Airhead question: 77 tranny with 82 flywheel

Can the input shaft of a 1977 transmission be converted to fit with a lighter later 1982 flywheel/clutch combo?

Yes.

The input shaft for a later light or no flywheel bike is shorter. The trans can be opened up and the suitable short drive shaft substituted for the long one. Of course you will have to swap over all the gears. If both drive shafts have gears it may be possible to do it by a complete swap but there was another change in I believe March of '82 when the three gears with angle cut teeth had the angle changed from 15 to 17.5 degrees. You can not mix these gears up and can use either set up but it has to all match.

IIRC, Snowbum actually proposes shortening the existing shaft with a cut-off wheel or similar device.
Shorten the pushrod accordingly and you should be good to go.

I've heard of doing this and agree it should work but I'm not sure the throw out rod also gets shortened. Have to look at them next to each other to see what way would work.

The '81 on pushrod pilots into the "ring" at the rear of the crankshaft. If I was doing this conversion, I would add all the updates BMW made here: the clutch assembly, input shaft, gearbox cover, pushrod and thrust bearing. It's definitely worth doing; the '81 on clutch will handle far more power and is more user friendly.

The input shaft of your 77 gearbox wont work as is and I never heard of the shortening method...but then again would all that be worth the effort? What are you trying to accomplish?

I'm told the newer style clutch is the better clutch and I can certainly vouch for the improvement in throttle response, as well as shifting, with the lighter /7 flywheel I just installed. I imagine the change would be even more dramatic with the even lighter later clutch assembly yet.

and here is the quote from Snowbums web page:http://www.bmwmotorcycletech.info/transmission.htm

8. In 1981 BMW made changes in the transmission clutch throwout bearing area, internally as well as the external shift lever arrangement. Also in 1981, BMW made a large change to the clutch and flywheel, which became a totally different design, and the flywheel is now called a Clutch Carrier....and the transmission input shaft was changed (shorter) to accommodate those changes. Transmissions before and after that date can be interchanged if the input shaft is the correct one. The input shaft is about 24 mm long on the early transmissions, and from 1981, it is about 19 mm long. It is also possible to shorten the input shaft on an early transmission, in order to fit it to a later clutch unit...by simply using a cutoff disc, etc., even with the transmission still together! Be sure to radius the forward edge properly. Within certain guidelines, such as the input shaft length, and internal gears angle cuts, shift kit or not, proper neutral switch...etc...as all mentioned in this page, most all the transmissions generally interchange.

I wonder if the flywheel and crankshaft are balanced together or balanced Independently.
My goal is to reduce the flywheel mass as much as possible by using the post 80's clutch and "flywheel".

Personally, if I rode a pre-'81 model swapping over to a late model clutch would be the first thing I would do to it. Well, that and converting my snowflakes to tubeless. Swapping over to a simplex timing chain would be a close third. The '81 on clutch is light years ahead IMO both in weight AND function. They can easily handle way more than stock power unlike the earlier units. The earlier units have enough trouble holding stock power!

They are balanced independently. They can be balanced as a unit but I see almost no advantage to it.

I have never shortened an input shaft and clutch rod. I have swapped many input shafts. That includes the later cover plate, clutch rod, lever, the whole deal from cable end to clutch. I recommend someone with experience if you swap shafts. Remember that with modifications supposedly most all transmissions interchange but their parts do not. They will fit together and work for a few thousand miles but then you will have a mess on your hands. Don't make the mistake of thinking that the changed helical gear pressure face angle is the angle of the helical. That is the same for both PFA's. The difference is in the shape of each individual tooth and it is hard to see just by looking.

" Don't make the mistake of thinking that the changed helical gear pressure face angle is the angle of the helical. That is the same for both PFA's. The difference is in the shape of each individual tooth and it is hard to see just by looking."

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The newer gears I have seen were marked with an X or have a line cut around the circumference of the gear teeth.

has anyone actually shortened the shaft?

IIRC, there is an identifying mark on the output shaft helical gear.
Is there one on the input shaft gear as well ?
How about the layshaft ?

All three gears are supposed to be marked. It's hard to spot on some. I am pretty sure I have seen some real late model gears that were not marked in any way that I could see. If you have known examples, you can compare and see the difference but it is tricky IMO.

Is that the X mark that Anton speaks of on his website ?

The 17.5 gears are supposed to have an X on them. I have heard the same thing SS mentions that there are a few with no X but are X gears. You usually can't see the X until the gear is off the shaft.

I didn't know before that the angle is a reference to the sharpness of the teeth and not the slant of the teeth. I thought it was the slant.

A lot of people think that. That's how the gears get mixed in some cases. That's why some people talk about the changed PFA effecting end play and axial load. It doesn't. What it effects is where each individual tooth along the height of the tooth contacts the next tooth and transfers the load.

so if you were to engage an X and a non X gear the teeth will not mesh properly while matching gears will ?
In other words, matching gear teeth will engage fully, down to the root of the tooth, while non matching gears won't.

I have seen it done numerous times working hear and there. They fit together just fine. They do make a bit more noise from the get go. From what I have seen, it takes 5 or 10k miles for them to eat themselves. That they will do! Very unfortunately, I have seen some pure unobtainium close ratio intermediate shafts F'ed up for it. Man I wish I had those F'ed up shafts now that I know you can swap those gears!

I would have thought that if the tooth angle is different that non matching gears would not engage the same depth as matching ones. What I'm getting is that You have a couple of shafts in your hand and you're trying to figure out whether you can replace one with the other by engaging the two gears and looking at how deeply they engage. I'll probably be doing that on Saturday since I have to replace the gear on an input shaft and will be looking at some used shafts to source the parts from.

anyone ?