38 replies to this topic

[purrlx]

how far do the pistons protrude above the block deck height??

[rexy]

Good opportunity to get rid of the Chevy valve relief pistons.

[TerrA LX]

No comp on no.2?
Was it due to the head gasket or the bore?

[Redslur]

Do any of the other pistons protrude above the bore? Generally as a rule, 304/308/355's don't normally even need valve relief pistons unless big cams are fitted. You can get away with flat tops. Anything out of the bore is different in my experience. Unusual for it to hit with valve reliefs though?  You would normally put that down to big end wear.

[mick_in_oz]

Are those valve reliefs a cast finish? cheap set of pistons? Maybe the piston is too heavy for the type of conrod and RPM it gets spun to, and the rod is stretching a little too far. Saying the piston is out of the block a bees d!ck does not help us identify the issue, please provide measurements. Could also be excessive piston to bore clearance causing piston rock, the reason only one side is touching, what thickness head gasket? Cometic and some others do thinner than standard gaskets, I think .027" being the thinnest I have seen.

[TerrA LX]

^ that's what I was thinking, worn bore causing the low comp, rocking piston...

[Struggler]

It looks to me that something has fallen into your engine, there appears to be an injury just above your thumb. This would be enough to hold that side down and force the opposite face into the head at TDC.

 

It has only happened a couple of times otherwise the piston would look a lot more second hand.

 

As for the leakdown, it is quite possible that the ringland has been squashed and pinched the top ring, can you move/spin the top ring around ?

 

Put that piston back in and measure how far it protrudes, if it is 0.005" or less you will be fine.

 

Is the engine a stroker ? If not then that must be a 327SBC piston.

[mick_in_oz]

Can you do a simple measurement with a steel rule across the top of the piston and a feeler gauge to the deck of the block, and average the measurement if need be to account for piston rock?

 

If it had .040" head gaskets in it previously, and the bearing looks fine, and it has as little as the .005" piston out the bore that you think it should have, then a possibility could be detonation on one cylinder due to poorly adjusted rockers on just that pot, causing less lift and less overlap letting it make too much cylinder pressure and that in turn the detonation and the piston rock mentioned above. I know a bit of  long shot but I have seen this on my brothers Boss 302 many years ago. The detonation could also account for the blown head gasket.

 

Any pics of the Gasket?

 

A few more thoughts, If the pistons stick out too far you can get them turned down that little bit for them to be at zero deck height, or -.005" if you're worried. Go for some new pistons for a Holden and forgo the reliefs and get some more comp in it and also probably end up with it being more tolerant of detonation due to the lack of the valve reliefs.

 

If its a real budget deal I'd probably just put it back together after checking all of the important areas for the correct clearance, big ends, piston to bore and ring gap etc ONLY as long as the pistons are very close to the zero deck discussed above.

[TerrA LX]

I assume the odd looking chamber belongs to No.2cly?
It is a different colour and has different valves?
The gasket doing funny things next to the bolt hole pictured at 10:30 around the chamber (top left) on the head.

[S pack]

Its lighter in colour due to the piston touching, bvut the valves are the same throughout just the light playing tricks

Looks to me like #2 chamber and valves are cleaner or oily compared to the others due to oil ingestion from the blown gasket.

I'm thinking the head gasket has blown first, then the piston has lightly kissed the head, if in fact it has actually made contact.

 

I reckon it would be a good idea to properly check #2 gudgeon pin and big end bearing clearances.