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Double Bass Most Common adjuster thread sizes

Chris Fitzgerald

Chris Fitzgerald

Student of Life
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Oct 19, 2000
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Louisville, KY
www.chrisfitzgeraldmusic.com
#1
A good number of my students (past and present) are interested in using the Full Circle pickup after playing the basses in my office that are equipped with them. I'm familiar with the most common diameter/thread pattern and the smaller metric size Fishman offers, but for years a fair number of students have had adjusters with bigger threads that seem to be made of or coated with some sort of synthetic nylon-ish material. Does anybody know the adjusters I'm talking about? I see them on a fair number of Shens and newer basses.

At any rate, I was just advising a student about pickups and noticed that Gollihur now shows Fishman as offering a third thread size for the Full Circle:

Screen Shot 2017-03-08 at 7.30.36 AM.png


I can't tell from eyeballing if the new 5/16" size is the same as these adjusters I've been seeing, but would love to find out so I know whether it's OK to now recommend the FC to students who have these adjusters. I can try to get a pic of a current student's bass if needed to help sometime this week.
 
#2
I don't know if this is the same as what you are seeing, but I had a Thompson sold by string emporium (supposedly made by a former employee of Shen) that had dehlrin adjusters. Sort of appears to be plastic. I contacted Steve of SE asking the size several times with no response so I bit the bullet and ordered the largest size offered of the full circle. It did not fit and once I had the bridge off I measured one to be 3/8, if I remember correctly. Gollihur accepted the return and sent me a Lifeline which I was able to make work.

YMMV
 
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#5
Chris Fitzgerald said:
A good number of my students (past and present) are interested in using the Full Circle pickup after playing the basses in my office that are equipped with them. I'm familiar with the most common diameter/thread pattern and the smaller metric size Fishman offers, but for years a fair number of students have had adjusters with bigger threads that seem to be made of or coated with some sort of synthetic nylon-ish material. Does anybody know the adjusters I'm talking about? I see them on a fair number of Shens and newer basses.

At any rate, I was just advising a student about pickups and noticed that Gollihur now shows Fishman as offering a third thread size for the Full Circle:

View attachment 1074615

I can't tell from eyeballing if the new 5/16" size is the same as these adjusters I've been seeing, but would love to find out so I know whether it's OK to now recommend the FC to students who have these adjusters. I can try to get a pic of a current student's bass if needed to help sometime this week.
Click to expand...

The largest (5/16x18) size has actually been out for several years. But it is still not as large as the delrin and wood adjusters. There is no Full Circle that will fit where those non-aluminum/brass adjusters have been. I would guess that the delrin cannot be machined as small as the metals can, I've never seen wood ones that small either.

If you want to figure out what size you need, we have an FAQ for choosing the right size Full Circle that should help.
 
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#6
It could be M8x1.25, which would be a logical choice for Delrin adjusters. I think that is what Yamahiko uses. If you have a metric ruler, hold the stud against it and you would see 8 threads per cm, and 8mm width. If this is the case, you can probably do a helicoil insert to bring it down to 1/4-20.
 
#7
Having access to a mill and a lathe I machined nuts to fit my adjuster into a sleeve and then machined a matching pocket into the bottom of the bridge to fit the sleeves into. This was because one of the wood threads had failed and I was just trying to salvage the bridge.

I could make sleeves like this in any thread size you can but a nut.
 
#9
MrMyopia said:
I don't know if this is the same as what you are seeing, but I had a Thompson sold by string emporium (supposedly made by a former employee of Shen) that had dehlrin adjusters. Sort of appears to be plastic. I contacted Steve of SE asking the size several times with no response so I bit the bullet and ordered the largest size offered of the full circle. It did not fit and once I had the bridge off I measured one to be 3/8, if I remember correctly. Gollihur accepted the return and sent me a Lifeline which I was able to make work.

YMMV
Click to expand...
The Delrin adjusters are sold by Marc at New Harmony Music and they come in two thread sizes: 3/8"-16 and 7/16"-14
 
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#10
Be wary of those delrin adjusters! They are smooth as silk and you can adjust them at full tension...and this happened while the bass was just sitting in the corner with nobody touching it. BAAAAMMMMMM!!!!!....and then the bridge dropped on the floor along with a new big dent in the top along with plenty of $#@!^&!!!!!....
never again, never again, never again......


B43.JPG


One small additional bit: there is an error in Chris' opening thread, bottom photo, last line: ".....5/8" post...." should read 5/16" post!

BTW: thx for the photo Chris, as it helped me solve a last minute Sunday evening repair conundrum; luckily I had a 5/16 tap in the truck tools!
 
