Piston Upgrade to a R75/5?

[jjwithers]

I have a R75/5 with slight pitting/scoring in the cylinders.

I don't know if this will work... but I'm thinking of getting R80 pistons and boring the cylinders to match. And It will be a nice upgrade too!

Are there specific years/pistons that I can use to upgrade this old /5 ?

I have a set or two of early '74 R90 pistons lying around. Not sure if those would work or if boring the cylinders would take too much material away for those pistons.

Checking to see if anybody did the homework already and if I can cheat off you!

[Duane Ausherman]

I have heard of several who went to the 800 cc pistons. I would think that going to 900 cc would be too far. The steel would be very thin.

[Kurt in S.A.]

There's a set of '74 pistons and cylinders on ebay...suitable in size for your engine case. Item #231485928629.

Kurt in S.A.

[Ken in Oklahoma]

In the past I've heard here on the forum that re-boring BMW cylinders is kind of iffy. That is the bore can get a bit 'snakey'.

I know that Harley Davidson cylinders require a special clamping fixture when re-boring, thus emulating the cylinder end pressures 'seen' with the head and cylinders properly torqued onto the engine. I don't know if I'm remembering it or not but I seem to recall that the same kind of fixture is needed for boring BMW cylinders for the same reason.

Ken

[Kurt in S.A.]

Ken -

I've heard the same thing about some kind of "fixture" when boring. I actually thought that boring the lower cc'd bikes was somewhat OK, but that even going 1st over on the 1000cc jugs was dicey. I think in that case it is about how much steel liner is left. I've never had the chance to measure the thickness of the liner for the 750 and 900cc bikes to see if it is actually thicker than the 1000cc cylinders. I would assume that BMW might have tried to minimize the liner thickness on the bigger engine because of all the added weight whereas they could keep a slightly thicker liner on the smaller engines. But that's just me talking out some orifice...

Kurt in S.A.

[jjwithers]

I know the fixture you are talking about. It is a torque plate fixture. There are two plates that are ground completely level/flat. There are holes for both ends of the cylinders. You torque the cylinders between the two plates and then you can bore them. Most shops that do boring should have them. I actually work with a person who has one. So getting the cylinders bored is no problem.
I just need to know what pistons I can get away with.

[Kurt in S.A.]

I just need to know what pistons I can get away with.

I think it's been stated...you could go to 800cc but I suspect going to 900cc would be pushing it. To go from 82mm (R75/5) to 84.8mm (R80/7) is 2.8mm in diameter or 1.4mm in thickness. Ask a machine/engine shop if they can take that much out of the available R75/5 liner and still be stable. To go to 900cc which is 90mm in diameter, that's 8mm or 4mm in thickness. I think the story gets pretty clear.

Kurt in S.A.

[pmonk]

I guess I am a little cautious, but why bore out for more than the 1st over pistions? I would want to conserve all the metal I could in case something happened and it had to be done again. Would you really get that much more oomph out of R80 pistons? I used to have an R75/6 and it seemed to carry me the wife and lots of camping stuff pretty good.

[jjwithers]

Extra oomph is always more fun!
And cheaper is better too! I am hoping to find a good used set of pistons vs buy a new set from BMW. And I doubt I will find a 1st over set on the used market. A R80 set should be a bit more common.
I'm trying to build a bike out of spare parts and only buying new parts as necessary.

[chasbmw]

Back in the day people would bore the bigger engines out to 1050cc and fit Venolia Pistons.

From what I remember results were not that positive as the liner depth was too thin and the bore would corrugate within a few thousand miles.

Buying a used set of Nikilsil r80 cylinders and Pistons, would be cheap enough, lots of people are fitting 1000 cc seibenrock cylinders and Pistons to 800s. You will have to relieve the crankcase opening.