Boxerworks.com

jtoml wrote:Some light aircraft engines (horizontally opposed fours) go through oil quickly if you fill them to the top. It blows out the breather tube. At 3/4 full, they stabilize at whatever their consumption actually is. Has anybody noticed this with airheads?

Yep, commonly described.

Manfred wrote:I would prefer an apple fritter, but haven't earned one.

I did find Duane's web page on top end issues, titled: BMW motorcycle piston ring compressor.

I replaced the pushrod seals a year ago and everything worked fine when I put it back together, so I might take each side apart, one at a time, and clean up the pistons, etc. But first, I need to have someone follow me at night to determine when I'm blowing smoke.

Which is easier to remedy - rings or valve guides?

With original equipment, when the rings are allowing some oil past them, it is usually higher mileage. To replace them is easy, but often the top ring land is worn too wide. I understand that it is possible to find a wider ring.

We replaced out own valve guides, but took the heads in for the valve seat and head work. A new valve guide will most likely have a different center and the only solution is to get them ground.

Be very careful who does your head work, especially replacing the guides. If they just drive out the old ones, run as fast as you can. The old guide will rip out some of the head aluminum as it comes out. They need to be machined away and then collapsed out of the head.

BMW had the guides made in 4 sizes, that is 3 oversizes. That was because of the issue of the base aluminum getting ripped out and the hole is too big for standard. The guide must go in quickly, while cold and the head is very hot, around "spit dance" temp, which is around 280 F.

There are a lot of methods used and some will result in far less miles from the head.

Many thanks, Duane.

I found this listed as the right kit for my bike: Part# 11251337398. Priced from $50 to $65 per piston.

after a complete top end, first over pistons, valves, guides, updated crankcase breather, and all oil seals replaced, my 74 R90/6 still uses oil. But at least it isn't dripping out anymore. I don't know that a quart every 1k miles is enough to worry about. On the other hand my 09 GS uses absolutely no oil. So the average is good.

Yeah, if compression is good, I wouldn't be worrying about 1 quart every 1000 miles. With the small oil capacity of these bikes, think of it as designed continual oil enfreshenment.

pmonk, how did you break it in? What procedure?

Here's what Snowbum says about decarboning an airhead. I can do this, but will it take care of that or should I pay someone with tools and know-how?

"Decarboning is done for various reasons, but especially to keep compression ratio from increasing, and for keeping hot-spots out of the cylinder head. Decarboning of the top end, heads, pistons, etc. on a BMW Airhead Motorcycle, is best done with the pistons and rings removed and separated from each other, and the head disassembled. However, I don't really suggest that you take the top end down that far just to de-carbon it. Leave the valves and springs in the head. Removing the head and cylinder enables the head to be soaked in something like Gunk (brand) Hydroseal (up to a week!); or Simple Green (50%, overnight)."

A head that needs to be decarboned has something wrong with it. The excess carbon is a symptom, not a cause.

Find and fix the real problem first. Then it will burn the carbon out in a few hundred miles.

Duane Ausherman wrote:A head that needs to be decarboned has something wrong with it. The excess carbon is a symptom, not a cause.

Find and fix the real problem first. Then it will burn the carbon out in a few hundred miles.

I agree, rings, valve guides, breather malfunction, of other problems dumped the carbon there. Cleaning it up is
just avoiding real problem.