possible piston to head contact.

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VonCramp

VonCramp

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In my ongoing quest to win the battle against my pro comp heads, they have thrown a new curveball at me. I recently pulled them off my '72 block 340 to get new springs installed for the new Comp Cams roller cam. Upon delivering them to the machine shop my machinist pointed out that there were shiny spots around the combustion chamber where it looks like the Pistons may have contacted the heads. The same shiny spots are also on the Pistons in the same "s" shaped pattern. My piston are 30 over flat top. Before I resort to changing or modifying the pistons, I was hoping that there may be a thicker head gasket available. Has anyone else encountered this particular issue before? Also, will the pro comp roller rockers be able to withstand the higher tension spring pressure without breaking in half? I do not have the spring ratings with me at the moment, but can post them later.
 

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340doc said:
What's your deck height on that? Just curious.
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I knew someone would ask this question. Honestly, I don't know. Never had a reason to check. All I can say is there is a significant difference between the chambers on the pro comps and the x-heads I was running.
 
X beads open chamber
Pro comp close chamber

You néed to measure the deck height on those pistons to find out what head gasket thickness is needed before they hit the head again
 
You can order any thickness head gasket from cometic you need, they are a little more $ than your standard gaskets, but cheaper than cutting down your pistons.
 
the pistons look like the L2316 trw/L2332 sealed power (numbers are stamped on piston tops) 1968-71 forged replacements, with compression height of 1.840". with an uncut block they are above the deck around .018", was the block deck surfaced? cometic makes thicker gaskets-kinda expensive
 
You have had some pretty serious contact, based on the pix. I would be checking the rod bearings.

As above, you absolutely need to measure the amount of height that the pistons come up above the deck with a dial indicator on the magnetic base. Set the mag base stably on the deck so that it does not rock, and check the top and bottom of each piston for its height relative to the deck. Rock the pistons back and forth at each measurement, note the highest and lowest heights, and take the mid-height as the measured height.

Then set the gasket thickness so you have .035" minimum piston-to head clearance, and a bit more, like .040" won't hurt. You will have a tight quench gap either way.

BTW, per nominal parts stack up, your Speed Pro pistons are nominally. .018" out of the hole. With a .039 Felpro 1008 compressed gasket thickness, you may have had just .020" clearance..... with that, it's pretty much a given that you will have piston-to-head contact at some point, with expansion and piston rock, etc.

Once you know that, you can calculate up the thickness you need. Cometic has them pretty much in .005" thickness increments in the range you likely need. You can order direct form Cometic and it takes just a few days for them to make the gaskets.

BTW, make sure you do indeed measure the piston deck heights; DON'T guess. It may reveal a lot of things. We found uneven deck slope top to bottom (deck not square to bores) so had the block decking corrected. We also found one crank journal ground about .003-.004" higher than the others, so had those 2 pistons milled a bit more. And you might find uneven rod lengths. With you running pistons with the heads above deck, getting this right on every piston is very important.
 
nm9stheham said:
You have had some pretty serious contact, based on the pix. I would be checking the rod bearings.

As above, you absolutely need to measure the amount of height that the pistons come up above the deck with a dial indicator on the magnetic base. Set the mag base stably on the deck so that it does not rock, and check the top and bottom of each piston for its height relative to the deck. Rock the pistons back and forth at each measurement, note the highest and lowest heights, and take the mid-height as the measured height.

Then set the gasket thickness so you have .035" minimum piston-to head clearance, and a bit more, like .040" won't hurt. You will have a tight quench gap either way.

BTW, per nominal parts stack up, your Speed Pro pistons are nominally. .018" out of the hole. With a .039 Felpro 1008 compressed gasket thickness, you may have had just .020" clearance..... with that, it's pretty much a given that you will have piston-to-head contact at some point, with expansion and piston rock, etc.

Once you know that, you can calculate up the thickness you need. Cometic has them pretty much in .005" thickness increments in the range you likely need. You can order direct form Cometic and it takes just a few days for them to make the gaskets.

