Replacing the VX head gaskets -job almost done

[Mr Blackstock]

So, I have bolted the heads down, installed rockers etc. Tommorow I should be able to finish it all off. I followed the manual's instructions regarding torquing the head down, 90 nm seems not alot. I can understand why people might be tempted to over tighten. I have tried to measure how far the piston's are proud of the block, 3/4 thickness of a 5 cent coin. See the pics.

 

[Grimes]

So, I have bolted the heads down, installed rockers etc. Tommorow I should be able to finish it all off. I followed the manual's instructions regarding torquing the head down, 90 nm seems not alot. I can understand why people might be tempted to over tighten. I have tried to measure how far the piston's are proud of the block, 3/4 thickness of a 5 cent coin. See the pics. View attachment 222910View attachment 222911View attachment 222912

Awesome, thanks. That’s about .035-0.037” against a coin I have here. Similar to mine before the block skim and quite a lot against conventional rules of thumb for piston to head clearance. Rule of thumb obviously didn’t apply to Ecotecs.

 

[Immortality]

I'd take that measurement with a grain of salt because of the carbon build up on the piston crown.

 

[Grimes]

I'd take that measurement with a grain of salt because of the carbon build up on the piston crown.

Allow -0.010 for the crud and squint factor and you’re still already at the accepted limit. Like I said earlier, once you skim 0.004-0.006 off the block, calculate the differential thermal elongation between the rod and block, allow for rod stretch under high revs, 0.003 for gasket crush and it’s just too tight.

 

[losh1971]

calculate the differential thermal elongation between the rod and block, allow for rod stretch under high revs and it’s just too tight.

If that was really an issue surely you would snap rings?

 

[Grimes]

If that was really an issue surely you would snap rings?

It’s only going to be an issue if the block is skimmed AND the protrusion is at the top end of tolerance AND the engine gets revv’d out a bit. I’ve never heard of any piston contact cases but then again every case I’ve known of where the block was skimmed also had higher piston clearances for boosting. When my machinist and I ran the numbers on mine there was no way we were going to chance it.

 

[losh1971]

Nah I was more thinking if rods stretch at high revs surely rings would snap on the ridge on a 250k engine?

 

[Grimes]

Dynamic stretch, not permanent. Everything is elastic. At the top of the exhaust stroke the rods function is to slow the piston down and reverse its direction. The piston has mass so the rod stretches elastically and then returns to its normal length. Piston to head clearance is there to allow for all these types of stretch factors. The rings are in no danger.

 

[Immortality]

Allow -0.010 for the crud and squint factor and you’re still already at the accepted limit. Like I said earlier, once you skim 0.004-0.006 off the block, calculate the differential thermal elongation between the rod and block, allow for rod stretch under high revs, 0.003 for gasket crush and it’s just too tight.

Nahh, I see carbon on that piston crown a lot thicker than 0.010"

 

[Grimes]

Nahh, I see carbon on that piston crown a lot thicker than 0.010"

You’re not squinting hard enough!