i can't seem to find this topic... well my 305 finally blew up and the heads still look ok, i will have them magnafluxed if i end up deciding to use them. i have some crap smogger 350 heads i forgot the casting number but i do remember that they were supposedly worse then my 416's, i was wondering with my stock dish type pistons how much compression would i be looking at running? this car will be a spring and summer/strip car only, no long drives with this baby. hopefully yall can help me out.

 

You'll want to put 1.94" intake valves in them, and do a basic bowl port and chamber unshrouding, at the minimum. They will end up at about 62cc.

Assuming a "typical" rebuilt motor with "stock type dish pistons": .030" overbore, .045" deck clearance (how far the pistons are "down in the hole" at TDC), .039" head gasket (FelPro 1004 or 7733PT2), 12cc dish, 62cc chambers; I come up with just under 9:1.

Certainly not enough to make some kind of street/strip terror; but A WHOLE LOT BETTER than the 7.9:1 you would get withthe same short block and, say, 882s or 624s. Definitely a step up from smoggers.

 

A typical stock 350 has -15cc dished pistons .025" down the bore at TDC.

If you install 58cc 305 heads on it using a thin .015" gasket (felpro #1094)
your cr will be 9.8:1.

if you want some power over and above what the increase in compression will get you, then you could fully port the 305 heads yourself, using larger 1.94x1.60" valves. (requires a new valve job)
(Do not use 2.02's). At that point,as Sofa suggested your chamber volume will be a little larger due to a little combustion chamber wall clearancing to fit the bigger valves. It will likely be 61-62cc's when done.
Your cr on the same motor using the same .015" gasket and 62cc heads will be 9.4:1.

You could always have the heads shaved a bit to adjust the cr if you want more.
if you max flat mill them right down to the valve seat edge you'll get 52cc. Now you have 10.5:1 compression. They'll be a little thin so.....

If you were to also drill and pin the rocker studs and shorten the valve guide boss height for high valve lift, you'd have a fairly decent street/strip head for not a whole lot of $$$'s.

 

Jee Gubber, what's this? Why, it's a junkyard stock 350 with a crylon rebuild and ported, big valve 416 heads!

 

I'm running 416's on my 350, but with flat top 5cc pistons.

 

If the motor's been rebuilt with "stock type" pistons, they're an extra .020" "down in the hole" below the stock .025". .045" total.

I would not put a steel shim gasket on a motor that runs on the street; ESPECIALLY not if it's anything approaching a daily driver. If you do, invest in some nice stout shoe leather. I've taken apart so many motors with steel-shim head gaslets in them over the years, I've long since lost count; I have NEVER, EVER, NOT EVEN ONCE taken one apart that didn't have leakage between cylinders. Usually, the "hot spot" between the 2 middle cyls. NOT ONE. Not good odds.

If the pistons have a 15cc dish, plus the .045" of deck clearance plus the .039" head gasket plus the 62cc chambers plus a .030" overbore, then the CR is closer to 8.7:1.

If the pistons are truly stock with 15cc dish, .025" of deck clearance, with a stock bore, then the CR will be 9 flat with 62cc chambers; and 9.3 with 58cc chambers (which won't flow, so that's not a real good option as far as making plans).

Real world, by the time you put this thing together, it's going to be right around 9:1, if you want it to be reliable.

Estimating high on the CR (or too low, for that matter), or taking risks with the head gaskets that ALWAYS blow, is a real good way to end up with a disappointment. Use logical, NOT the most optimistic possible, sensible, real-world numbers; and stay grounded to what's MOST LIKELY to happen, rather than what MIGHT happen in the most perfect possible world (which isn't the one we live in).

 

Sofa: If GM had even a small percentage of the problems you think will happen using thin head gaskets, they would have stopped using them 30+ years ago.

All the motors you've taken apart were old motors lilkely poorly maintained with old radiators etc etc etc etc and ready for a complete rebuild anyways.

I've been using these thin shim head gasket on big blocks and small blocks for many many years. I have never had one fail. Even at 12.5+:1 compression and nitrous oxide. Even with a few tuning mistakes while on nitrous.
Even had a motor blow up from lack of fuel octane using shim head gaskets. The head gaskets were intact with no signs of leakage upon disassembly.
The hole in the piston is another story....

If the conditions (warped head, warped block, severe detonation, overheating over advanced or retarded ignition timing) are right for head gasket trouble, you will get trouble reguardless of the gasket thickness.

You need to check your math or formula inputs for calculating compression ratio.
See the motor in the picture? That one is the real world. This is not the first one or the last one I've build over the years. The cr is 9.4:1 with stock GM dished pistons, stock block .015" head gasket and 62cc 305 cylinder heads.
Its 400+hp capable.

 

They did, in many applications... for example, most 400s got red Victorcore....

And yes, quite a number of the ones I've observed leakage in, were in fact GM. Old, poorly maintained, whatever, doesn't much matter; they were simply a REALITY. Quite a number of the others, were NOT GM, and definitely WERE NOT old or poorly maintained.

And the REALITY is, I have NEVER seen one motor, NOT EVEN ONE, that had steel shim head gaskets in it, and came back apart without evidence of leakage. Not factory ones, not ones I built, not ones that came from racing engine services, NOT ONE.

