R75/7 or R80/7?

[R75_7]

My R75/7 has worn pistons and rings. The mechanic says the taper is twice what the BMW specs allow. He reommends switching to R80 nikasil pistons and cylinders and heads. My bike is pretty restored already and I don't want to lose any value in loss of originality. Also it is to my undrstanding that the Germans went to the R80 set up to lower compression to use cheaper gas and get better mileage. I don't want to lose any pep that this bike might have in it although I haven't experienced it yet. What should I do? Can I get the reliabilty and the oomph without losing anything? My mechanic says he nikasil is the way to go because in the future I'll only replace worn rings.

[pmdave]

Externally, the all-aluminum jugs look identical to the old aluminum/iron sleeve jugs, so I don't see how they would affect the "originality". The new Nickasil cylinders do last forever, but the main point is that you just buy a cylinder/piston/ring kit and bolt it up. You don't have to wait for machining, and sweat the machinist screwing up your new Pi$tons or boring the hole too loose or too tight.

The old R75 cylinders were a cast iron sleeve in aluminum fins. That does create a boundary layer where the two metals meet--and that slows cooling--but with an R75 that's not an issue like it is on the R100. The main advantage I see with keeping the old cylinders and reboring for oversize pistons is that the sleeve is really massive, and that makes it very stable. It's not likely to warp or crack. They are also heavier, of course.

So, you can take your pick. And you may choose based on the cheaper price--whichever that is. If you're in a hurry, definitely go with the Nickasil kits.

pmdave :brow

[R75_7]

Thanks for the quick reply. What about compression or oomph. Are the R80 pistons sluggish?

[sgborgstrom]

IIRC not all R80 cylinders are the same, some were low compression models, others were not... I'm sure someone with a more airhead-guru-like aspect than I can tell you how to spot the differences.

That said, having done a 75 to 80 swap on my old R75/7 (don't know whether I got a high or low compression set, I had a choice of one pair) it looked the same but had more "pep".

The real question is: Will the airhead market ever get to the point of folks looking for "numbers matching" vehicles? :dunno

Steve

[R75_7]

Thanks for you replies. How can you tell the difference between the high compression and low compression pistons? The donor set up is from an R80GS. I can't remember the year but I believe it was an 80's model. We are thinking on rebuilding the heads and using R90 specs as far as valves and such. Is that a good idea? I want to get the most of the motor and newer technology without losing reliabilty or mileage.

[barryg]

I think you ought to use the cylinders you have. Get'em bored to the next size for the size pistions you need; probably 1st over. Get your heads redone at the same time, and you'll be good for 75,000 miles. I recommend Leo Goff at Memphis Motorwerks 901 757-1009. ps I would go to nikisil after my stock barrels are totally worn out. Good luck.