Need help understanding piston numbers

EuroIron · Post by **EuroIron** » Fri Nov 02, 2007 2:49 am

best to post good pix of the piston to show dome

and make sure other side matches before you stuff one in

I'd guess I have 75 pounds of BMW pistons

schrader7032 · Post by **schrader7032** » Fri Nov 02, 2007 7:10 am

Yes, the 71 97 is the diameter in mm. Here's what my R69S piston crown looks like:

The date is shown as 1/68, the triangle is the Nural stamp...never did figure out what the H stood for.

Here's a few pictures of the inside of the pison:

These appear to be mold information along with the date of the mold.

Kurt in S.A.

EuroIron · Post by **EuroIron** » Fri Nov 02, 2007 6:46 pm

I'll look and see what I have and I am sure several others will as well

ahdoman · Post by **ahdoman** » Wed Nov 07, 2007 1:31 am

Hey all - I got a hold of a set of "1st Oversize" pistons for my R60US. They measure 72.45mm (current size is 71.97). What should the bore size of the barrel be for these? How about ring size?

schrader7032 · Post by **schrader7032** » Wed Nov 07, 2007 4:58 pm

I've not seen this type of info for /2...I'm sure it's around. But judging from my Haynes for, say, an R60/5 using the "A" dimensions, the piston is 73.46mm while the cylinder diameter is 73.5mm. For an R100/7, the piston is 93.96mm while the cylinder diameter is 94.005-94.015mm.

The manual goes on to list the standard piston/cylinder clearance as:

/5,/6 - 0.035-0.045mm
R60/7 to R80/7 - about 0.020-0.040mm

Maximum permissable piston/cylinder clearance as:

/5,/6 - 0.12mm
All others - 0.08mm

Best approach is to give the pistons to the machinist and have them bore as required.

As for rings, I think you should just get the 1st over set of rings, and measure the end gaps. Gaps should be between 0.25 and 0.40mm. If your gap is too big with the 1st over set, you'd need to buy the 2nd over set and file them done.

The distance between the ring and the ring lands needs to be measured too. This ranges from about 0.07 to 0.10mm. I got this information on the rings from my Clymer manual.

Kurt in S.A.

VBMWMO · Post by **VBMWMO** » Mon Feb 15, 2010 2:52 pm

OK. I took the head and barrel off the bike (R60US) and found a shattered piston ring

. It also shattered the wall that separates the 2 scraper rings

. So, it looks like I need to find a new (used) piston. Anybody got one they want to part with cheap and help a brother out? The piston is a domed top and has the numbers 71 97 stamped in it. Is this the diameter? I measured it with my calipers and get that same value at the skirt. On the inside of the skirt it says "NURAL 7206". Right now I don't have the money to completely tear the engine down and do it right. Believe me, it really bothers me not to be doing this right but $ is in short supply and I need to get this bike running again. I'm in the SoCal area. Shoot me a PM if you have a piston to sell. I'll also need gaskets so maybe we can make a package deal.

Clyde1970BMW · Post by **Clyde1970BMW** » Mon Jul 12, 2010 11:07 pm

Hi...I'm looking for a used but usable right piston for 1977-1984 BMWR80, diameter 84.765mm. If you have a set and do not want to break up it up, I would consider buying two. I know the 1977-1984 dates may be a little new. I'm trying to exhume a 1970 750BMW750r/5 basket case that came with 750 cylinder machined out to 800cc. It has a good left piston and an unusable right piston. The good piston markings have brought me to the 1977-1984 BMWR80.

I'd sure like to take advantage of the existing cylinders. I can only assume they must have cost someone great deal!

Looking for any help or direction you can offer.

I'm new to this organization and hope I have posted this request in the proper place.

Thanks in advance

Clyde1970BMW

Vintage BMW Motorcycle Owners