- Cylinder and Piston Details
- Lubricants and Sealants
- Set Engine To Top-Dead-Center (TDC)
- Install Piston Rings
- Install Pistons
- Apply Gasket Sealer
- Install Cylinder O-rings and Push Rod Tube Seals
- Attach Connecting Rod to Piston
- Install Head Gasket and Cylinder Head
- Install Right Side Top End
- Set The Valves
The engine came with the stock 8.2:1 compression pistons. But the early RS motors came with high compression 9.5:1 pistons. The bike has 83,000+ miles on it. So I decided to install new high compression pistons. Tom Cutter at Rubber Chicken Racing Garage, told me due to the very tight piston clearances in the Nikasil cylinders and the amount of variation in piston diameter with the new pistons, the best way to proceed is to replate the cylinders with Nikasil and hone them to match the pistons to ensure proper clearance. So, I send him the new pistons, rings, wrist pins and old cylinders for this work to be done. I also had him vapor hone the cylinders to refinish them to the factory patina.
The other work needed was to the heads. When I tested them, the valves were leaking. So I sent the heads to Randy Long, at Long’s Mechanical Services, who is a well respected head rebuilder, for his opinion. We decided to replace the exhaust valves, all the exhaust guides, springs and keepers. I had him machine the heads for dual plugs. I have dual plug heads on two airhead bikes and I like the improved gas mileage. Randy milled the valve cover mating surfaces so they are flat and bead blasted the heads so they look brand new.
With the exception of the new 9.5:1 pistons that come with rings, wrist pins and snap rings, the other parts were included in the engine gasket kit I got from Euro MotoElectrics. I show the BMW part number for all the parts in the table below.
|11 25 1 337 175||PISTON – 93,97 E=9,5, High Compression, Nikasil||2|
|11 12 1 338 715||GASKET, Head:
In EME Gasket Kit
|11 11 1 337 567||O-RING – 93X2,2 (from 08/80), Cylinder:
In EME Gasket Kit
|11 11 1 262 141||GASKET RING – 11X2 (from 08/80), Top Cylinder Studs: In EME Gasket Kit||4|
|11 32 1 262 995|| PUSHROD SEAL (from 08/80):
In EME Gasket Kit
I use snap ring pliers to install the wrist pin snap rings. These can be set to compress or expand the ring with you squeeze the handles.
I’m trying out a new way to compress the piston rings. It’s a t-bolt ring clamp sold by Silicone Intake Systems. I got the 3.5 inch clamp (92-100 mm) that it is wide enough to cover all three piston rings.
This is the first time I used the strap above. It worked well.
I have used automotive ring compressors before. The cheap one I got is clumsy and doesn’t uniformly compress the rings, so it’s temperamental to use.
The following resources provide details about cylinders, pistons, heads and procedures.
- Robert Fleischer: Replacing Push Rod Tubes & Seals
- Robert Fleischer: Disassembly & Assembly of Heads
- Robert Fleischer: Pistons, Cylinders, Piston Rings, etc.
- Oak Okleshen: “Manual 1: Boxer Top End Disassembly, 1970-1975” and “Manual 2: Boxer Top End Reassembly 1970-1975” in a single bound booklet.
[Sadly, Oak died in April 2017. I understand the manual is still available. Request information with an email to: [email protected].]
- 11 BMW 1983 R100RS Remove Engine Top End
- 11 BMW 1977 R100RS Install Connecting Rods and Assemble Top End
- 11 BMW 1977 R100RS Remove Pistons, Connecting Rods & Inspect
- Nikasil replating and honing service; Connecting rod refurbishment:
Tom Cutter, Rubber Chicken Racing Garage.
- Head rebuilding and general engine machine shop services: Randy Long.
Randy does not have a web page. Here is his contact information.
Long’s Mechanical Services
74 Risbon Road
Honey Brook PA 19344-1754
Here is a video summary of the installation procedure.
VIDEO: 1983 BMW R100RS Install Engine Top End
Cylinder and Piston Details
The cylinders and pistons have various markings and features. It’s helpful to understand what they mean and what they are for.
The cylinders are identified with markings showing the left (L) and right (R) cylinder. These marks were added by the Nikasil plating company and are not found on stock cylinders, as they are not left or right handed.
There is a mark indicating the bore size classification, “B”. BMW uses three letters, “A”, “B”, “C” to group cylinders and pistons based on their diameter. Cylinders of a bore size class should be mated with pistons of the same class. My cylinders are “B” class.
