Hi, rebuilding my YZ125X (Noguchi 175 top end) Using DT clutch bits I found out the main clutch shaft from the YZ 6 speed transmission is 1mm larger than the1977 DT175 one. So the clutch boss doesn't fit.
I'm looking for a boss that has a 14mm ID & 17mm OD re the teeth. Does anyone have these measurements from the part number 525-1637-00 it may fit. 
Cheers

It's hard for me to know what parts you are trying to use as in the USA there was no such thing as a "1977" DT175.  And other countries had a twin shock or possibly a monoshock ("MX" designation outside the US) and they are different animals.

In any case, it sounds like you are looking for the 1G8 inner clutch hub that is splined to the transmission input shaft?    If so, it's a super common part used on the 1978-81 DT175, 77-79 IT175, and various other models.  In the UK, I would try a DT175MX (monoshock) inner clutch hub from the 1977 (if you had a 1977 monoshock model) or 78-81 era

Cheers Mark, the DT175 engine number is IGI 102043. I gather it was a mono shock. The shaft is 1mm smaller than the 6 speed YZ one.
The only markings stamped inside the DT boss are OG-1 (or could be O6-1)
I have another possible fit. 525 16371 boss. Do you recognise this number?

1G1 is a twin shock engine.  The monoshock engine had a 6 speed and is different internally from the 5 speed twin shock engine.    

The 525 inner hub number is almost certainly be what you have in your 1G1 engine?  It would fit the 1976 USA DT175 twin shock which was different from the earlier twin shock 175's and different again from the monoshock 175's.  As far as DT's go, the US 1976 DT175 had a "one year only" clutch and trans

Parts list says you need a 1G8 inner hub which coincides with my experience with swapping a 6 speed into an early 5 speed case.   

Also, Yamaha changed the primary gear ratio so you'll probably need to use the original YZ125x OUTER clutch basket...  which from memory may be the same as what you have in your spare 1G1 engine.  

The later monoshock outer clutch basket will have less teeth on the gear and won't mesh with the crank gear...  BUT there is a good chance you would be able to use the matching monoshock crank gear if you need to use a monoshock outer basket.  

Twin shock 5 speed DT175's had a 74 tooth clutch gear...  Six speed monoshock DT175 went to a smaller 71 tooth clutch gear and larger crank gear.  

Your 1976 YZ125x had a six speed with the larger 74 tooth clutch gear of the 5 speed DT...

1977 YZ125 was a "redesign" and basically became the DT175 monoshock engine architecture with the 71 tooth clutch gear and completely redesigned shifter without the horrible 74-76 "jack shaft" linkage.

In case I'm not being clear, your best chance of making this work will require the 1G8-16371 inner clutch hub and the "525" 74 tooth outer clutch hub/gear that was used on a very few twin shock models in 1976 (and likely later in countries that had twin shock 175's after 1976.)  

Cheers Mark, all very useful info and also a bit depressing.
My outer DT basket (525) has 74 teeth but is knackered. SO, I purchased a YZ125/175x 1976 (537) off ebay thinking I was doing the right thing. Now I've counted the teeth and it's 71 (aaaaaaagh) the DT crank gear has 19 teeth. On the bench it feels as though they mesh but what do I know. At this point It looks as though a need to get a larger crank gear? Is this correct.
This really is a 1 step forward 1 step back event. Still looking for a IG8-1637 inner boss. Cheers

We're flying blind here...  can't see what you bought or have.

I will say with a fair amount of certainty that you did not buy an outer clutch basket for the YZ125x/YZ175c.  Are you going by a casting number to identify it?

The challenge is the outer clutch basket is technically multiple parts.  The aluminum basket and the steel gear are the two major components...  A YZ125x would have a 74 tooth gear.  A monoshock DT175 (or 1977+ YZ125) would have a 71 tooth gear.  The inner aluminum basket was the same on several models and probably has the 537 number on it?  You can't identify a clutch outer basket assembly by a casting number on the aluminum basket.  

Technically the 537 prefix belongs to the YZ125c which also had a 74 tooth gear.  And to complicate matters more, the early YZ125c had a different clutch that was updated part way through production.

There were also two pushrod configurations in 1976 on the YZ125...  two piece without ball and two piece with ball.

Bottom line is you need to be diligent about what you're purchasing...  most ebay sellers don't have much of a clue.  

 

My memory tells me that the aluminum baskets that were mated to the two different gears might have been marked "525"?   Yamaha has always been pretty good about being consistent with parts...  to complicate things, in about 1975-76 motocross bike tech went ballistic.   If you were a serious racer, you pretty much needed to buy a new bike every six months as the manufacturers were making almost continuous changes and improvements.  Many of these changes are hard to document today as often the upgrade replaced previous parts.  For example, the original YZ125c clutch was discontinued.  You could still get frictions, etc, but if you needed a hub, you'd have to buy the new revised assembly that used more discs.  Modern parts lists often no longer show the original parts, they show the revised replacement parts.  

And you'll need a 22 tooth crank gear to use the 71 tooth basket.  In 1976, the gear should be keyed to the crank (earlier models used splines) so a crank gear from a DT175MX should work.

Thanks Mark, looking at several 22 tooth crank gears from 77/78 YZ125's . Is there anything else with the 71/22 combo that effects the rest of the clutch function.

I have also noticed that the 71T 537 alloy basket sides are 30mm deep & the 74T 525 is 26mm.
I gather this would need more plates.

I'm looking for another 74T 525 basket as was on the DT as the best way out.
It is a wonderful world of surprises this project. And your help has been invaluable.
Yes the crank is keyed.