Thin head gaskets

[Jacksdad]

I need to do something about the quench height and compression on a 360 without taking more stuff to the machine shop. I just want to build this one, get the car moving again, and start collecting good parts for the next one. I'm cc-ing the heads before they go on, but judging by the four huge valve reliefs on my pistons (Speed Pro H405P's. I know, I know - they came with a kit...), and the fact that even on a decked block they're 20 thou down in the hole I'm still going to need a little help squeeze-wise (okay - it's not a real word. I made it up..). I remember reading the MP shim gaskets are not available any more for the 360, so am I going to have to call Cometic? I have a set of Fel Pro's sitting around but I think they'll put my quench height at about 60 thou which is a tad more than I'd like. Thanks guys.

[dave571]

If you are using open heads, and your pistons are flat tops in the hole, then the quench isn't much of a concern. You won't really have any.

The piston has to almost touch the head to have quench effect. Either a closed chamber, or a big dome are required.

I'm not up on SB head volumes, but if we say 65 cc's, then your motor should be around 10:1 with the piston .020 in the whole, a .040 gasket, and 5 cc valve reliefs

10.3 with a .027 cometic

Here's a link to a calculator http://www.kb-silvolite.com/calc.php?action=comp

If you are using closed chamber heads with those then you hould try nto use the cometic, which will put your quench height at about .047

[Jacksdad]

The heads (pre magnum smog heads with 2.02s) are a bit of a mystery as far as chamber volume is concerned. They were machined before I got them, and I had them cleaned up by my machine shop, so actual volume is up in the air until I cc them. Gasket choice will depend on that, but I'm trying to work out my options first. I remember a thread that mentioned these pistons having a static compression of close to 8:1 (as opposed to the advertised 8.8), so I'll definitely need some help. The valve reliefs (four of the suckers. For the four valve engine that nobody makes maybe..?) probably lose more metal than the dishes in the old cast pistons.

[Jacksdad]

And thanks for that link. Appreciate it

[michaelmopar]

What year model is the motor. is it original or as it been gone thru. i ask this because i have a hole stack of manuals straight from mopar tech. from the 50's to the 90's and it list factory specs for "in the hole" and gasket thickness and head casting and even factory compression pretty helpful. i found out why my 318 was pinging i thought it was timing and come to find out it was sitting at 9.8:1 after the valve job and factory was 9.6:1 so i checked and the compression test proved that the info was accurate i found them in a old dodge dealership that was vacant.

[Jacksdad]

This one is a mix and match motor done on the cheap. That was the theory anyway. The original motor was a 78 Cordoba 360 (stock compression 7.8:1). The block and heads turned out to be unuseable, so the crank and rods were thrown into a 78 van block that was bored .030 over, along with a Summit Racing engine rebuild kit and hypereutectics. I searched for early heads but all three sets I took to the machine shop were cracked (just been real lucky with this one, haven't I?). I eventually stopped looking for old stuff and picked up a pair of reconditioned late model heads from the wrecking yard (forget the casting number - late 80s pre swirl if I remember right), blocked the EGR ports, hit them with a die grinder, and dropped in a set of stock 2.02s I had laying around. The machine shop guy said the marks front to back showed they'd had a cheap ass resurface done (he likened it to a big belt sander) so he recommended they be checked and redone if necessary. He took off as little as possible, but couldn't even guess how much had been taken off previously. When I cc them I'll have a better idea. I just ran some numbers through the calculator and doubling the piston head volume to 10cc for the four valve reliefs gave me 9.44:1 on a 65cc chamber and .040 gasket (I think the Fel Pro's are about that) which would be fine for my purposes. I'll check with Federal Mogul about the valve relief volume, but I guess it's really down to the chamber volume, and I should have a better idea when I get around to checking them. I know it's not that wild, but I have a MP 284/484 cam I'll probably use so I'll need some compression to stop the overlap bleeding off too much at low RPM's - this is going to be a regularly used street engine so I need some low end pull.