BMW R75/5 Top End | Brook's Airhead Garage

At long last, I’m ready to install the top end (pistons, rings, cylinders and heads) on the engine. Previously, I bead blasted the cylinders, measured the bores for ovality and taper, ground the ends down on new 0.5 mm oversize rings to get the correct end gap and had the heads rebuilt by Randy Long. The parts have been sitting in boxes patiently waiting while I got the engine and wiring back into the frame.

Here is the write-up on this part of the project:

11 BMW R75/5 Install Engine Top End

I had an adventure along the way with a new tool, a ring expander. Earlier in the project I used it to remove the rings and decided to use it to install them rather than use my fingers as has been my practice in the past. But, as I was “playing” with it with the the oil control ring, for some stupid reason, I squeezed the expander repeatedly to see how well it worked. If you expand a ring too much, you get this result. 🙁

Too Much Expansion Gets You a Broken Oil Control Ring 🙁

That cost me about $70 with tax and 2 weeks of lost time. This is yet another one of those experiences when I want to take a mulligan on what I just did a second ago :-).

Another surprise is the way the cam shaft and cam follower are aligned. I hadn’t expected the follower to be offset from the center line of the cam lobe, but it’s over about half way.

Exhaust Cam Showing Off-center Cam Follower

This is done so the rotating cam lobe causes a torque on the cam follower making it spin and in turn, spinning the push rod. The idea is to have even wearing of the cam lobe, the follower and the push rod. I was aware that the rocker arm tappet causes the valve to spin for a similar reason, but I wasn’t aware the same thing happens in the bottom end.

I followed Oak Okleshen’s documentation, “Manual 1: Boxer Top End Disassembly, 1970-1975” and “Manual 2: Boxer Top End Reassembly 1970-1975” available directly from him for a modest fee.

Email: “askoak” at “aol” dot “com”
Address:
Oak Okleshen #35
22637 S. Ridgeway
Richton Park, IL 60471

It covered an issue with aligning the rocker arms correctly on /5 heads that I was not aware of before. And, it has a lot of details that should help you stay out of trouble, if you don’t decide to “play” with your new ring expander 😉

Here are a couple of the other pictures from the write-up.