68442

Stock rebuild of a 70 W-31 350. Stock heads(CC'd at 64) Stock OEM w-31 cam. Basicly the only change is forged speed pro flat tops 30 over.
Question: Should i go with the factory steel shim head gaskets(NOS .017 i think) or the felpro set(Thicker .040 ?). The question comes into what my final comp ratio would be and it's effect on todays gas.

Ryan 1969 Chevelle SS396

I am interested and learning the same things. Do you have accurate numbers? CC'ed chambers, exact piston to deck height, piston combustion height, crank to deck height, etc?

Ryan

Octania

It would be a good idea to get all those numbers. From what I have seen, aftermarket 350 piston makers disregard the factory compression height [top of piston to center of pin] and just make it oh about 0.035" less than it should be, placing the piston that far down in the bore. If that were the ONLY incorrect measurement, one might overlook it, but these things add up.

coppercutlass

I found my speed pro's sat about .025 in the hole. Get all your measurements first. I would keep the compression about 10 to 1 so it can Atleast run on 93 octane. Unless you plan on running 110 all the time. Or e85.

1BOSS83

Are you thinking about re-using the old head gaskets? I ask, because, to my knowledge the factory style shim gaskets are not in production.

joe_padavano

SCE sells Olds copper head gaskets that are 0.021" compressed

brownbomber77

Can you re-use them with some of that Permatex copper spray...? Dumb question I'm sure...but it looks like they can be sandblasted and ready to rock...?

1BOSS83

Not being purposely redundant; These are the thinnest head gaskets available in production at this point, correct?

68442

I have sets of NOS new originals
Just mic'd the original steel gaskets and they come in at .020 compressed

olds 307 and 403

The Speedpro's usually sit .025" in the hole, multiple people have come up with that measurement. Measure where they sit anyways. I get 10.8 to 1 with the KB pistons calculator. I get 10.2 to 1 with Felpro head gaskets. If this is a drag strip car, race gas is around and the cost isn't a problem, use the shim head gaskets.

68442

Purpose of this car is a restoration. I do concourse correct resto's. This car is not a race car. Not looking to reinvent the wheel either. Looking to have a good reliable motor that performs well on todays gas.

Anyone know how reliable the factory 10.5 to 1 advertised numbers are?
Lets assume it's accurate. With the SP pistons sitting lower i'm actually dropping comp. to a degree just by that alone. So if i go with the steel shims maybe i get down to 10-1 ratio. Or with the felpros i may be 9-1 ???? guessing of course.

Then the question gets to how low can the compression be before the w-31 cam is not any good for you???

Maybe i'm answering my own question. The heads CC out to 64, nothing on the bottom end is changing from stock. I.m thinking the SP pistons alone are dropping me down enough if i use the steel shims to get CR down to 10.

Fun71

Yep, that is almost always the case with aftermarket cast pistons, but the Speed Pro forged pistons spec shows a compression height of 1.612" which is the correct height. If only all the other piston makers would adhere to the spec.

Octania

Since you have all gaskets known to man available....

I would measure *everything* then select the cam and gasket accordingly.

It sounds like you have cc'd the heads [ci'd?] already and have a number for that.

If the heads are cut much to resurface, that might lead to a thicker gasket just to get the intake alignment right and have the bolts fit in properly.

coppercutlass

The w31 cam would work with 10 to 1 compression. Im running a much bigger cam than the w31 with 10 to 1 compression and its a very stout runner. 10 to 1 would be the most i would run if you have 93 octane readily available.

Ok so punching in your numbers with the heads being 64cc's , the pistons .025 in the cylinder with flat tops and the bore being 4.057 you have 10.12 to 1 compression. Thats very close. You can probably go with a thicker gasket. Thats by the numbers calculated assuming of course . obviously if your block was decked , and it had an overbore they will change. But thinner gaskets wont help you run pump gas imo from the info you have given.

Octania

To muddy the waters, not knowing the OP's level of expertise...

I have seen folks refer to pistons with a [small] dish as "flat tops"

To me, a "flat top" has neither dish nor crown. Not any cc of dish whatsoever. Not even the 4-5cc of a 66-67 Toro 425 nearly dishless piston.

The 66-67 E400 and the W31 I believe were by this definition "flat top" pistons.

Ryan 1969 Chevelle SS396

Good stuff here:-)

Ryan

Flat top W-31 Piston :-)

coppercutlass

Looks like a huge chunk of the skirt is gone in that pic . I ran a set of original w31 pistons in one of my engines . Essentially turned them into cool ashtray after i broke skirts in 4 pistons after 2 seasons of hard abuse on the street and strip.

oldsmobiledave

Stock 1970 #6 heads should be 68-69cc. You state 64cc measured. So were your head milled at some point? Are You 100% certain that they are 64cc?

68442

Yes they cc out to 64, So looking closer compared to another set of untouched heads they look to be milled.
I will CC the others to compare

So i won't be using these heads.

The block has not been decked or anything machined still OEM inc. pistons, stock bore etc.

BTW these heads did not come from this motor originally nor was it ever run with them. Car was recieved as a basket case with parts galore. So whats been done to these heads is not indicative to the rest of the motor.

If i plug into the calculator what should be there(68-69 cc head, .025 down etc.) i come back with 10.3 with shims and 9.8 with felpro's.

Is 10.3 too much for todays gas ?

Octania

Hmmmm.... it is quite possible that the factory cut their #6 castings more than usual if they were destined for W31 engines. One of those little bumps that make a W31. Like the larger valves.

If your #6 heads have larger valves [2.000 +/-0.005" intakes] then maybe they ARE the engine's original heads. I don't know of any good way to tell otherwise.

For a correct concours restoration you will of course want #6 castings, shaved to factory specs, and sporting factory type valves.

Does the block have bore size stamps next to each bore- like "D" ? Most regular engines have "A" or "B" size bores.

1BOSS83

I am of the belief that it is too much for what the future holds for pump gas in the event this is a driver.

Not to beat a dead horse but my SP's came in at .025.

olds 307 and 403

I would say error on the safe side and run the Felpro head gaskets. Put enough paint on it to cover the gaskets.

68442

CC'd the other set of heads and they come in at 67-68(Little carbon buildup)

So, ya these are milled at 64

Run to Rund

Around 10 years ago, before ROL bought out Corteco, I bought a couple lifetimes worth of supply of the .028" head gaskets. Could part with some of them now. I like the composite type for good sealing, and the relative thinness of the #55661.

68442

Anybody have any experience with the Cometic head gaskets?

Looks like i could use their .051 gasket to get me at 10-1 with these milled 64 CC heads.
Any thoughts?

olds 307 and 403

Cometic gaskets are high quality MLS racing gaskets, perfect for a W31.

68442

O.K update

Got everything squared away back to stock. With the 68 CC heads(Had another set of W-31 heads)

Now with the steel shim gaskets(.020) i come in at 10.2
With the felpro gaskets(.040) i come in at 9.7

Using the factory W-31 Cam

Any thoughts and opinions on either gasket choice

THANKS

jensenracing77

Without having a fresh deck and head surface the steel shim gaskets make me nervous. I know many people get away with it but if it were me I would live with the 9.7 compression. I am not a professional engine builder so maybe I am wrong about this.

Run to Rund

How about the Corteco 55661 which is .028" thick? Combines medium thickness with good sealing when using iron heads.

distributorguy

Use the Fel-Pro and make sure the distributor is 100% (curve must be checked/modified) so you don't have detonation issues on pump gas. 10.2 is pushing it for premium if the distributor is not up to par. Plus the Fel-pros will seal for half the cost of Cometic.