Hi Everybody,

I've been working on restoring an older, non-export Bridgestone, apparently JIS, and the water bottle bosses- while still M5/ 5mm, seem to take a non-standard bolt-thread.

The rest of the frame's rack mounts, canti bolts, etc. are all threaded for either standard M5 or M6 bolts (no issues there); only the bottle cage mounts are different, a standard M5 bolt will bind after a few turns.

Any suggestions as to what this other thread dimension might be?

maybe you need to chase the treads . your lbs should be able to do it in a few mins .

I wish that were the case, but this isn't the first time I've encountered this... I had the same problem some years ago with an '84 Univega. On that occasion, I used a strange set of bolts that I'd had tucked away in a parts drawer (since they never seemed to fit anything else) and they turned out to be a perfect match; however, I never was able to identify the threading. I'm hoping that someone else might have a clue as to what it is.

I Googled "metric bolt thread pitch" and only one M5 pitch was listed as 0.8 mm for both ISO and JIS with no fine pitch option. However, I recall reading about an M5x0.75 threading and that may be what you have.

Shine a light in the hole before you start anything, BUT:

There are two common M5 sizes, M5x0.8 (coarse thread) and M5x0.5 (fine thread). I would find it unlikely that the bike was assembled with braze-in/riv-nut fittings that were not also M5x0.8. More likely, is that the threads are slightly rusty/have paint gumming them up. Without in person inspection, neither of those suppositions mean anything--so take a look. In either case, it is safe to carefully run an M5x0.8 tap through, if you determine this to be the problem.

Thanks; I'll look into it!

Try a 10-32. The thread pitch is virtually identical to .8M, but the diameter is slightly less.
A 5x.8 nut threads onto my 10-32 thread gauge sloppily.

Well... I've checked: the hole threads are clean-as-clean, no paint; no debris; no rust at all... and a 10-32 doesn't fit. In fact, it doesn't thread in even as far as a standard M5.

So... back to square one.

My first source didn't list the M5x0.5 threading but further Googling did come up with it as a fine thread standard. However, I would be very surprised if any bike maker would bother with it.

Use this tool for M5 x 0.8 (metric) thread cutting. The third one from the set of 3. Just to clear the threads. Put some oil onto the tool "teeth" and start screwing it in. 2 turns screw in, then half a turn screw out, to clear debris if the tool does any cutting. Until the tool clears the thread - do not tighten it.

Then your threads aren't .8mm pitch.

Just a crazy guess -- it's probably time for some careful measurements. [I know, I know.]

OK, please let us know how to measure thread pitch inside a 5mm or so shallow hole.

Go/no go thread gages (pic below). The one below sells for about a hundred bucks.

Or, more expeditiously and cheaply, you buy a bunch of different threaded screws and try each one.

Bill's right. A 10-32 (UNF) will fit somewhat loosely in an M5-0.8. An M5 will not fit in a 10-32. The pitch is ver close (0.7948 vs 0.8mm) and the 10-32 has an major diameter (OD) of about 0.190, with the M5 at 0.197, so that the 10-32 should have significant clearance.

If a 10-32 doesn't fit, the thread is either distorted or is not what you think it is.

People hate first hand knowledge because it gets too close to the truth. Random anonymous wild guesses are preferred, so here's another one: M4.5 x 0.75

Woah... M4.5-0.75 is pretty freaking rare; that's one of the obsolete JIS threadings, yes?

I'm going to try M5-0.5 & M5-0.75; they still make both of these, but (Yikes!) the latter I'd have to order in qtys. of 200 or more. (I'll write the company & see if I can get a single one as a courtesy sample.)

If this is something you're just "restoring" for fun, why not simply drill the old bosses out and replace them with something more common?

First hand knowledge is by far the preferred method....if you have a way to get it. Since we don't have access to the OP's bike and he says he's tried all the common bolts, we can only conjecture at this point.

Speaking of wild guesses, I will wildly guess that a hungry tiger will attack you if you were to enter it's cage. Would you prefer to get first hand knowledge to confirm or refute that guess?

4.5 mm threading was also used on Huret DT shifters. If you can get your hands on one from a co-op or a sympathetic LBS, you could use it to test the threading.
I did retap a Huret shifter band to accept modern adjusting cable stops; but this isn't a high torque situation. I suspect water bottle mounts re-taped in this way would be acceptable w/ Locktite as a backup.

I think that's what I would try. Drill to 11/64ths followed by M5 x .8 tap.