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Old 10-11-2012, 07:23 PM   #1
.fisher OP
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Location: Columbus OH
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R80 G/S to "R100 G/S" Conversion

I've searched and Google'd about this, and yeah there's even a currently active thread on adding a 1000cc top end to an R80. Bear with me here!

Locally there is an R80 G/S with a bottom end which went through a bit of a erm… catastrophic failure. My dad and I are looking to do what any poor unsuspecting saps would obviously do and drudge it back to life. Before sinking any cash into this project we've already come up with a few routes of action and I've searched most of our questions out but a few linger.

I've read many threads hailing the R80 engine's smoothness and how fantastic it is but many seem eager to replace the cylinders with the Seibenrock 1000 kits, etc. As the bottom end is on this thing is roasty-toasty it'll never be a number matching show queen. That's fine with me, all the more fun to have with it.

One of our options I haven't quite mapped out fully is to mate a post-90's r100 engine to the original transmission. I figure this is a good way to avoid the possible valve-seat issues of the 80's which I've read a bit of, sidestep the circlip hoopla as it's an 1981 and get some more power. Is this doable? Are we asking for trouble? Enough bench racing… any suggestions?


.fisher screwed with this post 10-11-2012 at 07:33 PM
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Old 10-11-2012, 09:38 PM   #2
supershaft
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I wouldn't worry about valve seats or circlips. The later trannies have other mods that I think are a big improvement. The valve seats sometimes last as long as any. Heads with 40mm carbs have monster big ports. I think they are border line too big and not the best shape.

Smoothness? Even my dad liked the R80 for it's smoothness. Personally, the main thing I notice about them is the lack of power versus a R100. They have always reminded me of a R90 that way. Why not go bigger than 94mm bores? Personally, I think BMW stopped at 94mm for a lot of very good reasons. I think BMW thought they went too far at 94mm. I think they took it as far as it makes sense to go.
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Old 10-12-2012, 06:27 AM   #3
.fisher OP
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Quote:
Originally Posted by supershaft View Post
I wouldn't worry about valve seats or circlips. The later trannies have other mods that I think are a big improvement. The valve seats sometimes last as long as any. Heads with 40mm carbs have monster big ports. I think they are border line too big and not the best shape.

Smoothness? Even my dad liked the R80 for it's smoothness. Personally, the main thing I notice about them is the lack of power versus a R100. They have always reminded me of a R90 that way. Why not go bigger than 94mm bores? Personally, I think BMW stopped at 94mm for a lot of very good reasons. I think BMW thought they went too far at 94mm. I think they took it as far as it makes sense to go.
Just the input I was hoping to hear!

Port-sizes are where my knowledge drops off significantly. So do the post 90's R100's have the 40mm ports? Are these what people put the larger European Bing's that I've read about here and there?

The main reason I've jumped to this is that my local BMW scrap heap (Repsycle) had a few nicely priced R100 short blocks on my last visit. I just want to be sure I'm not taking 2 steps forward and 1 backwards and mostly that it's a good solution.

Do you have any knowledge on how easily the R80 trans would mate to the R100 block? I'd love to have that extra power you (and many others) feel is missing off the bat, using BMW parts.
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Old 10-12-2012, 06:44 AM   #4
Airhead Wrangler
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Quote:
Originally Posted by .fisher View Post
Port-sizes are where my knowledge drops off significantly. So do the post 90's R100's have the 40mm ports? Are these what people put the larger European Bing's that I've read about here and there?
The large port (and valve) heads were used up until '83 or '84 on US models I believe. The general consensus is that the smaller port heads work better in just about every case. Even if you want to put bigger intake valves in, the small port heads work better.

Quote:
Originally Posted by .fisher View Post
The main reason I've jumped to this is that my local BMW scrap heap (Repsycle) had a few nicely priced R100 short blocks on my last visit. I just want to be sure I'm not taking 2 steps forward and 1 backwards and mostly that it's a good solution.

Do you have any knowledge on how easily the R80 trans would mate to the R100 block? I'd love to have that extra power you (and many others) feel is missing off the bat, using BMW parts.
No prob whatsoever. The blocks are identical as far as how the trans fits up. There may be a some minor differences in the blocks depending on the years, but nothing that will affect fitting it in your frame or mating your trans to it.
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Old 10-12-2012, 01:37 PM   #5
.fisher OP
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Quote:
Originally Posted by Airhead Wrangler View Post
The large port (and valve) heads were used up until '83 or '84 on US models I believe. The general consensus is that the smaller port heads work better in just about every case. Even if you want to put bigger intake valves in, the small port heads work better.

No prob whatsoever. The blocks are identical as far as how the trans fits up. There may be a some minor differences in the blocks depending on the years, but nothing that will affect fitting it in your frame or mating your trans to it.
Thanks for the info 'Wrangler, I was hoping you'd chime in.
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Old 10-13-2012, 12:18 AM   #6
Rob Farmer
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AW is spot on. Don't be tempted to fit 40mm bings instead of your 32mm carbs.

I was fortunate enough to own a GS imported from the States and a standard Euro GS at the same time. The 32mm equipped US spec machine was significantly faster than the euro bike. it pulled like a train up to 110mph and then simply wouldn't go any faster where the 40mm equipped bike didn't have as much midrange and would claw a few MPH past the 110 MPH mark. Since we don't ride airhead GS for their high performance ability I'd take the 32mm bike every time.


Gearboxes are identical on virtually all of the bikes it's the final drive (bevel box) that varies between the models as long as you buy a 1981 onwards box.

Don't sweat the circlip issue it isn't that big a deal.
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