The Nikasil cylinders have a groove machined at the base of the cylinder sleeve for a large O-ring to seal the cylinder in the engine block to prevent oil leaks. The Nikasil cylinders do not use a base gasket. There are also two grooves machined into the cylinder base around the the top cylinder stud holes. The top cylinder stud holes in the engine block provide an oil passage that lets oil flow around the studs to lubricate the rocker arm assemblies in the heads. A small O-ring fits in the groove to seal the top stud hole at the engine block to prevent oil leaks.
Pistons & Rings
I purchase “B” class pistons to match the cylinder class. There are three piston size classes, “A”, “B”, and “C” which differ by 0.01 mm, or 0.0004 inches, as in four ten-thousandths of an inch: Here are the piston sizes.
“A” – 93.96 mm
“B” – 93.97 mm
“C” – 93.98 mm
On the top of the pistons are several marks and information. The size class, “B”, the compression, “9.5:1” and the weight class “+”, are stenciled in black ink on the piston crown. The weight class of the piston (+) and the diameter (93.97 mm) are engraved in the crown. The black stencil will disappear as soon a the engine runs, but the engraved marks will remain so you can determine what type of piston you have.
Under the “+” is an engraved manufacture mark that looks like an “S” superimposed on a “K”, which stands for “Süddeutsche Kolbenbolzenfabrik” which was acquired by Mahle who sells the pistons and cylinders to BMW, but they still use the Süddeutsche Kolbenbolzenfabriktrademark mark.
There is an arrow engraved in the crown shown in the picture below.
It points to the front of the engine. The piston has to be installed with the arrow pointing to the front, or the pockets cut into the piston crown to provide clearance when the valves are open will interfere with the valves when they open.
Since the valves are different sizes-the intake is larger than the exhaust–if you reverse the piston when you install it, the intake valve will strike the piston crown. If you are just rotating the engine by hand, it will stop turning. If you tried to start the engine, you will bend the valve and damage the piston.
Tom added an “L” and another arrow to the top of the crown. The “L” means this piston was fitted to the left cylinder when it was replated with Nikasil and honed to fit the piston. The Sharpie arrow is easy to see when I install the piston.
The piston uses three rings, so there are three grooves for the rings. As I explain later, each ring is different. The top groove is for the #1 ring and the bottom groove is for the #3, or oil control, ring. Just below the three grooves is the hole in the piston with the bosses for the wrist pin the connects the piston to the connecting rod.
Wrist Pin Preparation
The wrist pins were marked by Tom for the connecting rod they go with, “L” and “R”. I freeze them before I install them so they will slide easier through the bosses on the piston and the connecting rod bushing. Before I freeze them, I install one snap ring on the wrist pin which makes it easier to install it as I explain later.
Lubricants and Sealants
I use Three Bond 1207B sealant to seal the cylinder to the engine block. I use Silicone Grease to lubricate the small cylinder stud O-rings and inside of the push rod tube seals. I use engine oil to lubricate the cylinder, piston skirts, the connecting rod wrist pin bushing and piston bushings, and the outside of the push rod tube seals.
Set Engine To Top-Dead-Center (TDC)
I’m going to install the top end on the the left first. I set the engine to TDC and I want to be sure the left side will have the valves closed. Each revolution of the engine closes the valves on one side, so being at TDC does not guarantee the valves are closed on the left side. I don’t want pressure on the push rods and rocker assembly when I install the cylinder head and torque it. I verify that the cam lobes are oriented so the ramp of the lobe is NOT under the left side cam followers which means the valves are closed.
Install Piston Rings
There are three pistons rings, 1st, or top, 2nd, or middle, and 3rd, or oil control. Each ring is marked at the factory on one side with “Top” indicating that side of the ring should face the top of the piston. Tom added “Top” with a Sharpie to the 1st ring, “2” to the top of the second ring since is looks similar to the 1st ring but really isn’t, and “Top” to the third so it is easy to see which side is the top side.
The top ring has a square profile, the second ring has a notch on the bottom side and the oil control ring has two pieces; an inner spring and the outer ring that fits over it in the groove of the piston.
Before I install the rings, I coat each ring with a drop of oil. I don’t want them dripping wet, just lightly coated on all sides.
I install the rings by sliding them down from the top of the piston, so the 3rd ring-oil control-goes on first. I remove the spring and expand it on the wire that goes through the spring so I can slide the spring over the piston. When it’s aligned with the lowest groove, I collapse the spring so the ends butt up against each other and I can’t see the inner wire.
