[304065]

Update:

Got the carbon removed from the head by soaking it in undiluted simple green for a few hours. That was enough to soften it and I was able to scrape it off with a wooden scraper.

Intake valve stem measured in spec as did the exhaust at right around 6.95mm. The tolerance spec is 6.96-.0015 or 6.958mm minimum. The ID of the guide measured up with a small hole gauge at 7.00mm. The spec for the gauge ID is 7 H 7, which is an ISO spec for a reamer size (the one you would use to ream the guide to final size) and equates to 7 to 7.015mm. So that is a valve stem clearance of (7.00-6.95= 0.05mm) where the wear limit is 0.1mm.

I also took a close look at the seats, there were a few spots either of carbon buildup or microwelding from seat to valve. I decided to lap the valve to restore the contact area. I did this with a conventional suction cup tool and some permatex lapping compound, and it only took about 10 oscillations of the tool to provide me with a consistent solid gray band all the way around the valve, about 1/8" wide.

I did thoughtfully consider sending the head to either Ted Porter or Max BMW to have the seat replaced, valve replaced and the head resurfaced at a cost of $750 per set of heads. However, after consultation with a long-time motorcycle racer, aviation mechanic and professional Porsche race crew member, I determined that the initial course of action would be as above. The valve isn't going to fail catastrophically as a result of any of these measures. The conditions for operation will be:

1) Leakdown test after initial break-in of 5% or better; and
2) Valve adjustment within specifications at 500 miles post-reassembly.

The early 80s airheads suffer from a condition known as "valve recession" by which the seats erode and the valve eventually becomes flush with the surface of the combustion chamber. This is evidenced by the clearance closing up over time. So it is possible to measure the clearance with a go/no-go gage at the valve adjustment interval to see what is happening.

A replacement cylinder arrives tomorrow; I will look at it and evaluate whether the pushrod tubes should be changed before installing on the bike. The existing cylinder with the deep pits has pristine pushrod tubes, so it would be a question of heating the heads and removing the tubes with a mandrel designed for that purpose, then reinstalling the good set in the good head.

So the parts tab for this will look like:

Rod Bolts
Rod Bearings
Cylinder o-rings (one large and two small)
Piston Rings
Valve Keepers (should always be changed as the valve rotates within these and they are cheap)
Head Gasket
Valve Cover Gasket

In the process of removing the head I had to take off the exhaust. Unfortunately for me, some PO had welded the left muffler to the left header and actually welded a sleeve around the left muffler to repair the rust. So the left header was unusable, and I had to cut through the rear crossover tube to get it apart. I found a used set of both headers and both crossover tubes in good shape; I am considering sending these out for Jet-hot coating prior to reassembly. New headers are $130 each but they are chrome, which will blue from the heat over time, whereas the ceramic coating may fare better according to those who have done it, and the all-in cost will be lower.

Stay tuned