I've pulled down my engine for my yearly rebuild and have found what look like small surface cracks on the no1 piston. They look like small thin cracks or scribe lines on the front two valve reliefs (one intake and one exhaust).




They almost look like scribe marks, which I'm sure I would have noticed during previous rebuilds. Other options are fatigue and heat.

If they are cracks due to heat, the only thing I can think of is that this engine is running an EWP with the coolant running in a reverse flow direction. As standard this would be be the coolest part of the engine, however with my setup it's probably the hottest. But having said that, my coolant temps at the output are lower than my previous setup so I can't imagine that this is the case.

I also find it strange that the cracks have occurred across different valves areas rather than on both exhaust valve reliefs.

The intake valve has hit the piston at some point and left a mark. This could have been during engine assembly or disassembly however I'm careful not to let the cams rotate when removing the timing so maybe it's due to valve float at 8500rpm. As part of this rebuild I'm switching in some Supertech duals to up the spring rate a little.

What do you guys think? Most likely I'm replacing this with a new piston, but it would be nice to know the cause.

Engine specs:
BP05, N/A, E85, High compression (around 14:1), approx 6 hours running since last rebuild.

 

Actually I'm thinking now it has to do with heat cycling creating small expansion / contraction cracking. It's right where the water pump sits on the block with the cracks forming in the area near the hottest part of the piston.

Still strange for a forged piston.

 

Do you have piston to valve contact, or is that just part of the relief cut for the valves?

 

Yup. One of the intake valves has been doing a bit of it's own thing. The valve spring for that specific valve is a bit down on spec and so it's probably been floating at high revs. I run such tight clearances on my valves due to the large cams so any movement in valve timing is bad.

Several of my valves have mushroomed a bit and have sunk into the head but that one is the worst. I'm going to have to replace the valves and upgrade the valve springs so I can run the revs I need. These are RB26 valves so they are around 2mm oversize.

As for the piston. I've spoken to the original engine builder and he said they are marks from checking valve relief clearances. I'm not sure why they are visible now and I didn't notice them on previous rebuilds.

 

Ok, I was going to say that they look like more like scratches than cracks, but with pictures it's always tough to tell. You need to fix that valve contact, I've seen a lot of cracked pistons because of valve contact.

 

Any abnormal marks on that cylinder wall to indicate an overly hot piston? I'd be curious if there are cracks on the underside or in the wrist pin boss as well.

 

Any machine shop should be able to red dye the top of the piston and tell you if it is an actual crack or just a scratch. Not sure how portable the motor is at the moment but that's an option if you really want to be sure. Shouldn't take the guy more than ten minutes