I found an online calculator but I'll have to dig up some numbers first.
Head gasket bore diameter...
Piston dome volume (negative for dished)....
Piston deck clearance......
These are the values I for sure don't know...

 

Just calculate the volumes of all the different spaces and add them up.

Piston dish
Deck height
Head gasket
Head combustion chamber
and, finally, Stroke

Divide the volume of everything, except for the stroke volume, by the volume of everything, including the stroke volume.

- Eric

edit: you found a calculator first, but you really don't need one.

 

You don't have a micrometer, do you?
That would be a very reasonable thing to pick up. That way you wouldn't have to trust other people's measurements.

You can get reasonably accurate digital vernier calipers, that don't even require lining up vernier lines, without spending too much.

- Eric

 

I have a caliper but I can't attest to it's accuracy. I'll see what I can pick up. So it's just measure everything, calculate the volumes, and apply the formula?
1. Piston dish - just the interior of the dish?
2. Deck height - the space between the top of the piston (at TDC) and the head mating surface?
3. Head gasket - the compressed height (which Advance auto emailed me and said it 0.038)
4. Head chamber - which I calculated to be about 66 CC
5. Stroke I assume I get from the manual?

This sounds like a job for my daughter. Put that math degree to work.

 

I wouldn't touch a digital caliper with a stolen 10' pole. Almost all made in China, and as accurate as you'd expect.

 

Professur I have 2 calipers both made in china. The upside is that I worked in a machine shop and they had an In house tech who would re calibrate all the tools. I like calipers for rough mesures where half thou. Wont matter as much. I got micrometers for that.

 

Macadoo stroke is 3.385.

 

I make no claims that they're accurate enough to build an engine with, but for the price, it's good to have something that lets you get a general idea.

My preference is for old high quality American stuff from flea markets and yard sales - there's always someone selling the old guy's stuff after he's passed away.



Yup. ∏R^2.



Yup. It's radiused at the bottom, so your number may be off by a hair.
Alternatively, you could CC it like the head.
Should be 24cc for a '71.



Yup. Use a depth gauge, exactly at TDC. Ideally measure all 4 corners (#1, #2, #7, and #8).



Exactly, using the radius listed for the gasket hole.



Right. The spec is for 64, but the common wisdom is that they run large, and in your case, even after the head was slightly decked, we can see that it's true.



Exactly. 3.385".



Ha!

Once you've done that, you'll know your static CR, and can make intelligent decisions about what else to do or not do.

- Eric

 

I needed a run out gauge for rebuilding a trans and had to grab a cheap chinese one with a magnetic stand (really useful with an aluminium case I tell ya)....I simply could not believe how far out it was. 10 thou out on 50 thou. Fortunately it was at least reliable ... it would measure the same repeatedly, so I would measure something .. then check that measure with feelers to know how much it really was.

 

Once again guys, thank you.
Eric, I thought about CCing the piston dish the same way I did the chamber but didn't like the thought of liquid in the cylinder, I may still try it though.
Prof, the feeler gauge is a great idea. Seems you're always solving something with feeler gauges.
Copper, thanks for the spec.

What would I want my CR to be? 8.5:1? A little higher? Keeping in mind Mark ground my cam for the stock CR.

 

There is a safe range in which your cam can be used a little less is usually ok but in most peoples opinion you'd rather have a litle more than less for power of course. I would be worried if you where at like 9.5 to 1 with a small cam made to work with stock compressiow. I'm sure you will be fine.

 

I would ask Mark about acceptable compression ratios for that cam for the particular octane you want to use.

At this point, with you considering varying head gasket height and possibly getting heads milled a bit more, you can pretty much make it whatever it needs to be.

- Eric

 

I'll send Mark an email and see what he thinks. Are the thinner head gaskets just as good as the Felpro blues are are they trickier to get a good seal?
Heading out to take some measurements. I'll report back....

 

Factory ran shim gaskets with no issues. Jm proformance "smitty" is said to sell them

 

Thanks Copper but I don't think a thinner gasket is going to be enough.
If my measurements and calculations are correct, my CR with the blue gaskets is at 7.68:1
Looks like I'll be doing more decking. Anyone care to ballpark my cost for this?
I emailed mark to see how much CR I should shoot for with the new cam. I'd like to use regular octane fuel (obviously for cost).

 

With that compression ratio, you'd be good with 54 octane.

 

Oh, ha ha :P

 

Interesting development; I just called the machine shop that worked on my heads and they were closed.....forever. That leaves us with one machine shop for an area with 200,000 people. Needless to say, they're backed up.

