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Finally took the cylinders off my '74 R90s last night. 129k miles and she's using oil, loss of power and pinging excessively.

Lots of oil coking on each piston and head. Pistons and bores are original. No complaints. That's a lot of miles for an air cooled motor.

After measuring the bores and pistons, I'm thinking I need a bore job and new pistons. Wanted to get a consensus from the experts here.

Left cylinder max. bore = 3.5455"
Left piston min. dia. = 3.5295"
clearance = .0160"

Righ cylinder max. bore = 3.5445"
Right piston min. dia. = 3.5275"
clearance = .0170"


Clearance appears way out of spec. I'm guessing only .002" to .003" max cylinder to piston clearance. If a bore is needed, do the numbers indicate how much? 1st over?, 2nd over?

thanks,

RG

RickR90s wrote:Finally took the cylinders off my '74 R90s last night. 129k miles and she's using oil, loss of power and pinging excessively.

Lots of oil coking on each piston and head. Pistons and bores are original. No complaints. That's a lot of miles for an air cooled motor.

After measuring the bores and pistons, I'm thinking I need a bore job and new pistons. Wanted to get a consensus from the experts here.

Left cylinder max. bore = 3.5455"
Left piston min. dia. = 3.5295"
clearance = .0160"

Righ cylinder max. bore = 3.5445"
Right piston min. dia. = 3.5275"
clearance = .0170"


Clearance appears way out of spec. I'm guessing only .002" to .003" max cylinder to piston clearance. If a bore is needed, do the numbers indicate how much? 1st over?, 2nd over?

thanks,

RG

What part of the piston are you measuring?

What's the ring gap, measured at 2-3 places along the bore?

Kurt in S.A.

Pistons measured parallel to the skirt at various depthts and 90 degrees from that. Measured in different areas to find the smallest measurement.

Rings were replaced about 10 years ago and filed to give about .012" for the 1st and 2nd rings. Oil rings was set a little larger (maybe .015") if I recall.

RG

Is this the same bike with the neutral-light issue?
Just wondered.
(129K miles is not a lot of miles, to my thinking. My R100s has 154+ K miles and is not showing that sort of oil consumption. )
Anyway, did you check compression before you took the engine apart?
The numbers you quoted as "max", what was the difference in each cyl? (The out-of-roundness)
First oversize up thru the R80 series is 0.5mm, and 2nd oversize is 1mm. For the R100 series the 1st over is 0.25mm. I thought I had listings in MY BMW manual for the R90, but NOT. Since your bike is a 1974, I'd guess 0.5mm and 1mm for the oversizes.
Clearances are on the order of 0.0012 inches for NEW; pretty tight. (All models) Note the pistons should be marked with letters: A,B, or C that designate the various sizes for the bores. It would not be unusual for you to have an "A" piston on one side and a "B" or "C" on the other.
NOTE I have mentioned both mm and inches in my discussion.

RickR90s wrote:Pistons measured parallel to the skirt at various depthts and 90 degrees from that. Measured in different areas to find the smallest measurement.

Rings were replaced about 10 years ago and filed to give about .012" for the 1st and 2nd rings. Oil rings was set a little larger (maybe .015") if I recall.

RG

Measure the piston 1/2" from the bottom of the skirt perpendicular to the pin to determine clearance. The piston skirt is machined oval and tapered.

Is this the same bike with the neutral-light issue?
Just wondered.
(129K miles is not a lot of miles, to my thinking. My R100s has 154+ K miles and is not showing that sort of oil consumption. )
Anyway, did you check compression before you took the engine apart?
The numbers you quoted as "max", what was the difference in each cyl? (The out-of-roundness)
First oversize up thru the R80 series is 0.5mm, and 2nd oversize is 1mm. For the R100 series the 1st over is 0.25mm. I thought I had listings in MY BMW manual for the R90, but NOT. Since your bike is a 1974, I'd guess 0.5mm and 1mm for the oversizes.
Clearances are on the order of 0.0012 inches for NEW; pretty tight. (All models) Note the pistons should be marked with letters: A,B, or C that designate the various sizes for the bores. It would not be unusual for you to have an "A" piston on one side and a "B" or "C" on the other.
NOTE I have mentioned both mm and inches in my discussion.

Yes, it is the same bike. New wiring harness installed a few months ago. Neutral light issue resolved.
Compression was 156 on L cylinder and 158 on R cylinder.
Both piston tops show significant accumulation of deposits (oil coking)

Left cylinder showed out of roundness of .001"
Right cylinder showed out of roundness of .0025"
Pistons are orginal as far as I know since I owned the bike (1980)

thanks,
RG

Is the wear on the piston ring grooves?

I would also think seriously about new exhaust valves and guides, especially if they have not been replaced yet

Compression numbers sound really good.
DO check the valves and guides as recommended; it would be a shame to do the cyls and get new pistons if the valve and guides were the real problem.
Possibly you just need new rings, too. NOT all the machine work other than honing the cyls.
Out-of-round numbers are OK, too!
PS. I came up with the clearance number of 0.0012inches by taking the cyl bore size and the matched piston diameter and subtracting. I thought it was too tight, but that's what was printed in the specs. (?)

I should have added that Leo Goff (of BMW Airhead fame) rebuilt my heads about five years ago. New guides, exhaust valves and lapping. Seats were excellent so weren't replaced. Compression numbers are a bit high IMHO. Based on a stock 9.5:1 compression and at sea level, I would expect about 140 or so readings. 9.5x14.6 (psi at sea level). The higher numbers indicate to me the existence of the carbon (coking) buildup on the pistons and heads.

The bike is pinging very badly. Even on slight acceleration in the proper gear, it's objectionable.

I think with this kind of mileage and excessive clearance. (I agree that .001" to .002" is about right), it's in need of more than just new rings.



thanks,

RG