TOM bridge specifications

[Simon Bancroft]

Hello all, I am a regular lurker on this board. I gratefully soak up all I can about flattop repair, my hobby.

Here's something new to me, and I can't find any answers on the net or in Frets.com. I've bought a new Gibson Tune-o-Matic bridge to replace the (non-original) one on a Harmony Rocket. What is the optimal geometry of the thumbwheels and the base? Specifically, how much space should there be between the top of the base and the bottom of the thumbwheel?

In the photo attached the action is 7/64 - 5/64. I want to sand the bottom of the base on the treble side so that 3/64 action can be dialed in. Do I sand (and scrape and fit) a little more to allow the guitar to age?

And while I'm asking, I see by Frank's pic in Frets.com that the intonation screws face the headstock as they do on the Rocket. I often see them facing the other way. Is there a right way and a wrong way?

rocket%20bridge.JPG

[Paul Verticchio]

Hi Simon (-:

Q, "Specifically, how much space should there be between the top of the base and the bottom of the thumbwheel?"

A: Just enough to allow for reasonable adjustments in the future.  I like to see about 3/16-1/4" which will allow the 12th fret action to be adjusted down by 3/32-1/8".

Q: "Do I sand (and scrape and fit) a little more to allow the guitar to age?"

A: If this is a vintage Harmony, it's done 'aging' or as we call it "settling in".  Yes, you sand the bottom of the bridge to lower the action in general.  "How" to do this correctly can be found on line. All you need is a couple of sheets of 80 grit abrasive paper & some low tack masking tape. You have some work ahead of you, but you'll love the results.

May I suggest: Set the thumb wheel washers 1/4" off the top of the base, put the TOM back on, put a couple of strings on it and you'll be able to judge how much of the bottom of the base should be removed. 

Removing 1/4" from the bottom of the base will lower the action at the 12th fret by 1/8".  Generally for every fraction of an inch that the bridge assembly is lowered,  the action will be lowered by half of that measurement at the 12th fret.  BTW: that's the only simple math used in this craft(;  Sand a little, check your results, sand some more etc., until you get it where you like it.  If you take off a little too much, you can always raise the action via the thumbscrews.

Q: "And while I'm asking, I see by Frank's pic in Frets.com that the intonation screws face the headstock as they do on the Rocket. I often see them facing the other way. Is there a right way and a wrong way?".

A:  Face the screws in a direction that allows the easiest access to them AND which allows an uninterrupted string break angle.  When the screws face the tailpiece, if the break angle is too steep, the strings touch the adjustment screws and that must be avoided.  I personally prefer pointing them toward the neck This method eliminates the string break angle potential problem.  Access ability varies from instrument to instrument.

I applaud your upgrading of this classic American guitar.  In the 60's, we peons held our Rockets in the same high regard as the privileged kids' 335's. They were, and remain, a GREAT classic American icon.

Best of luck,

Paul (-:

[Simon Bancroft]

That is the complete answer! Thank you very much, Paul.

I especially enjoyed your last paragraph. This Rocket was almost minty when I bought it on ebay three years ago and gave it to my son. It's had many exciting adventures since. 

Simon