R90S Motor Swap into R65LS - Can it be done?

[Justneed2]

Hi Everyone, I am in the beginning phases of researching motor swap possibilities for my '83 R65LS. Over the years I have been pushing the cafe racer aspect of her more and more, but I feel as though I hit the ceiling.

The next thing I want to do is custom make a new solo tail section, and do a motor swap to enhance the performance a bit. I have my sights set on doing an R90S motor and trans swap into her, but was wondering if anyone has done this in the past (I am sure someone out there has). And what difficulties they ran in to.

The one thing I do know that will be a major mod would be that the drive shaft will have to be shortened.

Any help or insight that can be offered will be greatly appreciated! Thanks.

[nhmaf]

I cannot speak from personal experience, but I do know that a more common modification is to fit an R80
engine into an R65, which is mostly just a swap-out type of affair.   I suppose it could also be done with an R100
engine.  One thing to note, those long-stroke cylinders/heads are going to be sticking out quite a bit further to
either side, so despite the fact that you normally don't want to raise the center of gravity, you may want to consider
fabricating some modified engine mounts to slightly raise the engine to regain some cornering clearance - I think that this is also
a racing modification trick with these.   This might also involve modification to the swingarm to compensate, if you raise the engine
enough to cause a significant angle change where the driveshart head bolts onto the transmission's output flange.   If you can find some
books on BMW history or BMW racing or older BOT races (there are some out there somewhere) that might give you some more visual
info/clues.

I don't know enough about the R90S transmission to know what swingarm changes would have to be made with it.

I think that the easiest change would be to swap over the R65 engine to an R80 or possibly R100, which could be as simple as
unbolt/bolton affair without even having to change the clutch/transmission.  Maybe you're looking for a more challenging
project than that, or have better access to metal-working tools or friends that have them, than I do !

[Justneed2]

I suppose it could also be done with an R100
engine.  One thing to note, those long-stroke cylinders/heads are going to be sticking out quite a bit further to
either side

Thanks for your help.
One of the reasons I want to go with the R90S motor is because like the R65 it is a short stroke. This, as you noted, helps with cornering clearance. I also happen to like the aesthetics of the shorter cylinder heads since they keep a more streamlined look.... which folds into the minimalist approach I have taken with this particular project.

As for access to tools and a few guys to lean on if I get stuck, thats not a problem as I work at a high performance auto shop, probably the best perk of my job!

I'd love to hear what others have to offer also. I consider myself a BMW enthusiast, but I am not an expert by any means!

[Ed Miller]

Thanks for your help.
One of the reasons I want to go with the R90S motor is because like the R65 it is a short stroke. This, as you noted, helps with cornering clearance. I also happen to like the aesthetics of the shorter cylinder heads since they keep a more streamlined look.... which folds into the minimalist approach I have taken with this particular project.

Huh?  I thought the R90S was the same stroke as the R90 and the R75?  Are you sure?

[Justneed2]

Thanks for your help.
One of the reasons I want to go with the R90S motor is because like the R65 it is a short stroke. This, as you noted, helps with cornering clearance. I also happen to like the aesthetics of the shorter cylinder heads since they keep a more streamlined look.... which folds into the minimalist approach I have taken with this particular project.

Huh?  I thought the R90S was the same stroke as the R90 and the R75?  Are you sure?

Your correct, my mistake.
R65, R65LS
82.0mm (3.23")   61.5mm (2.42")

R75
82.0mm (3.23")   70.6mm (2.78")

All 800cc models    
84.8mm (3.34")   70.6mm (2.78")

R90/6, R90S
90.0mm (3.54")   70.6mm (2.78")

All 1000cc models
94.0mm (3.70")   70.6mm (2.78")

[tagordon]

There is a cylinder/piston/connecting rod kit to bump displacement up to 805cc.
I saw one for sale on internet recently. Sorry can't remember where. Just thought I would throw this out, in case you had not heard of it.

[Bob_Roller]

I'm not so sure the transmission from an LS would fit an earlier engine.

I also think the clutch setup is different on the earlier bikes as well, and may not be compatible as well.

Troy ,

The kit that you are talking about had some longevity problems , apparently the bottom end didn't like the increase, and it was not uncommon to hear about lower end failures.

I was considering the larger displacement kit that CC Products was selling in the late '80's it was either an 805 cc, or 850 cc kit, until I heard about the serious engine problems that arose out of it's use.

[steve hawkins]

You can fit any engine to your LS, so I would be looking for a post 81 R100 engine as a base.  There just is not that much of a difference between the R80 and R65 power output, in my oppinion to warrant the grief.  Unless to are fitting it with a 1050 conversion.

As you have the lighter flywheel already, I think you should be able to swap your flywheel and clutch assembly accross etc to an earlier engine, but why bother?  Take advantage of the post 81 mods that made the later engines lighter, eg starter motor, flywheel, nickasil bores, airbox, etc.  This gets more complicated on pre 81 models as they have a special flywheel all of their own.

The other things you need to be concerned about is changing the final drive to take advantage of the extra power and also getting a new exhaust.

When you source your engine, make sure it comes with all the carbs etc.

Then you can worry about lifting the front/all of the engine in the frame.

You can 'soup up' an R65 engine.  But there is a limit to what you can do and if you go over that limit you will blow your motor.  Do your homework.

How deep are your pockets?

Steve H