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Captain Yesterday 11-11-2012 09:13 AM

R75 Pistons?
I need to swap one of my pistons out on my r75/5. My pistons are stamped 81,975, can I use a piston stamped 81,965? Does anyone know the significance of the numbers stamped on the pistons?


Renner 11-11-2012 09:23 AM

size in mm
as you well know, cylinder bores are matched to the pistons.
... just meaning to say it's a close match with a specific clearance tolerance.

So when a good shop hones your cylinders, they'll likely want to have the applicable pistons at hand so they can get the size and piston-cylinder clearace within the allowed tolerance.

those 81.965 pistons will be too small.

isdt BMW 11-11-2012 12:18 PM

If your bores are good and just need a light honing to break the glaze, you can reuse the existing piston. wouldn't hurt to get new rings. make rings for just about any motor. I will know by Thursday if I will have a spare piston. the base of your cylinder will be stamped A or B or C to note what the original size was. the size you quoted is a standard bore.

Captain Yesterday 11-11-2012 01:08 PM

ARG BMW :kboom

I guess I'll be on the look out for a B piston. If I can't find any thing I guess I'll have to go bore it one size over and get new pistons

disston 11-11-2012 01:11 PM

The pistons wear and the rings wear and the cylinders wear. Usually if done in time the first rebuild can be rings only. But if left too long or it's the second rebuild you need pistons and a boring, oversize. Now you need one piston. I don't like the idea of one piston. You should match it to the other piston. Things should stay balanced and matched. The smaller piston is a very poor match for the one you already have. And you prolly have enough cylinder wear that the original piston is loose in the bore? So try to find two bigger pistons than the one you have in fact it might be a good time to buy oversized pistons and get the thing bored.

I had second oversized pistons in my R90/6 but they were worn out. I was going to put rings on them but then another set of second oversized, not worn out, pistons showed up and they fit my already bored cylinders perfectly. talk about luck.

Renner 11-11-2012 03:39 PM


Originally Posted by Captain Yesterday (Post 20021016)
I guess I'll be on the look out for a B piston. If I can't find any thing I guess I'll have to go bore it one size over and get new pistons

the shop honed the cylinder which had the stuck piston, yes?
honed to minimum clean-up
how much was that? :ear

what is the actual value of the bore diameter, both cylinders.
Is there taper to the bore?
The shop you're using should be able to tell you.

it's not just a BMW thing.
but yes, pistons are damn expensive. :cry
That alternative, smokey R90 engine is an attractive option. :nod

Humboldt Airhead 11-11-2012 08:17 PM

I have 1 "B" piston left from my 70 R75/5. It is in good condition. It has the wrist pin but no rings. I can email pics if you like.

Captain Yesterday 11-12-2012 09:12 AM

Sent PM

isdt BMW 11-16-2012 01:43 PM

did you get a piston? I have a spare, for a B cylinder.

Captain Yesterday 11-16-2012 05:40 PM

Yeah I got one, should be hear in a day or two.


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