38 replies to this topic

[S pack]

Are the markings on this piston caused by contact with the head or could they have been caused by bits of head gasket getting mulched up in the cylinder?

[TerrA LX]

Are bore and piston skirt?

[mick_in_oz]

I didn't notice earlier that there is a mark near the 030" marking on the slug from the combustion chamber, and the larger mark on the opposite side is definitely combustion chamber, or more correctly the cylinder head surface, we can see the machining marks from the head surface in it.

 

An observation or expectation would be that if it was piston rock it should have LESS carbon left on the very edges on the piston rather than there being less closer in.

 

Are there any very minor surface markings on the bearings from that pot that may indicate some amount of detonation?

 

When I asked about a pic of the head gasket I was wondering we would see any sort of distortion of the other cylinders that would indicate detonation with it getting ready to push out in another area, but it looks pretty good, well, from the pics anyways.

 

It still could simply have been detonation on that one pot due to poorly set rockers.

 

As for the .005" out the hole, I agree that it should normally be all good. I have a 326" stroker that has 327 fuely rods and Keith Black hypereutectic pistons, with felpro gaskets 11.25 comp and a nice solid roller, used to get spun all the way to 8000 and it only just ever touched the head, just enough to clean the carbon off, but not enough to actually mark anything. Using that as my yard stick I can't see you having any trouble if all is correct.

 

It could even be poorly torqued cylinder head bolts, or old bolts, and it simply didn't clamp quite right, let the gasket go, and then as someone already pointed out, let some oil in and that caused the detonation, and the resulting marks on the slug.

 

Plenty of people also try to run too much timing in a Holden, if the heads are right they need very little, and being conservative will let it live a long time.

[Bernie]

Do you have floating little ends ?Is your con rod cracked on the little end ? Measure your piston against one of the others and compare them .Things can happen in the quality control dept .

Bernie

Edited by Bernie, 28 January 2014 - 09:21 AM.

[TerrA LX]

Piston to bore clearance is measured about 50mm down the bore and around the piston skirt, feeler gauges just won't cut it.

[mick_in_oz]

Those bearing shells look fine, bit of wear but no other marks to worry about.

 

As above, a cracked little end could be a culprit.

 

As above, the measurement point is normally across the skirts, in line with the balance pads, the flat area beneath the gudgeon pin, and yes you will need more tools than a set of feeler gauges.

 

If nothing else sticks out at you as being out of place then perhaps its a put back together and give it a go...

 

As for your mention of taking a little off the tops of the slugs, if you know someone with a big enough Lathe its not a big job, a Mill will also do the job but requires more time and effort, mostly due to setup.