Here we go again...

[Darrell]

Why did Frank Wheeler have all the luck with his Hodakas?

The piston went "groink" while riding my Wombat (with a combat piston and ported cylinder) this weekend. I pulled on the clutch fast and stopped the bike for awhile.

It was a temporary seizure -- been down this road before -- and I was able to ride the bike home. This time the ring appears to have survived, the compression hasn't dropped a bit.

I haven't pulled the top end off yet, but I expect all the damage is on the exhaust side. Looking at the photos: Does this look like a lean seizure? It's a bit off center for some reason.

Same old circumstances again, e.g., 94 octane fuel at 32:1 Yamalube 2R or Lucas semi-synthetic:

For a sidebar, I must add that the stock Wombat top end had a good long life and wore textbook perfect with no galling or scoring. This original top end had every right to go sour as it went through intentionally overly lean air and oil mixes to compensate for a bad condenser. Nowadays we recognize those symptoms as a failing condenser and point the finger at it right away. I'm on my third one now.

[Bill2001]

Looks similar to the seizure I had on my Wbat94 soon after I got it. The piston freed up easily and I suspect that it was "temporary", too. Also suspect that the engine had seized for the PO before I bought it.

I've fixed the top end and have rejetted richer for the break-in. One added accessory I've added is an EGT gauge to monitor engine conditions, we'll see how that works out.

[Bullfrog]

Darrel:

There isn't enough information provided by a peek into the exhaust port to definitely diagnose the cause of the seizure - at least not by me. So . . . all the usual suspects are, well, suspect. Lean jetting at some/several throttle position(s), advanced ignition timing, excessive compression, engine air leak, tight piston/cylinder clearance, bad fuel, etc, etc.

Ed

[Darrell]

OK, the piston is out now and revealing the secrets of its seizure:

Things to note:
- The piston is ART (cast) not a Wiseco
-There are five new marks: two beside the piston intake port; and three on the exhaust side (one wide mark flanked by two narrow ones.
-The top end has gone 500 miles at this point.
-There are two old seizure marks to ignore, if you can see them in the pics. One is at the pin and the other 180 degrees opposite. The piston groove was pinched from an impact to the piston crown. Caused by rough machine shop handling I guess.

If I had all the answers I wouldn't need to ask all the questions. Is this seizure pattern possibly indicative of too tight piston and cylinder interface? I specified .0025 clearance when I took the piston and cylinder in for boring.

Here come the pics. Many thanks for the wisdom from your experience.

[Darrell]

Well so far no opinions came through regarding this piston seizure damage. Hmm, wonder if I'm THAT Guy

In any event, I have some info now so that other may learn from my misfortune :

I took the piston and cylinder to the other machine shop. We have two and I went to the first one -- mainly because it was about a block and a half closer.

The four narrow marks, two on the exhaust side and two more on the intake side are indicative of a "Four-Corner Seizure" caused by too little piston to cylinder clearance. On inspection, I couldn't jamb a .002 feeler between the piston skirt and cylinder, even aft 500 miles of off-road riding to wear it in. I specified .0025 clearance back when I brought in the top end to the other/first machine shop. Maybe they took their reading from the top of the piston instead of the skirt.

This time I asked them to hone it up to .003 clearance this time -- which is the 1973 spec. Some of the latest thought is that a slightly tighter clearance transfer heat more effectively, that's why I originally specified .0025, but it looks like I didn't even get that much. We'll see what the original score was after the next machine shop brings it up to .003 thou.

It might be sort of a compound seizure with the wide bit of scoring over the exhaust port. I'm thinking or wondering if that seizure is somewhat independent, or at least only partly related to the four-corner event, I.e., too lean air-fuel mix, but the piston crown and plug reading don't indicate that. So, it's still a bit of a mystery to me.

[Darrell]

Conclusion and Resolution:

Alright, got the cylinder back from the (second/other) machine shop now.

To create a .003 piston-to-cylinder clearance they said they had to hone it out an additional .0015 to .002 . No wonder it seized .
As for every action there is also a reaction. According to Pi, that means the ring gap has opened up 3.14 times the added clearance .

Hope this helps someone somewhere. At least I can tell you where not to go for machine work in my hometown.

[ossa95d]

Good news Darrell! It's always good to identify the source of the problem. Glad you got it figured out.