I carefully expand the outer ring using a finger on each end to push the ring open just enough to slide it over the top of the piston. I wiggle it while I expand it just a little to move it down to the bottom groove. Then I orient it so it covers the spring.
Then I install the 2nd ring, again using my fingers to expand it just enough to slide into the 2nd groove followed by the 1st ring. I double check that the “Top” side of each ring is facing the top of the piston.
Ring Gap Orientation
The ring gaps should be placed at particular positions around the circumference of the piston. But, the location of the ring gaps on the left and right pistons are in different locations. The locations of the ring end gaps are identified by the position of the hour hand of an analog clock. Orient the piston as it will be when it is attached to the connecting rod so the arrow points to the front of the engine and the size marking (93.97) is on the RIGHT side of the LEFT piston, and on the LEFT side of the RIGHT piston.
LEFT PISTON RING GAP POSITIONS
1st (Top): 4:30
2nd (Middle): 10:30
3rd (Bottom): 1:30
RIGHT PISTON RING GAP POSITIONS
1st (Top): 7:30
2nd (Middle): 1:30
3rd (Bottom): 10:30
Before I install the pistons, I prepare the cylinder bore. I use 2 to 3 drops of engine oil and smear then on my fingers. Then I wipe the inside of the cylinder to introduce a very light sheen of oil.
This is sometimes referred to as the “dry method” of piston installation, but it’s really the “very lightly oiled” method. The reason to limit the oil on the cylinder wall is to help the ring seat quickly on the first engine start. The cross-hatch in the cylinder bore needs to cut the ring to introduce a labyrinth like seal across the outer face of the ring. It is this micro-abrasion of the ring that creates an oil tight seal.
Install Ring Compressor
I use a ring compressor to compress the rings so I can slide the piston into the cylinder. I align the compressor so it covers all three rings and tighten it until all three rings are compressed flush to the side of the piston, but I can still move the piston through the compressor
I install the pistons from the bottom of the cylinder where there is a chamfered edge on the cylinder wall to guide and compress the rings into the cylinder. I put the crown of the piston into the bottom of the cylinder with the edge of the compressor ring butting up against the edge of the cylinder. While push the piston into the cylinder with moderate pressure, I rock the piston back a forth a small amount to wiggle the rings into the cylinder.
If the piston jams and won’t move with moderate force, it’s likely due to a ring edge getting caught on the edge of the cylinder. Don’t hammer or bang on the piston as that’s a good way to break the ring. Instead remove the piston and rings, reinstall the ring compressor, and try again. Usually the piston will slide in the first time but occasionally you will have a ring catch on the edge of the cylinder.
Once I get the piston rings inside the cylinder, I loosen the ring compressor strap and remove it. I push the piston down to the edge of the wrist pin boss, but I leave it exposed so I can install the wrist pin with the piston captive in the cylinder.
Apply Gasket Sealer
I use Three Bond 1207B gasket sealer. I apply it to the base of the cylinder and the mating surface on the engine block. There is no base gasket on this engine. I like to warm the sealer before using it, so I put the tube in a glass of very hot water for 10 mins or so. This makes the sealer easier to apply and helps me apply it in a thin layer.
Before applying the sealer, I use lacquer thinner and a blue shop towel to clean the mating surface of the cylinder and the engine block. I want them spotless with no oil or finger prints so the sealer will bond with no gaps. I make sure there is no lint or debris from the shop towel on the mating surface.
Earlier I cleaned all the old sealer off the mating surfaces on the cylinder base and the engine block.
I use rubber gloves and apply a small amount of the sealer to a finger tip and then spread it on the edge of the cylinder base. Since the top cylinder stud holes are oil passages, I do not want to get sealer to get squeezed into those holes. So I keep it very thin when I get near those holes. Then I smooth out the sealant with my finger to cover the full width of the cylinder base trying to keep it out of the groove for the large O-ring.
If I get some sealant where I don’t want it, I some lacquer thinner to clean it up. I use a Q-tip on the cylinder sleeve and carefully remove the excess sealer so I don’t disturb the applied sealant.
I apply the sealant to the engine block mating surface. Again, I’m careful to keep the sealant light around the top cylinder studs so the sealant won’t squeeze out and block oil flow around the studs when the cylinder is torqued to the engine block.
Install Cylinder O-rings and Push Rod Tube Seals
When I’m done, I slip the large O-ring over the piston and the cylinder sleeve and use a screw driver to roll it down into the groove so I don’t disturb the sealer.