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#11
E61F3FFB-C30E-44CC-B2B6-DFE86ABA527D.jpeg


This is a 5/16 -18 adjuster with an aluminum shaft and pinned delrin wheel. The 8mm-1.25 adjusters are almost the same diameter and are 20 threads per inch. The same holes can be retapped to use either so I know the Full Circle can be used in an 8mm-1.25 bridge if it is retapped for 5/16-18. I just dealt with the issue last week.
 
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#12
I went along for almost 40 years trying to figure out thread sizes, going to the hardware store and trying nuts, etc., etc., and then I FINALLY just broke down and bought the damn set of thread gauges. With thread gauges and a caliper, you can MEASURE threads and determine what they are with CERTAINTY. I recommend that anyone involved in any kind of mechanical work make the small investment. You can easily download a chart of thread sizes, or if you're going to be involved in mechanical work you can buy a copy of Machinery's Handbook (it doesn't have to be the latest edition, UN and metric threads haven't changed since the 1950s). All the various kluge workarounds will end up costing you a lot more time and trouble than a few good measuring tools and the appropriate reference books.
 
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#13
Greg Clinkingbeard said:
View attachment 3737424

This is a 5/16 -18 adjuster with an aluminum shaft and pinned delrin wheel. The 8mm-1.25 adjusters are almost the same diameter and are 20 threads per inch. The same holes can be retapped to use either so I know the Full Circle can be used in an 8mm-1.25 bridge if it is retapped for 5/16-18. I just dealt with the issue last week.
Click to expand...

Why bother - the Full Circle is available in a 5/16x18 size - just get that one, right?
 
#15
Greg Clinkingbeard said:
That’s the point. If someone has a bridge with 8mm-1.25 adjuster threads, they can use the Full Circle, but will need to run a 5/16-18 tap through it.
Click to expand...

I wouldn't do that. Major dia. of a 5/16 thread is .313" and major dia. of a M8 thread is .315", in other words essentially identical. M8-1.25 has about 20 threads per inch and 5/16-18 has, of course, 18 tpi. This means when you run that 5/16 tap through the M8 hole you're going to be cutting away a substantial fraction of the thread. In fact, after 10 threads or so you're going to be cutting away the entire thread, as the 18 tpi tap and the 24 tpi existing thread go in and out of phase.

For a lightly stressed threaded joint in steel I'd be OK with that but for a fairly loaded thread in wood (wood!) that you expect to be sawing back and forth on when making the adjustments, I wouldn't. (Putting machine threads in wood is already bad practice when the threads are perfect; though obviously it can work OK-ish in something like hard maple.)

Personally I'd put a threaded insert into the wood and secure it with epoxy. There are a variety of versions of these, or you could take a coupling nut or something similar and just turn off the OD to jsut a little bigger than the major diameter of the thread.
 
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#18
Re Delrin, there are plenty of reports (like James's) about them being too weak to withstand pressure. New Harmony seems to make theirs 100% Delrin, while I think Chris is describing a Delrin wheel with an aluminum shaft. This might work, so long as the wheel and shaft are seriously properly bonded (and won't come apart down the road).

IIRC, Bill Lakeberg makes and sells another option: Delrin wheel and Delrin shaft reinforced with an aluminum pin (the pin runs down the centre of the Delrin shaft). This seems like the best case scenario for having an easy to turn shaft and avoiding a stress fail.
 
#19
eh_train said:
Re Delrin, there are plenty of reports (like James's) about them being too weak to withstand pressure. New Harmony seems to make theirs 100% Delrin, while I think Chris is describing a Delrin wheel with an aluminum shaft. This might work, so long as the wheel and shaft are seriously properly bonded (and won't come apart down the road).

IIRC, Bill Lakeberg makes and sells another option: Delrin wheel and Delrin shaft reinforced with an aluminum pin (the pin runs down the centre of the Delrin shaft). This seems like the best case scenario for having an easy to turn shaft and avoiding a stress fail.
Click to expand...
Hi eh_train,
Do you think that the Delrin adjuster failure (in James' photo above) was caused by the stress induced by the misalignment of the top part of the Bridge - i.e., being pulled by the string tension? The break, just above the adjuster wheel, indicates that the unthreaded shaft was misaligned and "pulled" to the point of breaking.
If the adjuster was some made of a non-Delrin material, it may not "snap" but there would be a significant amount of stress in that blind hole in the top part of the Bridge - Yes?
Just wondering. I may have watched too much "CSI Luthier".
Thanks.
 
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#20
[QUOTE="...there would be a significant amount of stress in that blind hole in the top part of the Bridge - Yes?
Just wondering. I may have watched too much "CSI Luthier".
Thanks.[/QUOTE]
Hard to tell from the pic, but if the unthreaded part is in the top part of the bridge I’d say that the adjusters are upside down! Can of worms, I know...

Adjusters do have to carry a lot of load, so a good fit is always going to be helpful. Delrin alone doesn’t seem to be the right material.

And CSI: Luthier? Great show!
 
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