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Old 01-28-2013, 02:10 PM   #1
boxerotto OP
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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?
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Old 01-28-2013, 04:01 PM   #2
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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.
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Old 01-28-2013, 04:15 PM   #3
Kai Ju
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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.
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Old 01-28-2013, 04:21 PM   #4
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Quote:
Originally Posted by Kai Ju View Post
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.
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Old 01-28-2013, 05:43 PM   #5
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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.
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Old 01-28-2013, 05:45 PM   #6
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Quote:
Originally Posted by boxerotto View Post
Can the input shaft of a 1977 transmission be converted to fit with a lighter later 1982 flywheel/clutch combo?
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?
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Old 01-28-2013, 06:29 PM   #7
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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.
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Old 01-28-2013, 06:39 PM   #8
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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".
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Old 01-28-2013, 06:57 PM   #9
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Quote:
Originally Posted by boxerotto View Post
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.
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Old 01-29-2013, 07:21 AM   #10
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" 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."

-----------------
The newer gears I have seen were marked with an X or have a line cut around the circumference of the gear teeth.
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Old 01-29-2013, 09:22 AM   #11
boxerotto OP
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has anyone actually shortened the shaft?
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Old 01-29-2013, 12:33 PM   #12
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Originally Posted by supershaft View Post
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.
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 ?
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Old 01-29-2013, 12:55 PM   #13
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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.
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Old 01-29-2013, 01:44 PM   #14
Kai Ju
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Originally Posted by supershaft View Post
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 ?
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Old 01-29-2013, 03:32 PM   #15
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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.
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