Then I put a small dab of Silicone Grease on the two small O-rings and carefully place them in the grooves around the top cylinder studs. The grease is a bit sticky so that helps keep the O-rings in the grooves. If the O-ring sticks to your finger use a screw driver to push the O-ring into the groove so you don’t get any Silicone Grease on the sealant.
I install the new push rod tube seals. They are not symmetric since the push rod tube meets the cylinder block at an angle. And there is a vertical bar on the outside of the seal which should be oriented at the bottom of the push rod tube and aligned vertically before installing the seal into the engine block.
I use some Silicone Grease, NOT Silicone Seal, to lubricate the inside of the push rod tube seal. The seal MUST be free to slide on the tubes as the engine heats up and cools down. I put a light smear of Silicone Grease on the inside of the seal. I put a drop of oil on the ribs on the outside of the seal that press against the push rod tube holes in the engine block and spread it around the outside of the seal.
Attach Connecting Rod to Piston
The cylinder is ready to install on the cylinder studs and the sealant is on the engine block.
I slide the cylinder onto the cylinder studs pushing it in until it’s next to the connecting rod. Then I verify that the piston is correctly positioned in the cylinder with the arrow pointing to the front of the engine.
I put a couple drops of oil inside the connecting rod wrist pin bushing and on the piston wrist pin bosses and with my pinkie finger smear it evenly on the inside surfaces.
Next, I put a drop of oil on the top piston skirt and smooth it evenly over the skirt. I put another drop of oil on my finger and spread it evenly on the bottom piston skirt. I want them lightly lubricated, but not with oil dripping off them.
I slide the cylinder further so the connecting rod is inside the piston.Then I remove the wire securing the connecting rod to the cylinder studs and carefully lower it to rest on the inside of the piston. I can’t put a rag under the rod to protect the engine block due to the sealant so I’m careful not to drop the rod while I’m installing the wrist pin.
I take the wrist pin out of the freezer, make sure it’s marked for the correct side, left, and insert the end that does not have the snap ring into the piston bushing facing the front of the engine.
When I get the wrist pin to slide into the piston boss, I pick up the connecting rod to align it with the boss and press the wrist pin through the connecting rod and through the piston boss on the other side until the snap ring is snug against the piston boss.
I use the snap ring pliers and install the second snap ring into the groove in the wrist pin. Then I use a small screw driver and rotate the ring while I look at it to be sure it’s securely in the groove.
Install Head Gasket and Cylinder Head
I need to push the cylinder up to the engine block. As I do that, the piston will push through the cylinder. However, the push rod tube seals can rub against the lower frame tube if they are oriented the way they should be with the bar centered at the bottom of the push rod tube, making it hard to push the cylinder toward the block. To avoid this, I rotate the seals 180 degrees so the thinnest part of the seal is above the frame.
Then I push the cylinder toward the block until the push rod tube seals have cleared the frame. Then I rotate them back to the correct position with the vertical bar centered and at the bottom of the push rod tube.
The piston is now all the way through the cylinder since I had the engine set to top-dead-center (TDC). Once again I verify that the arrow is pointing to the front of the engine.
I install the head gasket on the cylinder studs so the holes for the push rod tubes are at the bottom. It is possible to install the gasket backwards which will have part of the gasket cover the push rod tube hole. I verify the gasket holes align with the push rod tube holes.
I install the left head one the cylinder studs and push it home against the cylinder.
Install Rocker Assemblies
When I removed the rocker assemblies, I put them in zip-lock bags labelled with where they came from. The intake and exhaust rockers are different part numbers. I also labeled the push rods with masking tape to indicate which side they were on (L or R), and exhaust or intake. I put the tape on the top end of the rods. That insures the push rods go back into the engine in the correct push rod tube and orientation they came from.
The rocker arms have a casting number. The left intake and right exhaust rocker are the same and are marked with casting (#333) while the right intake and left exhaust are the same and are marked with casting (#367).
Note that the tappet finger that rests on the top of the valve stem is blued. This is caused by heat treating to harden it and is not a sign of a problem with the rocker.
The rocker assemblies are secured to the cylinder studs with special nuts. One face is machined flat and goes against the face of the pillow block.
The pillow blocks are machined on one end to fit over the collar in the head that the cylinder studs go through. This centers the rocker assembly in the head so the rocker finger will be correctly positioned on the end of the valve stem and the push rod will engage the cup on the tappet screw correctly and the push rod will not rub on the inside of the push rod tube.
