Here are the details on the engine I'm having trouble with. Its
a friend of mines and I'm the mechanic. Please see all pictures
attached.
Its an O.S. engine 15CV-x non turbo engine. Its the only engine
available because its a side exhaust for the NTC3 and fit in our stock
class rules.
The engine compression was lost when trying to restart one day. I
installed a brand new piston and sleeve. The compression was restored
and the fuel of choice was changed to 25percent Gen2 Byron. This is
the only fuel of choice available. Since then the engine
worked very well for twelve tanks after being broken in. Until
recently the engine started to blow plugs. The o.s. #8 plug is being
used. The plug would only last one race. It starts normal. Runs the
race and as soon as it gets into the pits and you shut it off the plug
is blown. So its impossible to get a restart. This has happened three
times consecutively. I concluded the engine was too hot and installed
a fourth plug to do an engine temp check. It didn't last 20 feet of
running. So there was no time for temp check. It was suggested to install one more shim to the head. I did this
and it didn't help. The same plug blew instantly. It was suggested by
O.S. tech dept that i check the bearings. So i tore the engine down.
Now this is my first time disassembling a engine. Anyways i found three
scrapes on the side of the piston. All evenly spaced. There is also a
little nick on the sleeve on the edge of one of the ports. Only one is
sorta deep compared to the others. They are mild. What do u think of
this? Also what are the little pits on the edge of the piston? I don't
think that is pitting because i think pitting happens in the middle of
the piston? Do u think the piston needs to be replaced? Could this be
the cause of the blowing plugs? And imagine this piston set is so new.
The sleeve is very shiny on the underside and top.

Now I moved on to check the bearings. I don't know how to check the
bearings but i removed the crank and inserted it from the front
bearing side and sorta wobbled it from side to side. It does have a
great amount of play. Same for the inner bearing. Is this normal? Do
they need to be replaced? What is the correct way to check the
bearing? They rotate smoothly

The last measurement i took was the bottom on the connecting rod. The
inner diameter is 0.157 inches. That is the brass part. Where it
attaches on the crankshaft the diameter is 0.153inches. Is that
acceptable? If i physically check for play. There is... but i don't
know what is normal. What u think?

I have colder glow plugs to try when we rebuild the engine.