Changes in RS Engine Design
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.
These pistons come with the rings installed. It is good practice to LEAVE THEM THERE and not remove them. These rings are easy to break when installing, so avoid the temptation of “fiddling” with them and just install the piston into the cylinder (YES, I’m Looking At YOU) 🙂
Based on the advice from Tom Cutter at Rubber Chicken Racing Garage, the best approach for longevity and an oil tight engine is to re-plate the Nikasil inside the cylinders and then hone them to get the correct clearance to match the actual piston dimensions. He said there is enough variability in the new pistons that this is the best method when installing them in the Nikasil cylinders for an oil tight engine.
Before I send the cylinders to Tom, I clean them, make some useful measurements, remove the two studs in each cylinder that help secure the head and remove the push rod tubes.
Video of the Procedure
Here is a short video showing how I remove the two cylinder studs and the push rod tubes. I show the “Risky” way to remove the tubes–and the unintended consequence–and then the safer way to do it.
Inspect & Clean Cylinders
The cylinders have oil baked on from the leaking push rod tubes and one has a fair amount of oil around the base of the sleeve that slides into the bore in the engine case. The push rod tubes are pretty rusty and I will replace them with new stainless steel tubes. I’ll show how I do that in another write-up.
I inspect the cross-hatch on the inside of the cylinders. The original cross-hatch is visible at the bottom of the bore as the rings never slide past this part of the cylinder.
Above that is the portion of the cylinder worn by the rings. You can see some faint cross-hatch, but this part of the cylinder is more polished than the bottom portion of the cylinder.
The four holes in the base of the cylinder are for the cylinder studs that secure the heads and cylinders to the engine block. The two holes that are at the top when the cylinder is installed are machined to accept o-rings.
Oil is sent up the annulus between the top studs and the hole in the cylinder to lubricate the rocker assemblies. The o-rings seal these passages at the engine block to prevent leaks. The bottom two cylinder stud holes are not machined, as shown below.
I remove the large o-ring that fits in a groove in the base of each cylinder to seal it against oil leaks. One of them is broken and that explains the baked on oil at the base of that cylinder around the groove for the o-ring.
Before removing the studs and push rod tubes, I clean the cylinders with engine degreaser and get all the crud and grime off them. It is common courtesy to clean your parts well before sending them for machine shop work.
Cylinder Stud and Push Rod Tube Measurements
I measured these prior to removing them so I can install them to the correct depth. That said, I found the push rod tubes were not installed at the correct depth in the cylinder holes as explained later.
Cylinder Stud Measurements
I measure the exposed threads of the studs before I remove. Then I can install them with the correct length of exposed thread. I measure 27 mm of exposed thread.
After I remove the stud, I measured the length and it’s 50 mm long.
Push Rod Tube Measurements
The push rod tubes fit into holes in the top of the cylinder. However, they are recessed in the holes and due to being at an angle, the amount of recess is different at the inside and outside edges of the tubes.
I measure the recess on both the inside and outside edges of the tubes. The inside edge is recessed 7 mm while the outside edge is recessed about 3.5 mm.
The outside measurement is what I’ll use to scribe a line on the new push rod tubes so they will be at the correct depth when I drive them in. If the tubes are too shallow, they will crack the seals and if they are too deep, there isn’t enough pressure on the rubber seals to seat them tightly and oil will leak from the bottom of the push rod tubes.
After I remove the push rod tubes, I can see where the bottom edge of the hole in the cylinder has created a sharp edge on each tube. I measure the short and long distance between the bottom of the tube and the sharp edge to see if I get the same difference I got when I measured the depth of the tube in the cylinder hole. The difference between these distances should be 3.5 mm the same as the difference in the recess of the inside and outside edges of the tubes.
The long side is 22 mm, and the short side is 18.5 mm, giving a difference is 3.5 mm. So the groove on the outside of the push rod tube is a reliable witness mark for how far the tube is driven into the push rod tube holes in the cylinder.
Correct Depth of Push Rod Tubes
The advice I received from Tom Cutter, at Rubber Chicken Racing Garage, about the correct depth of the push rod tubes is the have the top of the push rod tube collar be 1.75 mm BELOW the top of the cylinder gasket face (of course, the 1983 Nikasil cylinders don’t have a gasket, but the machined surface is still called the gasket face). I took a machinist flat and placed it on the cylinder gasket face, avoiding the ridge at the base of the cylinder sleeve.
I found the top of the push rod tube collar was flush with the cylinder gasket face.
That means the tubes where deeper in the cylinder holes than Tom’s recommendation; i.e; the tubes had been driven closer to the engine block. A common “fix” for customers who complained their push rod tube seals were leaking was to drive the tubes down out of the cylinder a bit to increase the pressure on them in lieu of replacing the seals. All the mechanic had to do was use a special tool BMW provided to fit around the collars and then drive the tubes closer to the engine block. Replacing the seals requires removing the top end and then having to replace the base gasket, or cylinder o-rings, and the head gasket. I suspect that’s what happened on this bike.
To be clear, the placement of the push rod tubes in the holes in the cylinder strives to avoid too much pressure that will crack the seals and too little pressure that will prevent them from holding a seal as the engine heats up and cools down. Tom’s recommendation is what I will rely on when I put the new push rod tubes back into the cylinders. I will adjust the measurements I took of the depth of the recess of the tubes inside the holes; I’ll reduce the amount of recess I measured by 1.75 mm which will place the top of the collars 1.75 mm below the cylinder gasket surface.
I use double nuts and put my ratcheting 17 mm box wrench on the bottom nut to remove the studs.
They come out without much effort.
Remove Push Rod Tubes
I use vice grips to clamp the tube just above the push rod seal ring. I make the vice grips snug, but not so tight as to deform the tube.
I heat the neck of the cylinder the tubes slide into with my MAP gas torch to get the aluminum hot so it will expand.
The picture below shows the way I removed the first two push rod tubes: THE RISKY WAY. I gave the vice grips a very hard wack. And on the second tube, my aim was off and I cracked off a small piece of the bottom cooling fin.
THE SAFER WAY that avoids cracking a fin is to put a scrap piece of plywood on top of the cylinder and hang it over the edge of top fin just a bit. Then firmly, but not aggressively, hit the vice grips with the hammer. A couple firm blows will remove the push rod tube. And, if your aim is off, you will hit the plywood, not a fin, and that should protect the fin from being bent or breaking.
Then I use a hammer and give a hard wack to the vice grips to pull the tubes out.
When I was removing the second tube, my aim was off and I cracked off a small piece of the bottom cooling fin.
I got it repaired by a friend who has repaired broken aluminum and cast iron cooling fins for me before.
So, after that lesson, I put a piece of plywood on top of the cylinder and hung it over the edge of the top fin a little bit to protect the fins from being hit by the hammer. This is the safer way to remove them. I held the board down at the back with one hand and with the other I used 2 or 3 firm, but not aggressive, hammer wacks to remove the tube.
Here is a push rod tube after it’s been removed. You can see the shiny part that went inside the hole in the cylinder and the sharp edge that is where the bottom of the hole in the cylinder rested against the side of the tube.