Before I install the push rods I put some engine assembly lube on the balls on the end of the rods.
Then I put the rods into the push rod tubes with the end that has the tape on the outside–I put the tape on the outside end of the rods and marked it “Top” when I removed them which makes it hard to install the push rods backwards :-)–verifying that I put the correct push rod into the correct push rod tube.
I remove the tape and then push the push rod down to seat the ball on the lower end of the push rod into the cup on the end of the cam follower.
I gently pull on the rods. I should feel resistance as I try to pull the rod out of the push rod tube. The resistance means the ball on the end of the push rod is seated in the cup of the cam follower. I have resistance for both push rods so they are seated correctly in the cam follower cup.
If the push rod pops out of the cup on the cam follower, I seat the push rod ball in the cam follower cup again and redo the test, but with less force so I can feel the resistance but I don’t pop the push rod ball out of the cam follower cup.
If the push rod comes out of the push rod tube with no resistance, it wasn’t seated in the cam follower cup and was riding on the lip. It I torque the heads down and then the cam tries to open the valves, the push rod will damage the cam follower cup and and/or the ball on the end of the push rod. That’s why it is critical to do this test to confirm the push rod is seated correctly in the cam follower cup.
I slide the pillow blocks over the cylinder studs and make sure the slit in the each pillow block points to the outside.
Left Exhaust Top Pillow Block Slot Points To Outside
I secure the rocker assembly with two of the special nuts being sure the flat side of the nut goes against the pillow block and finger tighten them.
Then I rotate the rocker so the cup on the end of the tappet screw engages the ball on the end of the intake push rod.
I repeat the procedure for the other rocker assembly.
I install the two thick flat washers and nuts on the head studs at 12:00 and 6:00 and finger tight them.
I use my socket wrench and a 15 mm socket and incrementally tighten–about 1/2 turn at a time–the four cylinder stud nuts on the rocker assembly in a cross-wise pattern to pull the cylinder sleeve evenly against the engine block. When the nuts are snug, I check to be sure the push rod tube seals are fully inserted into the holes in the engine block. Then I twist the push rods to be sure they spin freely to confirm that the cam has the valves closed on this side and the push rods are inserted correctly into the cup on the tappet adjuster.
If both push rods won’t turn, then the cam is trying to open the valves and you need to rotate the engine one revolution to take the stress off the push rods and rockers before you torque the head nuts.
I managed to let the left intake push rod fall part way out of the cup on the end of the tappet screw which meant the push rod would not turn. I loosened the rocker block nuts, seated the push rod ball back into the tappet cup, and hand tightened the rocker block nuts. Had I missed that and torqued the head down, I could have damaged the push rod and tappet cup.
I inspect the push rod tube seals to ensure they are seated all the way and the vertical bar is centered and on the bottom of the push rod tube.
Torque Head Nuts
The head nuts are torqued in three stages to 25 FOOT-Lbs.
- Stage 1: 10 FOOT-Lbs
- Stage 2: 18 FOOT-Lbs
- Stage 3: 25 FOOT-Lbs
I tighten then in a cross-wise pattern.
At each torque stage I tighten the nuts about 90 degrees and then move to the next one until I reach the torque for that stage. This ensures the head is compressed gently and uniformly.
The left side top end is now installed.
Install Right Side Top End
Before I install the top end on the right side, I rotate the engine one revolution back to TDC (OT mark on the flywheel) so the right side valves will be closed and there will be no pressure on the push rods and rocker arms when I torque down the right side head nuts.
Here is the engine with the top end installed. I put shop rags in the exhaust and intake ports of the heads to prevent anything from getting inside. I also put red tape on the temporary spark plugs and on the dip stick to alter me that I need to install new plugs, top and bottom, and oil before I start the engine.
Set The Valves
I wait 24 hours and retorque all the head nuts to 25 FOOT-Lbs as things will compress over night lowering the head torque. Then I set the valves. Since the heads have been rebuilt, I use looser valve settings for the first 600 miles. I set the intake to 0.006 inches and the exhaust to 0.008 inches. After the first 600 miles, I change the oil (again since I change it at 10 miles to flush all the assembly lube from the engine and 200 miles to clean out small metal bits created as parts mate) and reset the valve clearances: intake at 0.004 inch and exhaust to 0.006 inch then I balance the carburetors agian since the changed valve clearance will affect carburetion.
2020-03-08 Clarification about “L” and “R” marks on cylinders.