Alright, i am building an engine for my 79 4x4 tow-mud truck. Just got the engine back from the machine shop. I told the machinist that i wanted to have a compression ratio around 9.5:1. So he went ahead and picked the pistons (h345ncp30) he said that i needed and pressed them on the rods. After looking around and reading a bit it seems that with those pistons the cr would be closer to 8.8:1 with a 72cc head which he also recommended. The block has not been decked, the compression height on the pistons are 1.540, not sure about what head gasket to run. I would like to know what i need to do to get my cr close to where it needs to be. Should i just get a 64cc head or stick with the 72cc and run a thinner gasket? What are my options???

from what im seeing 8.6:1-8.8 depending on the gasket size. did you get your block 0 decked? or the heads milled? I don't know why he put such low compression pistons in a motor you wanted to be 9.5:1. are you sure the numbers are right? even if you run a thinner gasket you will not get the compression to 9.5:1

http://www.northernautoparts.com/ProductModelDetail.cfm?ProductModelId=10712

The block has not been decked at all, stock height. The heads would be milled to 72cc's. and yes #'s are correct.

You're going to have about .045" piston below the deck. Slap a little silicone on the deck surfaces and bolt them heads on. ;) Seriously, he should have set you up with the H631CP's instead, that would put the pistons .025 in the hole instead, plus they only have 2 VR's instead of 4. Not much you can do now but go for the 64 cc heads (not the best for quench) or better yet have the block zero decked and use standard composite head gaskets.

I need to buy a set of heads regardless, might be a good idea to go with some vortec's. Would it be better to deck the block and stay with a large chamber head for quench? Or go with a set of smaller chamber heads with not so great quench?

Stock is right at .025" DITH piston so taking .020" off your decks will put you there!

Then a .015" to .020" thick shim headgasket will give you a good quench.

Then figure out how small a chambered head you need to get you up to about 9.75 to almost 10 to 1 CR. They range from aftermarket down at 50 cc's to GM heads up to 80+ cc's on the 400's....

Use here to check your CR. by going backwards...

http://www.campbellenterprises.com/Race%20math%20calculators.htm

Unles you want to change pistons like was already mentioned earlier.

pdq67

I think i'm going to go with decking the block. Although i have never used a steel shim head gasket. Any tips for using these gaskets and getting a good seal?

why don't you just take the pistons and rods out, and bring them back to the machine shop as it appears it was someone elses' mistake? Maybe it will cost you 1 day of work, but who cares. Well worth it IMO.

why don't you just take the pistons and rods out, and bring them back to the machine shop as it appears it was someone elses' mistake? Maybe it will cost you 1 day of work, but who cares. Well worth it IMO.
^^--- have the machine shop make it right.

I always use shims.

I have used aluminum spray paint, both spray and dauber Copper-Cote and even good old Permatex, "Indianhead Shellac Gasket Cement". I've even double stacked Mr. Gasket's .020" shims too fine.

I'm almost anal about re-torquing them at least three times over that many days after heat and cool cycles to make darn sure the head-bolts stay where I want them..

Works fine!!

And I have re-torqued my darn header flange gaskets as many as 6 or 7 times before they finally seat and stop loosening up too. What a pain, but whatever works..

pdq67

PS., and I don't think you use anything on Fel-pro's .015" thick rubber coated #1094's..

Stock is right at .025" DITH piston so taking .020" off your decks will put you there!

Then a .015" to .020" thick shim headgasket will give you a good quench.

Then figure out how small a chambered head you need to get you up to about 9.75 to almost 10 to 1 CR. They range from aftermarket down at 50 cc's to GM heads up to 80+ cc's on the 400's....

Use here to check your CR. by going backwards...

http://www.campbellenterprises.com/Race%20math%20calculators.htm

Unles you want to change pistons like was already mentioned earlier.

pdq67


1.540 CH is .020" shorter than stock for a 350, putting the pistons .045" in the hole. My listing of the H345NCP has those pistons at 1.560 C/H, so all shouldn't be as bad as it first seems. You can go with the shim head gaskets, deck the block, or just bolt the heads on with stock head gaskets and be done with it. If you did a back to back test with all those different options you would not notice a difference in performance. From worst to best case scenario you're looking at maybe 15 HP difference. You could go with the Vortec heads, they will make a bit more power, but again it won't be night and day difference, and you need specific rockers, intake and valve covers besides the heads.