Compression Ratio Calculations and Head Gaskets

[Mark H.]

I've been working on a '64 L75 (300HP) engine and after a long hiatus started assembly. The crank and pistons are in the block, and tonight I started measuring deck clearance to help inform head gasket choice.

I measured deck clearance using a deck bridge and dial indicator. I took measurements at 12 o'clock, center, and 6 o'clock of each piston and then averaged. Results (also included head chamber volume in ccs):

Bore	12 o'clock	center	6 o'clock	average	head volume
1	18	19	22	20	64.8
3	14	16	20	17	63.8
5	15	17	21	18	63.8
7	17	19	23	20	64.6

My pistons are Keith Black KB 156 (dish = 7 ccs) and are 40 over. These values produced compression ratios as follows assuming a Fel-Pro steel shim head gasket

Bore	CR
1	9.6
3	9.75
5	9.73
7	9.6

Assuming a Fel-Pro perma torque gasket, I get this:

Bore	CR
1	9.08
3	9.23
5	9.21
7	9.10

Will measure the other side of the engine after dinner, but wanted to record results. Duke Williams has been patient with me for a decade -- this engine is getting a McCagh Special camshaft and I'm eager to get it installed and see what it will do.

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

Here are the measurements from the other side:

Bore	12 o'clock	center	6 o'clock	average	head volume
2	11	14	19	15	65
4	11	15	19	15	64
6	12	19	22	18	64
8	17	22	27	22	64.6

These values produced compression ratios as follows assuming a Fel-Pro steel shim head gasket

Bore	CR
2	9.68
4	9.8
6	9.73
8	9.61

Assuming a Fel-Pro perma torque gasket, I get this:

Bore	CR
2	9.16
4	9.26
6	9.19
8	9.09

I like the numbers from the steel shim gasket better, but that type of gasket really makes me nervous. Would welcome any thoughts/suggestions. Thanks.

[Duke W.]

Re: Compression Ratio Calculations and Head Gaskets

Glad to hear you're back at it, Mark. Are your kids out of college, yet?

Get a machinists bar and a feeler set that has at least a .0015" gage to measure the block deck and head mating surface for flatness. Placing the bar down the side of the surfaces and diagonally, if the thin feeler takes about the same amount of force to withdraw along its length, the surfaces are flat and a shim gasket should seal without issue. I think the Felpro 1094 is .015" compressed thickness and it has a polymer coating, so it can be installed "dry" without any kind of sealer.

Assuming OE 461 heads, I'm surprised the chambers are that large. From the plant they were likely in the 60-61 cc range, so I assume the valves are "sunk" due to multiply reseat operations.

Duke

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

Ha! Yes, they are.

The machine shop measured the chamber volume. I will try to spot check a couple and post some pictures of the chambers later.

i plan to check the mating surfaces using this method: https://www.youtube.com/watch?v=235BvFJw3Hc

is that what you had in mind?

Mark

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

I ran the machinist's bar across the block. The smallest feeler gauge I have is 2 thousandths. There were a few places where that would pass underneath the bar, but 3 thousandths would not:

IMG_2987.jpg

IMG_2989.jpg

IMG_2990.jpg

Should be good?

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

Edited to correct - A few spots where a 2 thousandths gauge will pass but 3 will not. One spot near #8 chamber where 3 will pass but 4 will not.

Pics of the odd-side bank:

IMG_2991.jpg

IMG_2992.jpg

Not sure what to make of these results.

[James G.]

Re: Compression Ratio Calculations and Head Gaskets

I noticed that around some of the bolt holes there appears to be a more polished area "a ring" around the hole, is this at all above the deck surface... AT ALL?
IF you take a short straight edge center the bolt hole under the edge, will it rock back and forth? OR if you drag toward the hole do you feel resistance as you get to the "polished area?
MEANING If the holes were chamfered slightly, would it help?

[Duke W.]

Re: Compression Ratio Calculations and Head Gaskets

I don't have enough bandwidth to watch videos. It's been a while since I tested for flatness, but I used a round machinist's bar and the .0015" feeler gage in my ancient Craftsman set.

I laid the bar across the cylinders, sides and center, and then diagonally across, so five total positions. At each position I would raise the bar, insert the feeler at three to four points along the bar and then withdraw it noting the force required. In all cases it took about the same force and my conclusion was that the surfaces were flat.

If you can slide in a .002" feeler you might have some warp.

Neither your heads or block deck appears to have been machined in the field. In my experience OE machined surfaces are flat as long as the engine has never been severely overheated.

