Gents,
After about 200 miles following a rebuild where work on the bores was only a honing to increase piston clearance and work on the pistons was a light filing to cleanup scoring from the tight bore, I took the bike up to 80 MPH, briefly. Then, there was some unusual noise, but not the klonking I had heard two rebuilds ago when the left-side big-end shell let go from lack of lubrication. A few moments after the noise began, and after I'd pulled to the shoulder, the engine flat died. Well, I was not particularly close to home, and my wife had no way to help me. I kicked it over with the plugs disconnected and things were not horrible, but there was an unusual sound, kind of a clicking (which I am yet to figure out.) I know it's almost a sin to think of riding your bike when you think something may have gone seriously wrong, but after a couple minutes of thinking about it, I decided to start it back up, if only to get off the interstate, which I did.
From the gas station near the offramp, I called a motorcycle recovery service for a quote. $200! for about 13 miles. Against all good reason, I decided to take the chance and ride it home very slowly, which I did at less than 30 MPH all the way. By the time I got home I was happy that nothing seemed any the worse. I knew that at least the top end must come apart, which it did. You could say I was ecstatic to find that the rods were stable on their journals. In fact, I was hard-pressed to find anything blatantly wrong until I noticed the black streaks on the right piston. (The other streaks are from before the recent rebuild.) When I wiggled that piston I found that it was a bit stiff rotating on the pin versus the free-moving left piston. Since I have billet rods, there is no small-end bush. I decided to slightly hone the small end to free things up, and that seems to be good (not running yet, of course).
Now, on to the photos. When I first looked at the top of the piston I thought it had a web-like pattern of hairline cracks only in the surface. Looking under high magnification I realized that the cracks were actually very tiny ridges. In the last photo of the piston top, you can see where I have taken a fine file to reveal that the ridges are aluminum. The whitish spots are actually semi-loose flecks of aluminum, and there are some near-micro beads of aluminum stuck to the top and spread about. I haven't seen any other photos like this, nor read anything about it, but I am guessing that this is the very first phase of reaching the melting point. I suppose Muskrat has seen this before, tagging it as maybe caused by overheating from pre-ignition. (Others may recognize it, as well.) Since the bike was running really well, I don't know that pinging would be the cause. I am more thinking that the heat would be due to the slightly tight bore running at a speed that the bike probably hasn't seen since 1970, but I am not going to ignore the possibility of pinging. I will he re-timing to be sure it's not too advanced. I don't think there has been any significant damage at this point, so I just plan to file down the ridges, scrutinize structural integrity and reuse the piston.
OK, this has been a long story, so I'll leave it here. Thanks for reading this far (if anyone has). Really appreciate thoughts of any kind. (Sorry, but "Don't you have anything better to do than write longwindedly on the forum?" is already taken.)
Richard L.
E, I think the photos might be just good enough to show what I'm intending, but the ability to zoom with the iPad might be cheating.