    Feb 4, 2008
    Lafayette, LA
    it works for strap buttons - it would probably work for a bridge as well since most of the pressure is lateral
  6. 202dy

    202dy Supporting Member

    Sep 26, 2006

    The question of whether or not toothpicks will hold the bridge on under string tension has been debated here several times.

    The goal of the remedial work is insuring the bridge is mounted to the body securely. The goal is not "tight screws". As a matter of fact, tight screws can be a real problem.

    Let's look at it from an engineering standpoint. Doweling and drilling to proper size allows the screw's threads equal 360 degree contact with the hole. Utilizing the dowels and boring creates a thin sleeve inside the hole that is completely adhered to the original walls of the hole. That is the way it is supposed to be if maximum strength is desired.

    Jamming a few toothpicks in the hole only allows for the threads to contact the hole at the tooth picks. That is considerably less than 360 degrees. Further more, with a couple of toothpicks inserted there is a lot more material in the hole than the counter bored dowel sleeve. When the screw is inserted There is a wedging effect on the body. This can cause cracks in the body. The cracking is rarely a structural problem. However, it will show up as cracks in the finish.

    Will it hold? Yes, most of the time it will hold.

    Would the method be acceptable to you if you took your guitar to a pro for bridge installation? I doubt that anyone on TB would be happy if they found out this is what their chosen tech did to their guitar.

    Doweling and boring is how a pro does the work. It is the precise way to accomplish the task. If you choose to do it differently that is your business.

    But it begs the question, what did you save by using this less than optimal method?
  7. cossie


    Apr 29, 2005
    Thanks for such a detailed reply 202dy, much appreciated. I probably won't get a chance to get the doweling etc until the weekend, but it gives me this whole week to prepare for it all.
  8. zagnut


    Jan 4, 2009
    Back in Detroit
    I drilled the holes out in my new bridge to accept slightly bigger diameter screws. I also used screws that were longer to make certain that they had substantial bite after all these years.

    If using longer screws, I would most certainly drill small pilot holes down into the original holes in the body to avoid the "wedging" effect previously mentioned.
  9. cossie


    Apr 29, 2005
    Bit of an update...

    I got 5 screws of the original bridge into the new bridge last night, simply by filing the holes enough to fit the screws - it wasn't a huge deal, just a few mms but I was really careful with what I was doing so it took a good chunk of time. The screws go in evenly and stay super tight.

    The next part is drilling 2 holes correctly for the remaining 2 mounting screws in the top left and right corners of the bridge. This is the part I'm a bit nervous about but I'll practice loads of any spare chunks of wood I can find!
  10. 202dy

    202dy Supporting Member

    Sep 26, 2006
    You have chosen Option B. A brave choice.

    Filing the holes works very well. The deluxe way to go about this is to counter bore the holes in the bridge on a drill press. There is one more way to do it safely. If you have a drill index and a vise this is easy. Find the bit that just fits the hole. Step up one size, which will only be a few thousandths larger. Chuck up the bridge in the vise and the bit into a drill and bore the bridge holes. Then move up to the next size and repeat until the holes are large enough to clear the screws. The result is clean if not as quick as using a drill press or a milling machine.

    As far as drilling the two top holes goes, using a level or a partner is still the way to go. However, depending on the length and penetration of the three screws in the existing holes the last two holes may not be necessary. More information about the new bridge, the guitar and body wood, and screw sizes would be helpful.
  11. cossie


    Apr 29, 2005
    Sure thing. I don't know the model of the bridge as I picked up at a guitar repair shop, it was going real cheap as it had no screws with it. It looks kind of like a chrome Gotoh Standard bridge but it has a lot more black (springs etc). It has 5 mounting screws at the back and 2 at the front - so 7 screws in all.

    The wood of the body is basswood - it's an Aerodyne bass.

    And the screw size is 3.5mm.

    And thanks again 202dy for your reply.
  12. 202dy

    202dy Supporting Member

    Sep 26, 2006
    If you have five in the rear, that's plenty. Three will probably do it if the original holes are not crossthreaded or damaged in any other way. If you're subscribe to the theory of "screwed, nailed, and glued", then by all means go for it.
  13. cossie


    Apr 29, 2005
    Sorry I rushed the reply before going to a meeting, I meant it had screws with it but NO packaging.
  14. cossie


    Apr 29, 2005
    Do you mean that the bridge could be mounted with the 5 rear screws and I wouldn't need to do the front 2? Don't worry I won't hold you responsible if I leave the 2 out and end up with a face full of bring at some point :p
  15. 202dy

    202dy Supporting Member

    Sep 26, 2006
    Bad Ass II bridges have five screws. Fender bridges have five screws. Original BA bridges have three.

    Five's plenty.

    But you know what they say about opinions on the internet.
  16. cossie


    Apr 29, 2005
    Got the bridge on - with all 7 screws - and put a new set of strings on it last night! :hyper:

    Boy-oh-boy was I nervous tuning those strings, I kept expecting a huge snap and seeing chunks of wood fly across the room. Thankfully nothing like that happened. I haven't had a chance to setup the action but I'll do that tonight.

    I was in a local music store checking out new gear and happened to mention to the owner I was in the process of putting a new bridge on my bass, he gave me that exact same advice - and he's a guitar tech. I did it before screwing my new bridge in and so far so good!

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