When BMW introduced the Nikasil cylinders in the US in the 1981 model year, they came with the 8.2:1 low compression pistons to help meet EPA requirements. So, my 1983 R100RS has low compression pistons. But, in Europe, BMW provided 9.5:1 pistons and rings for use in the Nikasil cylinders and they are still available.
The higher compression makes a useful difference in torque and horsepower:
- The 8.2:1 compression pistons produce 53-Ft-Lb and the 9.5:1 compression produces 56 Ft-Lb and, or about +5% for the higher compression.
- The 8.2:1 compression pistons produce 66 Hp and the 9.5:1 compression produces 70 Hp and or about +6% for the higher compression.
I intend to use this bike for two up touring and it has over 80,000 miles on it. So I am going to install 9.5:1 pistons sized for the “B” sized cylinders I have (part# 11 25 1 337 175) to get a useful boost in torque and horsepower.
For the best fit and an oil tight motor, the recommendation from Tom Cutter at Rubber Chicken Racing Garage is to replate the Nikasil inside the cylinders and then hone it to get the proper clearance that matches the actual pistons. There is some variability in the pistons and this approach mates the cylinder dimensions to the piston dimensions for a tight seal.
Here is the link to the write-up of the procedure.
And here is a link to a short video I made of the procedure. It shows the “Risky” way to remove the push rod tubes–and the unintended consequence–and a “Safer” way to do it.
VIDEO: 1983 BMW R100RS Remove Push Rod Tubes & Cylinder Studs