highschoololds

Quick question, I have a stock 350. Got to take heads off to get a broke manifold bolt out so I can finish my headers. What thickness can i use to up my compression a bit. It has #7 heads.

Rickman48

What thickness head gasket, or how much can you mill the heads??
Another 'might as well' would be a valve job with new valve seals!!

highschoololds

Yes you are correct, but I just want to do the headers right now. I've had them almost ten years! I don't have any of my other parts together right now, so the heads will come off again later for rebuilding. I was just gonna put some thinner gaskets on for now.

coppercutlass

Even if you mill the heads down to make up for todays thicker head gaskets you will be down on the compression. Your best bet is to use a flat top or small dish piston. By the time you mill the heads enough to get decent compression the intake won't fit adding to the cost., and possibly making those heads useless with another combo. I would mill the heads to make up for the diffrence and keep the compression where it's at save your money and get some forged flat tops.

coppercutlass

You could run corteco .027 gaskets but they cost near 200 bucks. The old shim style which where .022 are almost instinct. Felpros are the cheapest but they are .040

highschoololds

I have read all about the flattops and milling heads intake and decking block. I'm not ready for all that yet.

I have to have one head off so I can drill the exhaust bolt out. I was just gonna change the gaskets to give it a little bump while Ihad them off.

highschoololds

Just glanced at Summit. They list a .021 SCE copper gasket set for about $100. Any reason why these wouldn't work?

Thanks for all the responses!

coppercutlass

Cost vs. Gain aint worth it o.e gaskets where. Around .022 thick that slight diffrence won't do much

highschoololds

Thank you very much. Didnt know they were that thin as I was going to take head off tonight.

keithnh

Not sure what your bore is but a 4" bore and a .042 crushed gasket creates a volume of 8.6cc to add to both sides of the compression equation. A little more as the head gasket wont be exactly at the edge of the bore. Round to 10cc probably be close. Do a .021 "thin" decrease chamber volume by 5cc. On a 350C with 3.48 stroke that makes a 70cc head combo 10.12 with the thicker gasket and 10.65 with the thinner. Discounting dome/top ring volume /valve refiefs. A half a point approx. in my calculations. You might feel it.

coppercutlass

Yeah olds 350 have a 4.057 bore the 7 heads have more than the 64 cc chambers with the piston being around .025 to .030 in the hole and a 23 cc dished piston the compression is less than the factory posted compression of 8.5 to 1 even with a .022 thick gasket. Im at roughly 9.75 to 1 with flat tops being .025 in the hole and 64cc's with a .040 head gasket . There is no way around those 23 cc dished pistons. His compression would be at 8 to 1 with a .022 head gaket and the pistons being .025 in the hole and that can vary along with the cc of the chamber 442.com state 64cc's but they are closer to 68cc's

coppercutlass

Like i mentioned before between .018 -.022 they can vary. The last ones i tried to use where .022 un compressed and they turned out to be for a 330 so the didnt quite cover the water port and puked water every where. Even with a .018 those dished pistons are a compression killer. Some 14cc dished pistons can make some decent compression to run a good cam and make some decent numbers. He is trying to bump compression but if his engine is bone stock with the 23 cc dished pistons it's pretty much pointless imo. His compression would be 8.95 to 1 with his 23 cc pistons being at zero deck and his heads being a true 64cc and with a .015 gasket. It would be 7.84 to 1 with a .040 gasket, his pistons .025 in the hole and 68 cc's

captjim

Not sure I'd run .015 piston to head......

coppercutlass

I would not either . I was just showing what it would take to build up some compression with what he has and showing the diffrences with some variables etc.. He is better off running flat tops or a small dished piston. Run it as is untill the funds allow to use the right stuff.

keithnh

Im no expert on any engine make .. I did just have to learn about the compression ratio equation and found some tools I was using for estimating were off considerably. I am putting together 2 350s right now a chev and a poncho and have what was a good running olds 350 but water got in it and it froze. It may not be repairable and is a windowed mains block with 77 (think thats right) heads. Have the Mondello v8 mount kit for my 84 Cutlass with 3.6 /carb. I forget whats in the kit it may not work with a chev engine. But concerning this thread the young fellow was asking what a gasket variation would get him thick to thin and thats about a half a point in compression ratio which wherever he's starting from although not huge the change should be noticeable. I would be asking the same question in his shoes. Actually I was a little surprised when I punched in the numbers myself it was that much.

coppercutlass

The diffrence between a .018 gasket and a .040 gasket in compression is 8.16 vs. 7.84 the diffrence is .32 of a point assuming his heads are 68 cc and pistons are .025 in the hole with a 23 cc piston . Imo that wont be nothing you will notice on the street.

highschoololds

Got head off to get the broken exhaust bolt out, and man those are some huge dishes!

For future reference, if I go flattops with a zero deck, what would be the minimum advised gasket thickness? .040? Thinking along the lines of the XE268H, when I get that far (with upgraded gears and converter).

