The old Beemer’s valves and guides needed replacing. So Rich figured he might as well do it the hard way even though admitting a reasonable person would take the bike to a shop to have the BMW valve work done.
It is very technical work best left to the professionals.
There is a lot going on inside the head of a motorcycle engine, at least as far as moving parts that can wear and or fail. While most modern bikes are overhead cam designs there are still a few pushrod engines around such as older BMW “airheads.”
The rocker arm wear on my airhead BMW was why I went in for a look. One thing led to another and soon I was into a complete top end job. The valves were worn to the point of needing replacement and the exhaust guides also needed replacing, so I changed out the intakes as well.
Changing the guides must be done first; the BMW valve is lapped in using the new guides otherwise they won’t run true. The first hurdle is removing the old ones. First, the retainers must be removed, and then the guides need to be driven down and out of the ports after the heads are heated.
My Clymer manual says put them in the kitchen oven (with the proper permission of course) and heat to 450F. I thought using the kitchen sink to wash the cylinders last month was pushing it a bit so I decided to use an old Teflon coated electric grill.
The grill could not get the heads to that temperature but got them hot enough. I let them grill on “Sizzle” for about two hours (Note: there will be some fumes). With the help of a properly sized punch and a two-pound hammer they came out clean. By the way, if you spit on a cylinder fin and the spit boils instantly you are over 212 F. A “temp stick” is more accurate going to higher heats, or an infrared heat gun works well, but it must be good enough to read aluminum. Some cheaper versions have trouble with the shiny stuff. Again, there will be fumes. It’s just about impossible to clean the heads enough to avoid this but the cleaner the better.
The new guides need to be checked for size and reamed if necessary. I used a small 8mm ball hone and put in a crosshatch pattern. I made a valve install tool by cutting an old guide down and chamfering the inside diameter of both ends using my drill press. This is necessary because the new guides have a raised lip that you don’t want to squish. By sliding this adapter over one of the old valves, the chamfer on the other end centres it on the old BMW valve as it rides up to its head: we have a valve drift. Or you could buy one but that is less fun.
Once heated and tapped into the head the next thing is to get the valves sealing. A sane person would have taken the head to a shop to have the old seats cut to match the new valves and a quick lap job would have them sealing with little effort. But, that’s not my MO. I decided to lap them in by hand (sort of). With the help of some Permatex valve lapping compound and a cordless drill I spent hours getting a perfect seal. After applying compound and chucking the end of the valve, slowly work the valve in and then away from the seat occasionally reversing direction. This must be done very carefully and it is essential that none of the compound gets near the valve guides or they will get enlarged. Clean and check the seat often. Shine a flashlight in the port, and when you cannot see any light leaking through you are close. I like to use a red sharpie to paint the seat, a small back and forth motion (not a spin) will produce a pattern you can see. The people in the Nondestructive Testing industry use red because most people can see it the best in normal light. (They use greenish yellow for testing with fluorescents in the dark.)
When that looks good it’s time for the liquid test. I used Windex: it’s cheap and slippery. If any is leaking into the ports, it’s time to do it all over till it seals completely.
I know that sounds like a lot of work, and a reasonable person would have a shop do the BMW valve replacement. That’s one reason I wrote this article, most folks don’t realize why they are paying so much to have this done. The other thing is if you do mess up, it can cost you even more. For me, well, it beats playing Spider solitaire on the computer.
By Rich Burgess Canadian Biker Issue #312