Now, a hole in a piston is a different matter....

 

GM makes a very nice .028" head gasket for another option.

 

yea the 350 heads i have i'm pretty sure are 882's lol. could someone give me different comp ratios with different style pistons with different head gaskets?

 

http://kb-silvolite.com/calc.php?action=comp

Go nuts man.

 

sweet, thanks

 

This is a picture of a 350 "lightweight" smogger head and a 305 416 head.
See the thin deck meat right were the spark plug boss is (red arrows).
The felpro head gasket instructions even warn you of this potential trouble spot on the "lightweight" smogger 882,624 350 heads. Look at the 305 head in same spot. (the 305 head is the one on the right) A lot more meat., Not going to be a sealing problem.

 

Here's some typical head gasket leakage on some typical SBC heads (291 casting, double-hump no bolt holes), in the typical place. I just ganked this pic off somebody's eBay listing, so I have no knowledge of what gaskets were used here; could have been most anything. I'd bet money they weren't GM though, in this particular case.

Note that the places between cylinders in green rectangles are clean, but the ones in the magenta rectangles between the center 2 cylinders on BOTH heads, the "hot spot" location, are blown. Especially the one on the left.

This is what typically happens with steel shim gaskets; and where EVERY SINGLE HEAD with steel shim gaskets that I have EVER removed, showed leakage.

Interestingly enough, this is THE ONLY set of SBC heads on eBay this morning, that hasn't been cleaned up (though it looks like the owner has already sort of half-heartedly taken a Roloc pad to them, so in their as-run condition, they must have been pretty grim). Now that's not a very scientific sample, obviously, being only this ONE set; but for today, that's 100% of all used heads on eBay, have blown head gaskets. Note carefully that BOTH heads leaked at that spot, the one on the left so bad that it went all the way to the water jacket; and the one on the right, had leakage down at the bottom place, in addition to the center place.

This is one of the classic reasons why people complain that their engine doesn't idle smooth, has a little mystery "skip" or whatever, and so on; and of course, why people lose races to otherwise identical setups (on paper at leasat). This guy, whoever had these heads, obviously drove his car for A LONG LONG TIME with this going on, and quite likely had NO IDEA that it was like this. Again, pretty typical.

Next time you pop a motor apart, examine that place between the 2 center cyls REAL CAREFULLY, and see if it's got leakage on it. I cannot emphasize strongly enough that EVERY SINGLE motor I've ever pulled apart with steel-shim gaskets in it, EVERY ONE WITHOUT EXCEPTION, had leakage there, on at least one side, and usually both.

The palce where F-bird showed is another trouble spot, but for a different reason. That place, they changed the casting from the original design to the "lightweight" ones, as he points out; but there's a fair amount of variability from one head to another in the casting right there, and heads don't always fit the gaskets quite right. You can directly compare the "scalloped" shape of the outside surface of the casting on the heads he showed, with the "straight" surface on these. Also note the shift in the location of that little water jacket around the spark plug boss. The difference is obvious.

Incidentally, over-torquing (beyond 65 ft-lbs) just about GUARANTEES that this will happen, with amost ANY head gaskets. So for all I know, that could have been the REAL problem here; he might have used some other kind besides shim, and just incorrectly installed them. I have no way to know any of that.

 

You'll probably have some difficulty fitting both 1.94 intakes and 1.60 exhausts in a '416 head chamber. One or the other, but not both will fit in the stock positions unless there is almost no margin. I've been able to fit and unshroud 1.94s in a '416 head with a net of 59cc chamber volume (for a 350).

 

Funny the 350 head in my picture which came from a running stock 350, (had never been apart.) Had shim head gaskets and no sign of leakage between the center cylinders. It did have some signs of detonation on the number 8 piston.
i suspect that at some point the transmission modulator vacuum line probabily was leaking air causing that cylinder to run lean and detonate.
(The vacuum modulator line is connected on the manifold on the top of the #8 manifold runner.)
This is not a picture of some head off ebay which you have no first hand knowledge of, (You make a lot of assumptions about that head and how the dark spot got there and what gasket was on it). but from a motor i saw run and tore down myself.
if you look in the back ground you can see the zip gun used to start cleaning the decks of the head. Look at the shiny parts of the decks. The ebayer just didn;t finish the job before taking the pic.
Further look at the (lack of) available steel between the combustion chambers, Then look at the amount of space between the cylinders on a 305 head.
A lot more supporting steel for the gasket to seal on.
(the valve centerline spacing of a 305 head is closer together allowing more room between the cylinders and less valve shrouding by the cylinder wall.) The L-98 aluminum corvette head is the same.

Here is a pic of the center cylinder wall. Funny, no sign of gasket leakage.

There is no problem fitting a 1.94x 1.60 valve set on a 416 head. Only minor deshrouding of the combustion chamber wall is nesessary. No thin valve margins are needed. What the hell are you talking about?

 

Here is a pic of a 416 head. there is much more meat between the cylinders even after deshrouding for larger valves. You're not going to have a problem with these heads on a 350 block using shim head gaskets.
If your block or heads are warped, get it decked. No gasket will seal on a warped block.