Boxerworks.com

gspd wrote:

Max Headroom wrote:I gave my R90S 0.06 - 0.07mm piston-to-skirt clearance. No problems with seizing or running hot. Imperial equivalent: 0.0025" - 0.003"

I'd personally only go that tight on a water-cooled engine.

We experienced (ancient history) that less than 0.2mm or so on air-cooled VW sand drag engines produced less power and more heat, and often seizures/scoring on the first 1/8 mile run.
I think this was because the pistons expanded faster than the cylinders.
Hard plating (Nikasil equivalent) directly on ALUMINUM cylinders seemed to alleviate the situation and allow tighter clearances with no down side except the exorbitant cost.

What does the 'BMW book' recommend?
I'd wager it's in the 0.2mm range for iron bores and 0.1mm for Nics.
Any takers?

I'd like to know what the BMW book recommends! Haynes says what I put up before, Duane appears to have the Haynes info, and Snowbum doesn't have it that I see.

I called Google and found a R69 builder who was pleased to have .003" after honing, and got directed to a Triumph 650 spec of .0046" minimum. I'll try to narrow the search down a touch, and see if I can find the spec outta Munich (Spandau).

My BMW manual which doesn't cover the 90/6 says: page 11-0/14.

R60/7 Piston fitting clearance mm (in): 0.020...0.040
(0.00079...0.00157)

R75/7 Piston fitting clearance mm (in): 0.019...0.041
(0.00075...0.00161)

R80/7 Piston fitting clearance mm (in): 0.023...0.047
(0.00091...0.00185)

R100/7/S/RS Piston fitting clearance mm (in): 0.028...0.047
(0.00110...0.00185)

For all the above models...
Total permissible wear clearance of piston and cylinder, mm (in): max. 0.08 (0.0031)

My Clymer manual :
Piston to cylinder clearance: 1970-1976 models: 0.035-0.045 mm (0.0014-0.0018)
max. allowable piston to cylinder clearance: 1970-1976 models :0.120 mm (0.0047)

When comparing the /7 specs from BMW and Clymer, they weren't much different, especially the smaller models where they were close to identical.

Hope that helps.

Thanks, SteveD!

They do seem a bit tight, BUT:

It's a cast piston in a Biral cylinder. Cast pistons don't expand quite so much as forged, and the alu fins won't strangle the bore quite so much as an all-iron cylinder.

I did find a Vech quote that /2 get .0025", so a wee spot tighter seems reasonable.

For 750 to 1000cc motors, I have set them up at .0015 for the last 20 years and have not had a problem.

gspd wrote:Are you talking about piston to cylinder clearance?
If so, somebody messed up on the decimal point.
.0007" and .00016" are both way too tight, it'll seize in minutes.

I work in metric;
I'd shoot for 0.2mm. (0.1mm for nikasil)
Up to 0.5mm will run fine
Ring gap MUST BE AT LEAST 0.1mm. a bit more, say 0.15mm is safer.

I've got to go sit down and refigure out the decimal point thing.
I know the numbers, but I seem to have messed the point.
I'll be back.

robtg wrote:For 750 to 1000cc motors, I have set them up at .0015 for the last 20 years and have not had a problem.

Thanks Rob! Pistons are at the machinist with the jugs going for .0015".

SteveD wrote:My BMW manual which doesn't cover the 90/6 says: page 11-0/14.

R60/7 Piston fitting clearance mm (in): 0.020...0.040
(0.00079...0.00157)

R75/7 Piston fitting clearance mm (in): 0.019...0.041
(0.00075...0.00161)

R80/7 Piston fitting clearance mm (in): 0.023...0.047
(0.00091...0.00185)

R100/7/S/RS Piston fitting clearance mm (in): 0.028...0.047
(0.00110...0.00185)

For all the above models...
Total permissible wear clearance of piston and cylinder, mm (in): max. 0.08 (0.0031)
My Clymer manual :
Piston to cylinder clearance: 1970-1976 models: 0.035-0.045 mm (0.0014-0.0018)
max. allowable piston to cylinder clearance: 1970-1976 models :0.120 mm (0.0047)

I said 0.20mm instead of 0.02mm; at least I knew '2' was the right number.
Me and Dewey never got along

Not much progress this week. Got the left shock mounted (thanks, Ken!), and left the heads with a Simple Green soak last night. It worked well for the left head, but the liquid seeped past the R head exhaust valve, so I was only able to scrub one head clean:



The L intake also passed some fluid overnight, so I'm going to give them all a lapping before it goes back together. My barrels should be bored this week, so I would expect to be getting the motor back together next weekend. Do I dare to fire it with no trans or exhaust? Probably not.

Mel, please assure us that you'll be checking the valve guides as well, before re-assembling the engine?

Inquiring minds want to be reassured.