 

This is the problem with pulling the heads on these later engines.
Usually the actual compression ratio (allowing for manufacturing tolerances that usually mead the combustion chambers a little bit large) is right around 8, and anything that takes away from that leaves you in the 7s.

People doing full rebuilds have several options available, including using pistons with less of a dish, but you can only use a thinner head gasket, or mill the heads.

If I'm not mistaken, milling it 0.030" will remove about 7cc (perhaps a bit more if my guestimate of CC diameter is off).

- Eric

 

Figures. No telling what kind of work they did in their final minutes, then, but, on the good side, you're damned lucky you got your heads back at all.

- Eric

 

Yeah, I considered that also. I'll have this other shop inspect the work.

 

So it looks like in order to get back to the factory 8.5:1 I need a combustion chamber volume of 54 CC. Is that going to be doable? And is there a way to calculate how much needs to be milled or will the shop know that?

 

So you want to remove 12cc of volume.

You could get a sense of how much to remove by filling it up with 54cc and seeing where the line would be - is it in the roughly round area, or is it lower?

That sure sounds like you need to remove between 0.050 and 0.070", which is a lot.
If you use a thinner head gasket, the need to mill will be proportionately less.
You will be close to where you also need to mill the intake surface of the heads as well, but I don't think you'll be quite there yet, considering that 0.030" of that total is caused by the thicker head gasket, meaning that you're actually moving the head only about 0.030" down toward the crank.

Definitely get some input from other members here before doing anything final, as I have not had to do this before.

- Eric

 

Nope milling the head that much will do more harm than good. At this point going to flat tops might be better but I belive their is a set of small dish cast units which will not require balancing but will require being careful during any process such as pressing on. Or you can dis assemble the block deck it to zero and have bearly enough compression there is no way around those toilet dish pistons.

 

I would get. Set of smaller dished pistons pay 80 bucks to have em swapped over then pay 150 to deck the block to have .010 left on it and you should have some decent compression. I think member dmfranco was selling those cast units for 75.

 

Yes, I've been reading up on all that you've both mentioned. I want to start by asking; what was the CR before I started all this? Am I right that the original shim gaskets were 0.011 compressed? If that's true, the CR would have been 8.1:1. I could live with that. I'd rather build another motor than get in to replacing pistons and/or decking both surfaces of both heads on this motor.
can we still get the shim gaskets? I've been looking but no luck.

 

Smitty at mj proformance sells em .

 

That's what I found out too. Their website is down but they give a phone number. I'll call tomorrow. Anything special I need to know about shims? Spray with copper? Re-torque after a heat cycle?

 

I sprayed mine and I always check \ re torque my heads after 1 season only when it's new sfter that that's it.

 

I've read the cooper stuff is the best. Is that what you used?

 

I agree that if you can get the original gaskets, that's your best bet in your current situation, where you don't want to change or mess with too much.

You had a good running low-mile engine with a valve problem, so you took off the heads and had the valves taken care of.
As Copper pointed out, Next Stop: Full Rebuild, so if that's not your destination, better get off now.

- Eric

 

Yeah, it's easy to get sucked in. I just want it back to where it was. The original shims will get me back there and that's good enough. Like I said, once I'm back on the road I'll think about building something with muscle.
I just had no idea that replacement gaskets were so much thicker. Live (and build) and learn.
And I know I say this a lot, but thanks fellas for helping me steer in the right direction.

 

Interesting side note: I have seven "C" bore sizes and one "B". Is that common?

 

B and C means select fit components, common...and its good your getting a thiner gasket.

 

Thanks Lars

 

Smitty's shim gaskets are on their way. Nice guy. Not cheap but glad to have this resolved.
The "plan" is to have this back together and painted by Christmas. But not much going to happen today. It's 5 degrees here in central Il. and I can't get the shop above 50. I'd like it to be 60 if I'm going to use rtv.

 

I've used RTV at 50° with no problem, but there's nothing wrong with being cautious either.

- Eric

 

Yeah, I think given enough time to cure, temp doesn't matter too much. I just don't feel like freezing my toes! Lol.

 

By the way, if I may ask, what's Smitty getting for those things?

I've got one slightly leaky valve and will need to pull one head (planning to just swap in a spare, unless it's an easy fix), and, obviously, would prefer both gaskets to be the same thickness when I'm done.

- Eric

 

$105 for the pair, shipped. Certainly caught me off guard...but still worth it for my situation.