Hey everyone, I have run into a wall here. I have a 75 250B Mx I am going through. I have the cases split and was inspecting the gears for the transmission. The main clutch shaft has a gear that is cracked in two places and the shaft is bad. I looked the parts up in my factory parts list manual and it says the shaft is supposed to have 16 teeth. My shaft has 18 teeth, and the second gear is supposed to have 32 teeth and mine has 31 teeth. I contacted a seller on Ebay that has both gear clusters for sale and asked him what the tooth count was for the main shaft and the second gear. He told me that the main is 18 and second is 31, and he also looked in the book and it calls for the 16-32 combo.
I have looked in all he online parts suppliers and for the gear, number 17 in the transmission list, calls out for 31 teeth but there is no tooth callout for the main shaft.
Anyone have any insight for this? I did a Microsoft AI search for any Yamaha Enduro, Mx and YZ bikes that might have a 18 tooth count main shaft and there isn't one.
Thank you for any and all info on this.
Don.

www.cmsnl.com/yamaha-mx250-1975-usa_model8540/partslist/ not sure if this helps. I'm on my phone which is limiting me, plus old age lol. The link above show 18 teeth on first gear that is part of the main shaft. I'll look on the computer tomorrow as I can get it all on one screen. But I'm thinking if yours has 18 then this link should also show the content number of teeth on the other gear you want. You say second gear or "the second gear" so I'm thinking you mean first gear big wheel that runs against your 28 tooth. Anyway. Sleep time & I might be totally up the creek  I did have & race a "75 MX250B new in "75 so I'm an old farther now but have fond memories of that model. Worked first the dealer back then. Hope my link takes otherwise go to cmsl & 1975 & I picked the link on the right. Doesn't say B model but looks like.

MX250 and YZ250 had an 18 tooth mainshaft in 1975 and 1976 at the very least...   Factory Yamaha Tech sheets and specs verify this as does RT325's find on CMSNL

The most important question that comes to my mind is the ("second" or "driven") mating gear for the 18 tooth first gear which shows as 33 teeth in the specs...  I'd want to make sure the first gear "set" is correct...  in this case 18/33

But I think you might be jumping to asking about the actual second gear driven wheel in the transmission which shows as 31 teeth in the specs.

In my experience, it's always best to buy used gears as a set if possible...  they wear into each other and sometimes swapping one gear to replace a bad gear in a used pair doesn't last long before that gear pair fails again.  Sometimes it's best and even cheaper to find a whole transmission set on ebay and use the whole thing or the matched pairs of gears.

P.S.  MX250A had a different first gear with a 16 tooth mainshaft...  the parts book you have might be incorrect for your B model...  Yamaha always had lists of parts books errors and changes coming out as sometimes the books were not updated to match new model changes.

This is the first I have seen a call out for the 18-tooth main shaft ger, thank you.

I have checked the mating gear for the18 tooth main gear, it is 33 teeth, so it is 18-33. What got me jacked up is that the factory parts list book calls out for the main to be 16 teeth and the mating gear to be 33. That is also what had me going because if one gear was even a tooth off the mating gear had to be different too. Just like my book calls for the number 17 gear in the diagram to have 32 teeth and mine has 31, the mating gear has the same tooth count as my book calls for.  
I found a guy not far from me that has a couple complete transmissions, I am going to grab one tomorrow.
 I have 3 1975 250B Mx bikes I am building. One will be bone stock like it can from the dealer except for a IT400 front end. The other two will be modified like they could have been back in the day.
They will have IT400 front ends and modified aluminum swing arms. and custom colors. One of the builds. I have checked the mating gear for the18 tooth main gear, it is 33 teeth, so it is 18-33. What got me jacked up is that the factory parts list book calls out for the main to be 16 teeth and the mating gear to be 33. That is also what had me going because if one gear was even a tooth off the mating gear had to be different too. Just like my book calls for the number 17 gear in the diagram to have 32 teeth and mine has 31, the mating gear has the same tooth count as my book calls for.  
I found a guy not far from me that has a couple complete transmissions, I am going to grab one tomorrow.
 I have 3 1975 250B Mx bikes I am building. One will be bone stock like it can from the dealer except for a IT400 front end. The other two will be modified like they could have been back in the day.
They will have IT400 front ends and modified aluminum swing arms. and custom colors. One of the builds.

Keep in mind that gears do not like even fractions of each other for wear and harmonics.
16/32 is asking for trouble.
16/33 is OK but not the best.
18/32 is a no no for wear.

Usual targets are at least one prime numbered element in the pair or mating parts.

cliff

Keep in mind that gears do not like even fractions of each other for wear and harmonics.
16/32 is asking for trouble
16/33 is OK
18/32 is a no no for wear.

Usual targets are at least one prime numbered element in the pair or mating parts.

cliff