HMJ

Greetings...

Today I decided to work on early 90's Trek 7000 MTB I picked up in 2022. Upon getting it into the light I discovered the seatpost clamp is missing. I figured this would be easy enough to replace: just go to the parts bin and pick one...but I figured wrong. The OD of the seat tube is 33.4 mm, which appears to be a very odd size since there near zero references (that I can find) to a corresponding seatpost clamp.

There is a not-so-close-to-me bike co-op that I could rummage through next time I'm in the area. I thought about trying to "expand" a 31.8 mm clamp but not sure how that would turn out. I also have a 34.9 mm clamp that I suppose I could somehow shim. I did find a couple of (somewhat expensive) 34.0 clamps on ebay that may or may not work.

Beyond those options, am I SOL?

Many thanks in advance.

FBinNY 

I suspect that there's some measurement error, either your mistake or some distortion at the top of the tube.

If this is a metal frame, measure the middle of the seat tube where you know it's OK. Since the tube extends to the end, that's what the right clamp would match.
Or-----
Measure the seat tube after a proper fitting seatpost is in place, checking that it's snug and can't rock, nor are there any visible gaps.

Of course there's always the option of finding a published spec. online.

Worst case, if it actually is a screwball, you can build it up to a standard diameter with multiple windings of tape.

mpetry912 

what is the measurement (Outside diameter) of the seat POST ? I suspect it's 31.8

that's the measurement you need. then go to eboy and type in "seat post clarmp 31.8" and see what you get.

seat post clamp 31.8 for sale | eBay

/markp

bboy314

The clamp needs to be sized to fit the seat TUBE of the frame, not the seat post.

Salsa makes a 33.3mm clamp.

FBinNY 

Sorry, but no.

You have to keep the OD of the post distinct from that of the tube it fits into.

Frame seat post clamps are sized by the OD of the frame tube, not the post, they fit.

So, assuming a 31.8 (1-1/4) post, we'd expect a seat tube with that ID, a and most likely a 1/16" wall and 34.9mm (1-3/8") OD.

JoeTBM 

Let us see if we have one in our bins, I will check later this morning for you

Crankycrank

A shim of 0.75mm thickness is all you would need the reduce the 34.9 to 33.4, about the thickness of some aluminum cans which you could use. I had a bike for 20 years with an aluminum can shim for the seat tube clamp and no issues.

wesmamyke

Can confirm the bonded Trek frames take an odd clamp. I usually kept one in the seat clamp bin as they are hard to find. I might have one let, would have to check. An older Trek dealer, not a Trek branded store would probably have one somewhere.

HMJ

Many thanks for your feedback. I probably should have inserted a pic of the caliper measurement originally but I will post it here for posterity in case anyone else stumbles upon this oddball:




Above is at the top of the seat tube where it is smaller than the rest of it to accommodate the clamp (note lip below the caliper). There does not appear to be any undue "stress" - either inward or outward - on the clamp area as the slot opening looks parallel (to my old man eyes). The rest of the seat tube appears to be 35.2 mm FWIW. Frame is aluminum.

HMJ

Thanks for confirming!

I had briefly considered calling a pre-Trek-store shop and asking if by chance they had one of these laying around but thought the odds were pretty long...BUT...after reading your post I started to think the odds have to be a bit better than rummaging through bins at a co-op

FBinNY 

Yes, we're past that now. As confirmed by another post earlier, Trek, for whatever reasons of their own, opted to neck (or turn) the tube down slightly to make it require a unique clamp. Then, of course, stop making that clamp.

Either shim it to accept a standard 35mm clamp, or visit an old time Trek dealer and hope they still have the right clamp.

FWIW I'm impressed, though never surprised at the lengths companies will go to create oddball sizes.

bboy314

Another option: use the readily available clamp in the correct size manufactured by Salsa.

HMJ

So they do...many thanks for the intel!

I'd have to check the width of the Salsa clamp to make sure it is not larger than the clamp area of the Trek's seat tube, since it is smaller than the rest of the tube (see above). I wouldn't think it is an issue...but then again I didn't think getting a clamp for this bike's seat tube would be a big deal either

veganbikes

I would have high doubts that Salsa clamp wouldn't fit it is such an odd size that it would be silly of them to make it incompatible with the Treks which have that odd ball size and might be the only ones.