I’ve done work on the front and rear of the engine replacing various components. You can read about that work here.
- 11 BMW 1977 R100RS Replace Rear Main Seal & Oil Pump Cover O-Ring
- 11 BMW 1977 R100RS Replace Timing Chain, Crankshaft Timing Gear & Nose Bearing
- 12 BMW 1977 R100RS Replace Engine Electrical Components
- 21 BMW 1977 R100RS Remove, Refurbish, Install Clutch
It’s time to pull the pistons and connecting rods and inspect them.
The 1977 R100RS has “Al-Fin” cylinders, aluminum pistons and chrome top rings. The Al-Fin cylinders have a steel liner bonded to aluminum cooling fins. The compression ratio is 9.5:1 which is the highest compression ratio used on airhead engines. This is true for the standard engine and the CFO version I have.
But, this bike is not standard. The original cylinders, pistons, rings and gudgeon pins were replaced at some point, for reasons unknown, with the later Nikasil cylinders, pistons, rings and gudgeon pins. Nikasil cylinders do not have a steel sleeve. They are aluminum with a thin, but very hard coating Nickel Silicon Carbide applied inside the bore called Nikasil.
The rings are cast iron. The idea is the rings wear and can be replaced as necessary, but the cylinder is so hard in comparison that it doesn’t wear. Nikasil cylinders were introduced by BMW for the airhead engines in 1981, so this change to the bike occurred sometime after that.
The upside of the Nikasil cylinders is they are lighter, hold there dimensions very well and will last for many, many miles. The downside is in the US, the pistons that go with them reduce the compression to 8.2:1 with a corresponding reduction in torque and horsepower from the original 1977 engine which had pistons providing 9.5:1 compression. However, I learned from Tom Cutter at Rubber Chicken Racing Garage there are European piston, ring and gudeon pin sets for Nikasil cylinders that deliver 9.5:1 compression. Unfortunately, this bike has the US pistons installed so the compression is only 8.2:1.
BMW has two sources for these cylinders commonly known as Nikasil. Nikasil is a registered trademark of Mahle Gesellshaft. The other name for the same process is Galnikal which is a registered trademark of Kolben Schmidt. The trademark of Kolben Schmidt is an “S” and “K” overlaid on each other. This bike has Kolben Schmidt cylinders and pistons.
These are resources about airhead cylinders and pistons and engine specifications. They are the sources I used for this write-up.
- Bob Fleischer: Pistons. Swapping heads. Cylinders. Piston rings, pins, clips, keepers, de-carboning; Honing, ETC.
- Bob Fleischer: Cylinders (iron, steel, Nikasil, Galnikal)
- Duane Auscherman: BMW Engine Specification
The connecting rods use special “use once” stretch bolts that have a 12 sided head called a serrated head. I got the wrench that fits it at my local NAPA store, a 10 mm wrench.
The pistons are secured on the gudgeon (wrist pin) pin with an external C-clip. I have a set of pliers with various pin sizes to fit a variety of hole sizes in the pins.
Remove Carburetors, Exhaust, Heads and Cylinders
The procedure for the R100RS is the same as the R75/5 and R75/6. The one difference is the retaining C-clip for the gudgeon pins. The R75’s have an internal C-clip while the R100RS has an external one. This is how I did it for these bikes.
- 00 BMW 1973 R75/5 General Tear Down
- 1975 R75/6 Rebuild: Project Index
Remove Pistons and Rings
Here is a short video of removing a piston on the R100RS with the exterior C-clip.
As I remove each piston, I use a sharpie to mark which side they came from on the underside of the piston. I put the gudgeon pin, c-clips and rings in a labeled bag so I can be sure the piston, rings, gudgeon pin and c-clip are matched to the cylinder they came from.
Piston Cleaning and Inspection
The carbon on the piston crown isn’t too crusty.
I remove the rings. The bottom oil control ring has an expander behind the ring. I use my fingers to carefully expand each ring and slide it off the piston.
The ring grooves are not caked with carbon, the skirts have no carbon and show no signs of gouges or deep scratches which is a good sign the rings seated well in the cylinder and carbon or other debris has not gotten through the rings.
I soak the piston crown and grooves in Berryman Chemtool B12 carburetor cleaner over night in a plastic container with a lid. I soak one piston at a time so I don’t confuse which side they came from as the Chemtool B12 dissolves the marks I made on the under side of each piston. As a precaution, I put the plastic container in a steel pot in case it leaked overnight.
Berryman Chemtool B12 is not to trifled with. It should not contact your skin. It will eat through nitrile gloves. When working with it use rubber gloves, wear eye protection and make sure there is good ventilation. Use a container with a cover to prevent evaporation and to prevent fumes accumulating while the pistons soak.
After soaking overnight most of the carbon came off. I used a brass wire brush to clean the last of the carbon off the crown and sides of the piston.
On the top of the piston, in the middle, is an arrow which points to the front of the engine so the piston is installed in the correct direction in the cylinder. At the top is a marking “93.96” which is the diameter of the piston in millimeters. Pistons come is three sizes, “A”, “B”, “C”, as do the cylinders. This piston is the “A” size. Both pistons and the cylinders are typically of the same size. Underneath the size is a “-” which indicates the weight group of the piston; these are “lighter” weight pistons. Both pistons should have the same weight group mark.
Both pistons have spots of discoloration. This is likely due to engine sitting for some time and moisture condensing under the carbon deposits and corroding the surface.
Remove Connecting Rods
The connecting rods are attached to the crankshaft crankpin with special 12 sided serrated bolts.
I use a special tool, a 10 mm serrated wrench, to remove the bolts. I got this at my local NAPA dealer, part# 2305.
The shaft of the wrench fits inside a 13 mm socket.
I suspend the rods with a piece of wire through the gudgeon pin hole of the rod and wrapped around the top cylinder studs. This prevents the rod from falling an nicking the edge of the engine case. I heated the rod around the bolt briefly so it would be easier to remove the bolt. I use my impact wrench to loosen the rod bolts.
Here is a short video of removing the bolts on one of the connecting rods.
Here is the rod and bolts after removing it.
Inspect Rod Bearing Shells
The left rod bearing shells look fine, but the right have a score in them from something hard that passed through the bearing.
Here is a short video of inspecting the rods and the bearing shells.
I inspected the cranks throw for damage and did not see any scores, pits or corrosion. I will order new bearing shells.
Measure Crankshaft Throw Diameter
I use a 1-2″ micrometer with a 0.0005″ precision to measure the diameter of the crankshaft throws. Here is a short video of how I made the measurements.
I took 5 measurements of the diameter of each throw and averaged them.
- Left Crankshaft Throw Diameter: 1.8896 inches
- Right Crankshaft Throw Diameter: 1.8894 inches
The published range is 1.8888 – 1.8894 inches.
Despite the slightly larger value for the left crankpin than the published range, the error precision of the micrometer limits how precise the measurements are. I expect the error limit for the micrometer is easily +/- 0.0002 inches.
My conclusion is both crankpins show no appreciable wear.