BTW, make sure you do indeed measure the piston deck heights; DON'T guess. It may reveal a lot of things. We found uneven deck slope top to bottom (deck not square to bores) so had the block decking corrected. We also found one crank journal ground about .003-.004" higher than the others, so had those 2 pistons milled a bit more. And you might find uneven rod lengths. With you running pistons with the heads above deck, getting this right on every piston is very important.
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X2, If the rod bearings are really trashed you should also check the rods for straightness and the upper bushings.
 
What's the problem? Just slap it back together. It ain't tore nuthin up. That's what you call REAL quench.
 
You didn't clay it up to check? Or take any measurements? If those are the Chinese pro comp rockers they should last anywhere from 5 seconds to 5 weeks.
 
What do the heads look like? You'd better measure and see what you need for a head gasket thickness. What rods are you using?
 
Closed chamber heads with the early 340's are usually a no go. The KB's I run on the '68 340 in my Duster came out to be exactly .018" over the deck, with no deck surfacing at all. I ended up having a set of open chamber 308's ported for it. Late now, but Edelbrock makes a special version of its aluminum heads specifically for the early 340's. They're opened up to 65cc's and have enough clearance for the .018" above the deck pistons that are the norm for the earlier 340's.

The thicker head gasket should solve the issue, but of course it will lower your compression ratio too. Assuming of course that you haven't hurt anything important with that contact.
 
I built a 400 B engine once that had low compression 440 pistons in it. They came out of the deck 0.057! I had some 906 heads relieved to accept them....
 
mguner said:
I built a 400 B engine once that had low compression 440 pistons in it. They came out of the deck 0.057! I had some 906 heads relieved to accept them....
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Wow! lol
 
70aarcuda said:
Felpro makes a .053 head gasket
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Would that be the 8553PT Perma torque? I have a set of these and am trying to figure out piston to head clearance on my set up. After searching it seems they are lots of results between .045-.053"

Or are you referring to 521SD? Can't seem to find much info at all on that one.

There visual appearance seems different than the OP's. Maybe he had .039" or even thinner, in which case that would be a simple fix with the right head gasket. The 8553PT might be enough, either way he needs to figure out how far out of the hole those pistons really are.
 
the 8553PT......

dial indicator and magnetic bridge to check deck height..shown above the degree wheel
 

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I'm going through this with my 340 right now also. I used a magnetic base dial indicator to find the distance the pistons are out of the block. I used a 1/2" piece of keystock and layed flat across the cylinder surface then set the dial indicator at zero on top of the keystock and rolled the engine over and measured the total height out of the block. I would assume any solid straight edge would work. Mine were right on the money at .018 I had a set of magnum gaskets that mic'ed out at .065 and after tourqed to specs they compressed to .055 using feeler gauges to check the distance between the block and head. I also had a set of the felpro .053 gaskets but I liked the the thicker ones to be safe. BTW I'm also using magnum heads on the 340 similar to your's
 
jammied said:
I'm going through this with my 340 right now also. I used a magnetic base dial indicator to find the distance the pistons are out of the block. I used a 1/2" piece of keystock and layed flat across the cylinder surface then set the dial indicator at zero on top of the keystock and rolled the engine over and measured the total height out of the block. I would assume any solid straight edge would work. Mine were right on the money at .018 I had a set of magnum gaskets that mic'ed out at .065 and after tourqed to specs they compressed to .055 using feeler gauges to check the distance between the block and head. I also had a set of the felpro .053 gaskets but I liked the the thicker ones to be safe. BTW I'm also using magnum heads on the 340 similar to your's
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What is the part number of your head gaskets?
 
not sure of the part# they were in a gasket kit for a 5.9 magnum I ordered a few years ago on ebay I have seen that gasket kit on there recently though. The headgaskets look identical to the mopar factory gaskets for the magnums only quite a bit thicker
 
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