C1 1958 3755550 heads have varying combustion chamber CCs?

[Ian G.]

Hi,

I recently bought the correct 3755550 heads for my 59 290 HP fuelie. I decided to figure out the Compression ratio on them and came up with only about 7.7, which ends up being about 265 HP. I measured the CCs of the heads at 69cc. Would GM have machined different sizes of combustion chambers on these heads for the various horse power engines on corvettes in 1959? I'm afraid I got the right heads, but machined for low horse power, which being a newbie I didn't account for Is it possible (or even advisable) to have a machine shop fill in the chambers and machine new ones?

I have another set of heads, which were wrong, but even those are 64cc, which also seems too big.

Also, the pistons sit very low, about .05 under the deck, but I measured with the bottom of a digital caliper, not having a micrometer yet. Seems like I'm going to have way too much quench too, even with a .015 shim...

I got my numbers like so:

piston bore: 3.915
stroke length: 3
gasket bore: 4.1 (Felpro metal shim gasket)
gasket thickness: .015
combustion chamber: 69
dome volume: -5
deck clearance: .05

So now I'm wondering does it make more sense to have the engine rebuilt with domed pistons or to try to get the right RIGHT heads (even though I paid to have these rebuilt already), or just take my beating and put the heads on and shrug?

[Joe L.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Hi,

I recently bought the correct 3755550 heads for my 59 290 HP fuelie. I decided to figure out the Compression ratio on them and came up with only about 7.7, which ends up being about 265 HP. I measured the CCs of the heads at 69cc. Would GM have machined different sizes of combustion chambers on these heads for the various horse power engines on corvettes in 1959? I'm afraid I got the right heads, but machined for low horse power, which being a newbie I didn't account for Is it possible (or even advisable) to have a machine shop fill in the chambers and machine new ones?

I have another set of heads, which were wrong, but even those are 64cc, which also seems too big.

Also, the pistons sit very low, about .05 under the deck, but I measured with the bottom of a digital caliper, not having a micrometer yet. Seems like I'm going to have way too much quench too, even with a .015 shim...

I got my numbers like so:

piston bore: 3.915
stroke length: 3
gasket bore: 4.1 (Felpro metal shim gasket)
gasket thickness: .015
combustion chamber: 69
dome volume: -5
deck clearance: .05

So now I'm wondering does it make more sense to have the engine rebuilt with domed pistons or to try to get the right RIGHT heads (even though I paid to have these rebuilt already), or just take my beating and put the heads on and shrug?

Ian-----


Combustion chambers in GM cylinder heads are a feature of the CASTING; they are not MACHINED (i.e. like some modern cylinder heads that have their combustion chambers CNC machined). As a result, the combustion chambers may vary slightly from design spec. However, the 3755550 heads had a design spec of 60 cc, nominal, combustion chamber size. They would not vary by as much as the +4 to +9 cc's that you've found. So, I'd say you're looking at one of three things:

1) Your measurement is inaccurate;

2) The heads have had multiple valve jobs and the valves are now much deeper into their seats than they were originally;

3) The combustion chambers have been modified by grinding. This should be readily apparent as the natural, as-cast surface texture of the combustion chambers will be rendered much smoother.

Once you KNOW that the combustion chamber volume measurement is accurate, you can have the heads surfaced to reduce combustion chamber volume and, thus, restore the original combustion chamber volume.

[Ian G.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Thanks Joe. Maybe I measured them incorrectly. I could swear it was 70 but I'll try again. They do not look smooth like they were ground or machined extra. I'll look at the valves more closely as well.

[Ian G.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Thanks for the sanity check I remeasured and it is indeed 60cc. So my compression is 8.6. Mostly due to the deck height being so high at .05. With my shim gasket of .015 that makes the quench .065. If that quench is accurate, will it cause issues?

When my friend comes over we'll remeasure that too. Its probably inaccurate.

[John N.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Ian
Have you made a decision not to use the 290HP domed pistons?
Regards

[Joe R.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Ian,

Have a couple of questions for you.

1. Does your Corvette run well?

2. Do you NEED to have more horsepower?

3. Does your Corvette sound poorly!

4. Does your Corvette LOOK any different?

5. Does it have any trouble getting into the trailer?

Just a few thoughts to ponder!

JR

[Ian G.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Good point JR But I haven't actually run it, since it was partially disassembled when I bought it and I just continued the process.

John, The previous owner rebuilt the engine about 5 years ago, so I don't want to have to rebuild it again... It had the wrong heads, so I got the right ones, and just trying to figure out what I'm doing

[Duke W.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

Thanks for the sanity check I remeasured and it is indeed 60cc. So my compression is 8.6. Mostly due to the deck height being so high at .05. With my shim gasket of .015 that makes the quench .065. If that quench is accurate, will it cause issues?

When my friend comes over we'll remeasure that too. Its probably inaccurate.

Don't worry about the "quench". It's a way overrated parameter until you get to very high CRs. Most OE engines were assembled with quench clearance in the 50-60 thou range. As is your engine will probably run on regular unleaded.

You didn't give the make/model of pistons, but they are obviously designed for low compression with less compression height than proper high compression pistons.

Get some proper compression height pistons - maybe even with domes - like were used on '61 315 HP. Evaluate various piston/head gasket options and target the CR at 10.25, assuming you're running a Duntov cam.

Duke

[Ian G.]

Re: C1 1958 3755550 heads have varying combustion chamber CCs?

But Duke, Joe Ray said I should be happy with what I have :P

I think I'll put it back together with the new heads and see how it goes and then if I'm not happy I'm definitely going to redo the block with domed pistons.

~ian