Valve Seats: "a tricky job" on Airheads

[Air4Life]

From Snowbum's site:   "If the valve seats were replaced, keep a close eye on the valve clearances, check them more often, for maybe 5000 miles.....there have been instances of the seats moving; particularly if not installed 100% perfectly, a tricky job on a BMW Airhead."

http://bmwmotorcycletech.info/setvalves.htm

Would anyone know what makes these valve seats especially tricky to install?

[Bob_Roller]

I personally don't know, a lot has been written about the heads need to be done by someone that is well versed in airhead cylinder heads .

I've got an independent airhead mechanic that I take sub assemblies to, when specialty tooling is required .

I asked him about the critical nature of the heads, his comment was that they are no different than air cooled VW or Porsche heads .

He sends cylinder heads to a local shop that specializes in air cooled VW engines, claims that he's never had any issues with the parts or workmanship in the 30 years he's been dealing with them .

So take that for what's it's worth !!!!!!!!

I think the key element here is to take them to a shop that's familiar with air cooled engines .

[tvrla]

One reason they're tricky is because the seats are an interference fit. The aluminum must be heated to expand the hole, the seat dropped in, and when the metal cools, it grips the seat tightly.

The problem is that the head, especially the exhaust valve, is the hottest part of the engine. If the job wasn't done properly two things can happen - the seat drop out when it's hot, or if the receiving area was cut too small (too much interference), the head can crack when it cools and shrinks.

There are other considerations, and booby traps to avoid. One of those is to not use the BMW valve guides since they're pre reamed for the valve stems. The problem with that is the guide changes when installed due to it's interference fit.

I'm no expert and can't tell you a whole lot more than that, but I do know it's a specialty job. I'm sure a shop that does a good job on Porsche or VW Bug heads could handle ours without a problem. Water cooled heads don't see the heat ours do, so don't require the exacting precision of air cooled heads.

[Air4Life]

Thanks Bob and Wirespokes.

Oh, I thought it was something specific to these Airheads of the r65 that made them "tricky".  For this certainly is not a new concept in handling these metals, outside of the manufacturing process that is.  I guess, like many other things, these old skills are dying off.    

Well, it sure is a good thing there are a whole LOT of those VW's putting around.

[bruce_launceston]

I had a new valve seat move into the combustion chamber about 4mm. When I asked the machinist what interference they were installed at he said the milling machine was preset to suit the valve seats.
This is obviously not tight enough for an old airhead with soft alloy heads.
They redid both exhaust valves seats, fitting larger valve seats into newly machined holes to the interferences that BMW specify (that I supplied along with the preheat temperatures etc) and I have had no further problems.

This is why all the airhead gurus say to only use machine shops experienced in working with airheads, if not beemers at least old VW's, they apparently have the same issues.

Cheers Bruce