I’m going to have the heads on this project dual-plugged. Before I sent them to Randy Long who does this work, and more, I did an inspection of the valves to see what I could learn. I documented this work here:
I’m going to have the Nikasil cylinders on this bike replated and honed to match the new 9.5:1 pistons I’m going to install in this build. I’ve learned from a well versed airhead mechanic, Tom Cutter, at Rubber Chicken Racing Garage, that the connecting rods are subject to deformation over time such that the distance between the hole centers of the big end and little end of the rod becomes a bit longer than when new. It’s worth having them checked and the flat of the connecting rod cap machined to achieve the design distance between the hole centers. This is not very expensive and ought to reduce wear and tear on the wrist pin and crank shaft throw.
It’s quite straight forward once you have the top end removed as I document in this write-up:
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.
New 9.5:1 “B” Size Piston Kit For “B” Size Cylinder
New Piston Kit Contents-“B” Size, 9.5:1 Piston (93.97 mm)
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.
This may be helpful to you if you are doing a frame off rebuild/restore, or you need to know how to remove various subsystems. The bike, with the exception of the body work, fits into a reasonably small pile of boxes … sort of a boxed, boxer if you will :-).
A Boxed “Boxer” 🙂
Onward. But first I’m giving “Grover”, the 1973 R75/5 some well deserved attention.