Main Seal Driver

The main seal at the back of the R100 motor is large (more than three inches on the outside diameter) and narrow, making installation a challenge without the proper tool. When removing the transmission for a main shaft circlip upgrade, I noticed that the main seal that I'd installed with a drift and hammer was leaking between the engine case and the OD of the seal. More than likely this was do to bending the seal body with the drift, since it's almost impossible to keep the seal square to the bore all the way around using that method. While I was at Ted Porter's BMW shop I noticed a main seal installation tool laying on his bench, and asked if I could copy some dimensions from it. Ted readily agreed, and even offered to read the dimensions off as I wrote them down. Unfortunately, his eyeballs are as bad as mine, and it took both of us to read the caliper! Regardless, on my return home I sat down at the computer and knocked out a drawing, simplified from the driver on Ted's bench to make fabrication in my small shop a bit easier. You can jump to a larger version by clicking on the thumbnail at left.

I chose to make my driver from some aluminum plate I had laying around, which meant sawing out a square and then rounding it up while it was held in a 4 jaw chuck. That's a lot of work but it was faster than running around looking for three and a half inch bar stock, then getting nailed by minimum order charges. For me the most challenging part of the job was getting the five bolt holes properly located in the plate. I was able to find some information on the web that let me calculate a bolt circle, but without a dividing head and a milling machine the information didn't do me much good.

Fortunately, BMW uses a washer plate under the five bolts that secure the flywheel to the crank, and that washer plate makes an excellent template to locate the holes. I started by boring the 0.880" center hole. Next I made up a punch with a 0.440" OD so it would self center in the washer plate bolt holes, then machined a very short conical nib in the center to mark the bolt hole locations. I used drill rod and hardened the end so it would last more than one or two hits. In use the washer plate gets centered on the 0.880" hole, then the punch gets dropped through one of the washer plate bolt holes. A light tap is all that's needed to get an indent where the bolt center goes. Over at the drill press, I drilled out the hole to 0.440", then chamfered the hole edges. For the second hole I used a flywheel bolt to align the washer plate with the hole I'd just drilled, made sure I had good alignment with the 0.880" hole, then marked the second hole on the other side of the diameter of the tool. I repeated this process for the remaining holes. To make the drawing I measured the distance between adjacent bolt holes, then used the following formula to calculate the bolt circle:

5 Bolt Pattern - pentagon
B = distance between bolts, on center
D = diameter of bolt circle
R = radius of bolt circle

R = B / (2 * sin 36°)
D = 2 * R

The plate I started with already had a hole drilled and tapped into it, pretty close to the bolt circle. I worked around it to make sure that none of the holes intersected it, and didn't give it another thought until the first time I used the tool. The seal went in fine, but then I found I didn't have the hand strength to remove the tool! About that time I saw the threaded hole, and inserting a bolt into it and screwing it down so that it pushed against the crank popped the tool right out. I didn't even need a wrench on the bolt. The upshot is that the location of the threaded hole isn't that important, and neither is the thread size. If you don't have an M8 x 1.25 bolt laying around, tap the hole for a bolt of about that size that you do have.

With the outside diameter turned and the five bolt holes drilled I next worked out the shoulder for the seal. That's the 3.145" diameter, and it's pretty straight forward to make. What wasn't obvious when looking at Ted's tool was the counter bore in the underside of the tool, and I missed recording the depth dimension. That's important because this tool uses the bottom of that bore as a limit when driving the seal. I could have just as easily made the tool a little larger in outside diameter and used the shoulder to limit inward travel, but since the BMW tool uses the bottom of the bore and that was my pattern, that's what I did as well. As is often the case with these projects, it was Sunday and Ted's shop was closed, so I machined that depth by feel. That meant I made a series of trial cuts, stopping when the underside of the shoulder was even with the rear surface of the engine case. That results in the main seal surface installed even with the rear engine case surface. Chamfering the edges of the bolt holes and cleaning it with solvent and compressed air finished up the tool.

I soaked the new seal for an hour or so in motor oil I'd warmed to 225° in the kitchen oven. I used some heavy duty aluminum foil to make up a shallow tray just larger than the seal, minimizing the amount of oil needed for the soaking. Tom Cutter recommends warming the engine case when a heat gun as well, but at the time I didn't know about that tip and installed the seal cold. I fished the seal out of the oil, slipped it over the driver with the open end toward the engine, and then installed the five bolts through the tool and into the crank. I drew them down finger tight, verifying that the seal was centered in the bore and that the shoulder was square across it. Then I used a socket wrench to tighten the bolts, working a turn at a time in a star pattern across the bolt heads. When all the bolts were snug I backed them out and used the extra threaded hole to pop out the driver. I found a small bit of plastic clinging to one side of the seal; apparently it had been scraped off one side of the seal on the way in. Perhaps heating the cases would have avoided that, but after several thousand miles the seal is not leaking.

This was a fun little project, but if I could have borrowed a seal driver it would have been the way to go. All of this machining took quite a while, and it didn't help that I started with plate instead of round bar. And in fifty thousand miles of use, I've only needed to install one main seal (the first one would still be going strong if I'd used this tool instead of a hammer).