I decided to install 9.5:1 high compression pistons used in the earlier RS engines instead of the original 8.2:1 pistons used in the 1983 RS motor. Due to variability in piston diameter and the tight clearance required with Nikasil cylinders, the advice of Tom Cutter at Rubber Chicken Racing Garage is to replate the cylinders with Nikasil and hone them so they provide the optimal clearance with the new pistons. So I sent the cylinders and new pistons to him to have this done, but before I did, I removed the push rod tubes and the head studs from the cylinders. You can see how I do that here:
Now I am ready to install new stainless steel push rod tubes and the original head studs into the cylinders.
I use a mandrel sized for these push rod tubes (18 mm). I got the mandrel from EME. It’s made by Siebenrock in Germany, but EME is now producing their own tool. BMW changed the diameter of the push rod tubes so check your tube diameter before you pickup a mandrel to be sure you get one sized to fit.
I use a machinists parallel, or flat, to help set the depth of the push rod tube. I got this one from Cycle Works.
I use a Popsicle stick to set the depth of the tube correctly. Popsicle stick thickness varies from 1.8-2.0 mm. I use a caliper to select one that is about 1.85 mm thick.
I use a wire brush to burnish the push rod tube holes in the cylinders. You can get a set of these at Harbor Freight.I decided to try some temperature pencils to measure how hot the cylinders are. They melt at a specific temperature. I make a mark on the cylinder with the pencils near the push rod tube holes to determine how hot the metal is.
I am replacing the original push rod tubes that are a bit rusty with new stainless steel ones.I purchased a set of four stainless steel push rod tubes from Euro MotoElectrics (EME).
This is a short video summarizing the process.
VIDEO: 1983 BMW R100RS Install Push Rod Tubes & Head Studs
The cylinders should be clean and the holes for the push rod tubes scrubbed clean and free of any oil, grit or grunge. I had Tom vapor blast the cylinders so they were returned clean. I burnish the push rod tube holes in the cylinders with a suitable diameter wire brush.
Prepare Push Rod Tubes
I use 600 wet/dry paper to remove the sharp edge on the end of the tube that fits into the hole in the cylinder. These tubes have a nice taper to help center the tubes in the hole, but I don’t want the sharp edge to catch on the side of the hole.
I learned the push rod tube collar is an interference fit on the tube and is designed to butt up against a ridge machined into the outside of the tube. One collar was not flush with the ridge. I used a long socket, slide it over the end of the tube, and tapped on it to move the collar up against the ridge. Later I show how to secure the collars.
The push rod tube seal will push the collar against ridge when the head nuts are tightened so the collar can’t move.
The steel push rod tubes are a shrink fit in holes in the aluminum cylinders. I will heat the cylinders to 275 F and freeze the tubes so minimal shaving of aluminum from the inside of the holes occurs when the tubes are driven into the holes. But, the tubes have a thin wall and so they won’t stay cold when they contact the hot cylinder.
To keep the tubes cold for as long as possible, I fill the end that fits into the hole in the cylinder with paper towel, soak it with water and then freeze the tubes until the the water is frozen. The water soaked paper helps keep the tube cold while I drive it into the hot cylinder.
Select Popsicle Stick To Set Push Rod Tube Depth
The push rod tube has a collar that pushes against the push rod tube seal. The top of the collar should to be from 1.8-2.0 mm below the top of the cylinder flange. A Popsicle stick, although the thickness varies from stick to stick, is in this range. I measure a couple until I find one around 1.8-1.9 mm thick.
I put the stick under the edge of the machinist flat and adjust the tube depth until the top of the tube collar touches the bottom of the Popsicle stick while the flat is resting on top of the cylinder flange.
There is a ridge on the cylinder flange, so avoid putting the machinists flat on top of the ridge when making the measurement.
Install Push Rod Tubes
I am using the technique Tom Cutter recommends: Red Loctite #263 is put on the outside of the tube where it slides into the hole in the cylinder. This secures the tube in the hole and prevents any oil leaking between the tube and the cylinder.
On a previous project, I put high temperature RTV on the tube before driving it into the hole in the cylinder to seal it against oil leaks.
- 11 BMW 1977 R100RS Install Connecting Rods and Assemble Top End
So far, those tubes and the push rod tube seals are not leaking. But, RTV does not secure the tube in the head as Red Loctite does, and Tom’s technique ensures the tubes are secure in the cylinder. From now on, I will use Tom’s technique.
I heat the cylinders in the oven to 275 F for an hour to be sure they are heated completely. I use a temperature stick to verify the cylinders are at 275 F.
I set the oven to 285 F. I put a mark on the aluminum next to the push rod tube holes with the 275 F pencil. When it melts the aluminum is at least 275 F. You can use an infrared non-contact thermometer but readings are often low due to the reflectivity of the shiny aluminum. Try pointing it in between two fins to get a more accurate reading.
When I’m ready to install the push rod tubes, I put all the tools I need out on the workbench as I have to work quickly. I use two hammers, a carpenters hammer to make small adjustments and a 2 pound maul to drive the tube quickly until it’s close to the edge of the cylinder flange.
I take a hot cylinder out of the oven and a push rod tube out of the freezer. I put the hot cylinder on the workbench.
Then I put Red Loctite #263 around the tube about 1/2 inch from the end that goes into the cylinder hole for a width of 1/2 inch.
I use Permatex 27100 Red thread locker. Based on published specifications, it appears to be similar to Loctite 263.
I insert the mandrel in the collar end of the push rod tube and align the tube in the hole and set it with the carpenter hammer. When it is seated, I use the 2 pound maul to drive the tube into the hole until the top of the push rod tube collar is about even with the top of the cylinder flange. Then I switch to the carpenter hammer to adjust the depth of the push rod tube collar as required so it the top of the collar just touches the bottom of the Popsicle stick.
If you drive the push rod tube too deep, you can insert the mandrel into the top of the tube–pushing the paper towel down out of the way with the mandrel–and tap the tube move it a bit out of the cylinder hole. You have to make this adjustment quickly as you don’t want the cylinder to cool down nor the tube to heat up a lot.
After I get the push rod tube depth correct, I mark the top of a fin next to the other hole with the 250 F and 275 F temperature pencils. If only the 250 F pencil melts, I likely can install the second tube if I move quickly. IF neither melt I have to reheat the cylinder before I install the second tube. That said, it’s safer to just return the cylinder to the oven for about 10 minutes and very it is heated to 275 F.
While the first cylinder heats up, I take the second cylinder out of the oven, another frozen push rod tube from the freezer and install it in the other cylinder. Then I return the second cylinder to the oven to heat up to 275 before I install the second tube.
When the heads have cooled down but are still pretty warm to the touch, I put a small amount of Red Loctite #263 on the face of the push rod tube collars. It will wick into the gap and help keep the collars from moving.
Install Head Studs
I clean and wire brush the threads of the four head studs. I test fit them into the holes to be sure there is nothing in the threads. Since Tom vapor honed the cylinders, the stud threads are clean. I use Red Loctite #263 to secure the studs in the cylinder–or, in my case, Permatex 27100 which has very similar properties.
I put Red Loctite #263 on the threads and thread the stud into the hole leaving 28 mm of thread exposed. Since the stud is 50 mm long, a little less than half the threads are in the cylinder.
I clean up the excess thread locker so it won’t harden on the top of the machined surface the head gasket fits on as I don’t want any high spots in the gasket when I install the head.
I let the cylinders sit for 24 hours to let the Loctite cure completely. Here are the vapor honed cylinders with new stainless steel push rod tubes and head studs installed. They look much better than what I started with.