I suggest you discuss your method and results with your machinist and get his opinion.

I notice that the bottom face of your exhaust valves have a machined depression. I don't think the OE valves did and this might account for the higher than expected chamber volumes.

Duke

[Jeffrey S.]

Re: Compression Ratio Calculations and Head Gaskets

I've been working on a '64 L75 (300HP) engine and after a long hiatus started assembly. The crank and pistons are in the block, and tonight I started measuring deck clearance to help inform head gasket choice.

I measured deck clearance using a deck bridge and dial indicator. I took measurements at 12 o'clock, center, and 6 o'clock of each piston and then averaged. Results (also included head chamber volume in ccs):
Mark,
You made a very smart decision to work with Duke and to use the McCagh cam. I did the very same thing several years ago on my '69 base engine car and couldn't be more pleased with he results. Duke has been "pestering" me to have the engine dyno'd but I haven't yet. But the seat of the pants feel is enough for me with smooth as silk idle and good low end power. Good lick with your build.
Jeff

Bore	12 o'clock	center	6 o'clock	average	head volume
1	18	19	22	20	64.8
3	14	16	20	17	63.8
5	15	17	21	18	63.8
7	17	19	23	20	64.6

My pistons are Keith Black KB 156 (dish = 7 ccs) and are 40 over. These values produced compression ratios as follows assuming a Fel-Pro steel shim head gasket

Bore	CR
1	9.6
3	9.75
5	9.73
7	9.6

Assuming a Fel-Pro perma torque gasket, I get this:

Bore	CR
1	9.08
3	9.23
5	9.21
7	9.10

Will measure the other side of the engine after dinner, but wanted to record results. Duke Williams has been patient with me for a decade -- this engine is getting a McCagh Special camshaft and I'm eager to get it installed and see what it will do.

Mark,
You made a smart decision to work with Duke and to use the McCagh cam. I did the same thing several years ago and couldn't be more pleased with the results. Duke has been "pestering" me to have the engine dyno'd but I haven't yet. The seat of the pants feel is more important to me than the numbers. Smooth as silk idle and great low end power.
Jeff

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

If you can slide in a .002" feeler you might have some warp.

. . .

I suggest you discuss your method and results with your machinist and get his opinion.

Will do. I do not want to have to take this thing apart again once it is installed. If I have to disassemble to take the block and heads back to the shop, I may just bite the bullet and buy some KB 157s to solve the compression ratio issue and give myself some cushion with a thicker head gasket.

Do I recall 10.3:1 was your target CR?

Mark

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

I noticed that around some of the bolt holes there appears to be a more polished area "a ring" around the hole, is this at all above the deck surface... AT ALL?
IF you take a short straight edge center the bolt hole under the edge, will it rock back and forth? OR if you drag toward the hole do you feel resistance as you get to the "polished area?
MEANING If the holes were chamfered slightly, would it help?

Good suggestion. I will check all surfaces with a razor blade this evening.

[Duke W.]

Re: Compression Ratio Calculations and Head Gaskets

Will do. I do not want to have to take this thing apart again once it is installed. If I have to disassemble to take the block and heads back to the shop, I may just bite the bullet and buy some KB 157s to solve the compression ratio issue and give myself some cushion with a thicker head gasket.

Do I recall 10.3:1 was your target CR?


Mark

10.3 would be the "not to exceed" maximum for the McCagh Special camshaft with 93 PON fuel, and reduce 0.1 for every point lower octane. For example, 10.1 if you lived in CA with its 91 PON premium.

Duke

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

Is there a maximum variability in CR across cylinders?

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

I noticed that around some of the bolt holes there appears to be a more polished area "a ring" around the hole, is this at all above the deck surface... AT ALL?
IF you take a short straight edge center the bolt hole under the edge, will it rock back and forth? OR if you drag toward the hole do you feel resistance as you get to the "polished area?
MEANING If the holes were chamfered slightly, would it help?

I ran over all the surfaces with a razor. Only detected one or two burrs, otherwise smooth.

[Mark H.]

Re: Compression Ratio Calculations and Head Gaskets

Well, after much consideration I think the following is the new plan:

Take the block and heads back to the machine shop and have the decks straightened out (hopefully without harming the stamp pad - it is not an original motor but it is period correct).

Ditch the KB 156 pistons for KB 157.

The heads had already been ported, polished and port matched. Get the head volume after whatever machine work is done.

Get the rotating mass balanced. Again.

If my calculations are correct, doing the above should put me in the 10.0 to 10.2 range using a Fel-Pro PermaTorque head gasket.