When I was disassembling the engine, I noticed that the center forward head nut (the one accessed from the bottom) was finger tight.  Like I really mean finger tight as in no wrench needed.  I suspected that was not good.  It wasn’t too long ago that I had checked those two lower nuts and they were both good and tight.  I have lapped three of these heads now.  Each one displayed the same low spot just above the exhaust port.  Even under optimum conditions, it looks like the heads give a bit due to the high operating temp in that area (just above the exhaust port).  When I started lapping this head, it was evident that the area just above the exhaust port was low in relation to the rest of the sealing surface.  This is an obvious weak link in the LS650 head.

Generally, a good indication that you’ve been getting things too hot is coking on the underside of the piston.  Mine didn’t exhibit any coking.  I am guessing that’s because this failure was a one-time deal.  It’s not something that has been degrading over time, not repeated operation at elevated temperature.

Forth, I’m just windin this sucker up too tight.  At 8000 rpm the piston speed is 4933 feet per minute.  Recommended threshold is like 4500 feet per minute for a street engine.  On each one of my LSR pulls I had the thing above 6K for around 25 seconds.  That top ring is the heat sink for the piston crown, the hottest component in the engine.  It’s soakin up heat like a dry sponge soaks up water, and that heat must transmit through the rings to the cylinder and out to the cool air.  I think the failure occurred on the intake side of the piston because that area of the cylinder gets the least cooling air.  I know the fresh charge is on that side, but I think that’s mostly cooling the head.  The piston is mostly relying on the cylinder to shed heat.  It also might not get as much oil thrown up from the rod bearing.  All you have to do is look at the failed piston to see which side runs hotter.
 
Since I was runnin on the ragged edge, and I didn’t have the benefit of the additional fuel to help keep things cool, that poor piston simply went past the limit and started getting soft.  The ring got so hot that the gap closed and butted, then all hell broke loose.  The Fastman was right about the ring butting.  It’s not that it didn’t have enough gap, I just took it past it’s allowable limit, generating so much heat that it couldn’t handle it.  The ring butted and bound up and as the piston moved down the ring stayed put and ripped the top off the piston.  I guess I could set it up with a lot more gap to provide a margin of safety, but maybe it would be better to set it up with sufficient fuel and a reasonable redline, or maybe all three (fuel, redline & butt gap).
 
All along I’ve been concerned about detonation with this tight-quench setup.  But I don’t think this failure was detonation.  Nothing else is beat up.  I’ve reviewed a ton of photos showing detonation damage.  There’s plenty of pistons with similar damage, but all of them show ring lands folded down.  None show the land folded upward.  And there is generally lots of other areas pock marked and beat up.  I don’t doubt that there may have been some detonation going on in my engine (due to the heat), I just don’t think detonation initiated the failure.

The big carb test will have to be done over.  I’ll be putting it back together with a Wiseco.  The test data I’ve collected so far on the carburetors is nice to have but won’t lend itself to a fair test.  I have to look at the bright side, now I will be able to do a comparative test of the Wiseco vs machining the cylinder to increase compression.  That should be interesting.

Hope you find this report informative.  I haven’t given up on my tight quench motor, just have to figure out a few things before I try it again.  I learned a lot from this experiment, and I look forward to learning more as I forge ahead.  Knowledge is power.

Best regards, Mike

DBM:

I think you analyzed it right - the failure is located at the thin area near the valve pocket.  The small amount of material overheated and liquified.

I wonder if it would be wise for you to use one of those thermo coatings on the top of the piston?  They all tell a good story about how well their products work - but I have never seen anyone provide a photo of the piston after it has been used.

Sorry for your loss. I am sure the next round will go better.  I have a couple used stock pistons if you want them......not sure about the quality of them.

I believe Muratic Acid will remove the aluminum from the valves without doing much of anything to the valves.....apply it with a Q-Tip to the aluminum.  (Flat trackers used to do this trick to remove scorched pistons from cylinder walls).

Thanks for that thoughtful and thorough analysis DBM. It's a reasonable hypothesis and I loved how you walked us through your thinking on the diagnostic process. It's all to our benefit as interested observers. Albeit at your expense. But, a wonderful education for us all. Thank you!

DragBikeMike wrote on 02/09/20 at 15:37:46:


I picture you saying this climbing out of the wreckage of some sub-orbital X-plane, pilot-engineer headed aloft to see what all the guff from the test pilots was about.  Data collected, back to work, in quest of better.

Yes, some use muriatic acid to clean pools/concrete. You can get it at most hardware stores in a gallon jug pretty cheap.