Probably a dumb question but......Is there anyway to tell if my pistons have ever been oversized? Performing a top end gasket replacement and I was just wondering. The bike has about 120k miles on it. Can post pictures if that helps.
I'm just guessing here, but I'm going to assume that "oversized" pistons are going to require a larger bore than stock. Stock bore is 82mm. If you have calipers, check your bore. Additionally, you can measure the piston with it at TDC to double check against stock size.
From Gemini via BMW MOA data:
The standard nominal bore size for a stock BMW R75/7 motorcycle is 82 mm, and the actual stock piston diameters are stamped on the piston crown (typically sizing designations like 81.965 mm, 81.975 mm, or 81.985 mm depending on the specific factory cylinder grading) to provide the necessary running clearance inside the 82 mm cylinder. [1, 2, 3]
Thanks. I will check the diameter. Can't read any stamps on piston head.
looks like they're both about 81.9 mm.
If you have calipers, check your bore. Additionally, you can measure the piston with it at TDC to double check against stock size.
The standard nominal bore size for a stock BMW R75/7 motorcycle is 82 mm...
This is almost correct.
Due to the immense heat in the combustion chamber, pistons are tapered in shape, with the largest portion (the portion to measure) of the piston being at the "skirt"... opposite the dome (combustion chamber) end. To measure this, the cylinder has to come off.
To reduce drag, pistons are also ground oval, so you can't just measure anywhere on the skirt. You'll need to measure 90 degrees to the piston pin... and at the skirt.
The 82mm dimension is the bore itself, so that's what you want to be measuring. There will be crud from combustion at the top of the bore which will throw off your bore measurement.Â
Overbores will be in 1/4mm sizes. A millimeter is 0.0394", or very roughly 0.040". So this makes each 1/4mm overbore roughly equivalent to the 0.010", 0.020", 0.030" overbores used on American and older British vehicles.Â
Piston-to-bore fits are conserved by the excellent top end cooling found on Airheads. 250,000 miles between boring jobs is not unheard of. In order for the average Airhead to have needed a "bore job", you would have almost had to run the top end without motor oil.Â
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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