How Much to Mill Heads? 73 Heads on 71 Block?

Does anyone have any idea how much milling is safe to do on the heads? The block is getting a .010 to clean it up. That will raise compression to 8.35 actual.

What is the head cc change by taking say .010 off the heads? I believe they are 58cc stock?

Also, can I use a set of 73 heads on a 71 block? When/where do the coolant sealing issues arise?

 

Last one I did ('70 heads) I took enough off the heads to get 10.5 w/a steel head gasket & Sealed Power Hyperutectic pistons - Was an orig. '70 SP block so didn't deck it at all. I can't remember for sure but may have been .050" off the heads. Easy enough to roughly calculate the CC difference/.001".

 

i think you will have to use the 73 sandwich gaskets for earlier heads. memory is questionable, almost 50 yrs ago. lol

 

Not sure I follow your formula. Can you give me a hypothetical if I shave the heads .010 how many CCs that takes out of the heads?

I somewhat feel the first few cuts will be equivalent to reducing the head gasket thickness since the chamber is largest there. So, if decreasing the head gasket from a .040 to a .030 raises the compression .16, then shaving the head .010 would likely raise the compression about the same amount. I tried it in my spreadsheet and it looks like .010 off the heads would yield a 2cc reduction to get a similar compression increase as thinner head gaskets.

I have a set of what I think are 73 heads for my original 71 block. They are in different buildings and I haven't had them side by side yet.

Another question, how much can I take off the deck/heads/thinner gasket before I need to start milling the intake or intake side of the heads? So far, if I take .010 off the heads, .010 off the block and use .020 steel shim head gaskets, that's a total of .040.

 

Same calculation as head gasket volume.

π*r²*h = volume of a cyl. & you'd have a cyl. that's .010" high. I don't recall the diameter of the head chamber (3.8"?) & even if not perfectly round or a straight wall will be extremely close.

CI/.061 = cc

So if the the chamber is 3.8", .010" off would be 1.86 cc.

 

I don't remember milling my intake but also don't remember how much I took off the heads not can I say the intake shouldn't have been milled - In retrospect it probably should've but that engine sure ran good!

 

Pretty close to what I came up with.

 

My current heads I had cut .030

 

It add$ up decking blk, cutting heads & intake, etc., .020-.030 or more.
Autotec or Diamond pistons with your spec'd comp height will eliminate any fiddling around with intake fitment, plus you'll have pistons with new forging materials to run tighter bore clearances & thin rings.
You can bump comp up or down abt 1/2 pt. using steel or fel-pro gskts.

 

I'm trying to avoid new pistons. I could if it needed boring but if it cleans up with a hone, which the machine shop said it would, I'm going back with the originals. It specs out to about an actual 8.5 with .040 gaskets and 8.8 with .020 steel gaskets.

It is just a street motor. I keep trying not to go down that rabbit hole even though I already have a bigger cam, headers and aluminum intake, I want to keep it cheap gas friendly. I figure anything less than 9:1 should run 87 octane if I have to.

My Stage 1 455 with aluminum heads is right at 9.5 and it runs on 87 pretty well.

When all is said and done, I want it reliable and a little hotter than stock but it won't see a drag strip often if ever.

 

Have you checked into the stock V6 hypers with valve reliefs, they used to be abt half of forged.
I ran a set from Poston years ago.

 

It it comes back measuring standard, I'm going to use the originals. Cheap and they worked before.

 

I think part of what Mart was getting at is that if you tally up the costs properly, that new set of custom pistons really isn't as expensive as you think. Particularly if you are adding power or trying to get squish and compression for better economy. Plus when you figure it over the life of the engine it becomes a fairly minor expense. It only seems big because you have to pay it all at once but it can be one of the biggest bang-for-the-buck items on your list. And don't forget to subtract the cost of that set of cheapo pistons you were considering.

Jim

 

My stock pistons cost me nothing so I save even more. If it is down 10-15HP because of 1-1.5 points of compression, I'm OK with that. It will still be better than it was and I mostly wanted to get rid of the leaks at every place it could leak.

 

Is the 8.8 before or after milling/decking?

 

That's all in with milled block, milled heads and .020 steel shim head gaskets.

 

What Jim said....
Compression without a doubt, wakes up any aftermarket cam, big time... I'd say more like 50-60 h.p.
Stock '71 8.5 comp rated at 260, Stock '70 10.0 comp rated at 315.

 

ALSO need to take into account the costs of decking the block & milling the heads. Usually the cuts are made in .005" increments with the costs rising for every cut for the average man. So you need to think about these costs as well so the better pistons bring your Quench/squish into the proper area for more detonation resistance as well as MORE POWER & getting the compression to a MORE DESIREABLE area.
People forget some of these things when planning.
So bottom line is the cost is usually not that much more & you will have a much better combo.

Just my thoughts on the subject.

Tom T.

 

I put your numbers into the Wallace Racing Calculator. .011 off the heads would be 8.55:1

 

If you're tearing it down, I have this minimum of 10.0+ in my head....