Thanks, this is a great site that I struggled to find for many years!

coppercutlass

I have flattops with them being .025 in the hole and the heads at 66cc's im at 9.98 to 1 compression i run a comp 280h . I would just square the deck usually takes a few thousanths . Then when you do your valve job have the heads milled . I had mine milled .022 with bigger valves im at 66 cc's rougly . Some of the example i was giving with the compression ratios in the other post is based on numbers that can vary like head cc's, deck height, piston cc's gasket thickness. These are all factors that go into an engine build .

keithnh

At 4.057 bore discounting the gap back from the bore to the gasket the .018 gasket will add .2326 CI or 3.81 cc the .040 .5169 CI or 8.47 cc . The thinner gasket less than half the volume of the thicker. Its free compression as long as it seals reliably and the gasket costs are not way more no matter what the other variables are.

coppercutlass

I never said the thinner one wouldn't yeild more compression. just that it don't make sense in a stock engine with out a big cam etc etc.

bigD

i agree with coppercutlass, for this application the high dollar thin gaskets would be a waste, 1/3 of a point of compression isn't going to be a factor on a stock engine. i also see no need to zero deck a 350 olds when you look at the compression possible with the flat top pistons and then consider that most sbo builds run iron heads, factor in the commonly availably fuel quality, if it's a street engine you're asking for detonation problems...

highschoololds

Why run .025 in hole and then cut .022 off heads? Why not just deck block and then not have to worry about the intake and heads lining up?

olds 307 and 403

On a stock 350, small cam motor with pistons .025 to .030" in hole, would thin gaskets hurt? Even with the .011" thin gaskets from M&J Proformance, you still have .036 to. 041" clearance, isn't that ideal?

cutlassefi

Uhhh if you take .020 off the heads or the block it's the same thing. You've still changed the relationship.
When I shave heads I cut the intake side of those instead if the intake itself. That way you're not rendering the intake useless for another build.

coppercutlass

Olds 307 and 403 I ran .015 gaskets on a 9 to 1 combo with stock deck height and the heads milled .005 with no issues.

captjim

So why suggest a variable that is not a safe option? Makes zero sense, like reccommending cutting the chambers down to 50 cc. Will lower Cr all right, but not a viable option.

coppercutlass

I never said it was all right to do I was just showing the extremes that would be needed to be done to gain compression on those 23 cc's I never once said go ahead and do that . So what's your point .

jag1886

IMO you have hit the nail on the head, I see this again and again where people say it makes no difference weather you mill the heads or the block, if your pistons are .025 down the hole and then you slap on a .041 head gasket you end up with .066 squish area, if you mill the block to 0 instead then slap on your .041 head gasket you have .041 squish which can do a lot for cylinder mix and controlling pinging.
Oldsmobiles don't have much squish area in there heads to start with so making what you have work to it's full potential is important.
I only bring this up because people talk a lot about compression and it seems they completely forget about the squish area.

coppercutlass

My reason to cut .022 was because the engine I was buliding was suppoused to be a simpple re ringand bearings . I was going to use the 14cc pistons . So I got the heads milled .022 and used a shim gasket . It turned out the gaskets where for a 330 . So I got a set of cast flat tops from a w31 engine and replaced the 14cc ones and used the felpro .040 gaskets . I'm stuck with my heads and intake because of the machining.

coppercutlass

Joesw31 yes I would asume. With the bigger valves and carbon build up. Closer to over 10 to 1 but I didn't cc the heads . My heads are probably closer to 64 cc's but again I'm assuming I should have cc'd the heads but I was running out of time at that time. It runs fine on pump 93 with no issues so it can't be that high.

coppercutlass

yep i still run the same cam. Im building another short block dunno if im usuing slighly used .030 425 pistons i already have or forged standard bore this one will sport a lunati cam.

coppercutlass

Im in the process of building so to speak not together yet. But i picked up a good short block and a lunati braket master 2 cam 496 /520 same as edelbrock with those pistons for 350 from a local racer . The short block i picked up is in really good shape and is perfect for a standar rod and main with a standard forged piston. It would cost me about the same to run a forged vs the 425 slugs with boring it and parts .

coppercutlass

2.072 and 1.63 if I remember correctly.

380 Racer

I will never have any block 0 decked. What happens down the road if something happens and the block needs decked again? Suddenly the piston is X amount out of the block. My custom pistons have been made to have .010 clearance "just-in-case" Again this is JMO.

captjim

I disagree. Factor in the rim of the piston where it meets the quench pad on the head and this is a very small percentage of the bore. Having such a miniscule quech area makes it LESS important, not more. The more quench area on the head/piston, the more important optimum piston-to-head clearance is. IMHO. Having said that, if you are building an engine, might as well get it .040 while achieving the desired Cr.

bigD

i'm of the same mind set as 380 racer... besides for a street engine the difference in gasket thickness isn't going to affect performance enough to even worry about. if it's that big of a deal then compromise and mill the heads .010" to offset some of the gasket thickness...

jag1886

This sight can be very confusing to people at times, you say you disagree but in your last sentence you agree with what I said.

captjim

If you are starting from scratch, sure, get it right. Why not? But I personally do not put a lot of importance on that in our engines with those slow burning chambers. Just my opinion.