Mmm, do I need to run the 6 plate clutch, due to the extra power. You mentioned slip. Only because mine will be a ported Noguchi 175. So closer to 28/30HP (allegedly) This would work with the 537 basket I've got (it's deeper) Will this work with the 1J4 boss (and the 22T drive gear of course) ?

I would be concerned about slipping and would try for the 6 friction clutch.  A stock DT175 engine with the stock 5 plate clutch sometimes slip...  I never had the same slippage issues on the later DT175 (monoshock) with the updated 6 friction clutch...  even when modified for a lot more power than stock.  

I'm kind of annoyed....  I completely emptied the shed where I thought the box with the later 175 (monoshock) clutches was hiding and I didn't find it.  

That shed has about 30% of the parts boxes along with a lot of household stuff...  I have 4 other places to look.  

I DID find my 78 DT125 engine (monoshock) with the 1J4 clutch hub.  I can confirm that the 1J4 inner hub has the ~ 14mm by 17mm spline but is too short to use 6 friction discs.  

(The corresponding aluminum clutch basket is a 525 casting but it is mated to a 71 tooth gear instead of 74 tooth used on the earlier models.)

I will keep looking...  I didn't realize how much stuff I had and how much it got disorganized when we moved here.  

Here's the 1J4 hub. About 24mm tall splines which goes with the shorter 525 outer basket.

Morning Mark, thanks for the measurements re the 1J4 boss. I've ordered the 6 plate clutch kit so I'm committing my deeper 537 basket. So all I need now is the 1G8 inner hub/boss.
Good luck in your shed. You are my only hope at this point. No pressure.
I'll keep searching the inter web. Aaaaagh

Make sure to measure the thickness of new frictions when you get them.  They need to be 3mm thick and not 4mm thick.  Yamaha made this change right around 1976 on the the 125/175 and sometimes you'll get the earlier 4mm thick frictions from a supplier....  thicker ones will fit the basket just fine as the diameter and tabs didn't change, just the thickness.  

I am continuing my search... 
 

Found the 1G8!  It has 1G8 printed on it in ink and 537 cast in.  Came from a 1980 DT175(MX).  Splines are 14mm x 17mm.  Disc stack height is about 28mm.  




 

Wehay, brilliant. Mark your a genius. Are you willing to part with this unicorn and how would one arrange the purchase?
Cheers

Here is a list from CMSNL of which other bikes used that part, just in case!

BOSS, CLUTCH for Yamaha - order at CMSNL

• RT180 1993 (P) USA
• RT180 1995 (S) USA
• RT180 1996 (T) USA
• RT180 1997 (V) USA
• YZ125 1977 USA
• YZ125X 1976 USA
• YZ175 1976 USA
• IT175 1977 USA
• IT175 1978 USA
• DT175 1978 USA
• YZ125 COMPETITION 1978 USA
• YZ125 COMPETITION 1979 USA
• YZ125 COMPETITION 1980 (A) USA
• DT175 1979 USA
• IT175 1979 USA CANADA SOUTHAFRICA
• MX175 1979 USA CANADA
• DT175 1980 (A) USA
• IT125 1980 (A) USA
• IT175 1980 (A) USA
• MX175 1980 (A) USA
• DT175 1981 ( USA
• IT125 1981 ( USA
• IT175 1981 ( USA
• MX175 1981 ( USA
• IT175 1982 (C) USA CANADA
• YZ100 1982 (C) USA
• IT175 1983 (D) USA CANADA
• YZ100 COMPETITION 1983 (D) USA
• RT180 1998 (W) USA
• RT180 1990 (L) USA
• RT180 1991 (M) USA
• RT180 1992 (N) USA
•  

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