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The machine shop told me they decked my 1968 327 block to 9.005" (both banks). I am wondering what thickness head gaskets I need to use.
 

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The machine shop told me they decked my 1968 327 block to 9.005" (both banks). I am wondering what thickness head gaskets I need to use.
Considering that your piston is only in the hole, .005, I consider this a time to take advantage of bringing your piston to head ''quench" in line to keep detonation down. Depending on who you talk to , .039 felpro head gaskets will do just that and at the same time , increasing compression within favorable limits. I decked my block to 0 and used felpro 1010 head gasket, putting the desirable between .035-.045. Just a matter of preference.
You could use the steel shims at about .017 but I think that's a little tight unless your are racing and have good fuel. .
 

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We don't know how far the piston is in the hole unless the OP has measured with the rotating assembly installed.

Assuming it was .010 at 9.010 is just that an assumption.

Have you changed the pistons? Has the block been align bored? Rods resized?
 

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Discussion Starter #5
For stock engines, I've heard a gasket thickness of .040" is used. I also read that stock height is 9.025". Assuming this is actually true and mine is 9.005", would I need to use a .060" head gasket? Or would the recommendation be to stick with stock .040" and happily accept the small increase in CR?

I guess I am looking for confirmation from experts that a 0.040" gasket thickness is typical for stock, and that 9.025" is the stock "0" height for decks and if a change in gasket thickness is necessary when you deviate from this.

Thanks
Mark
 

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My 327 has never been decked and I am using TRW forged flat top pistons 2165, pistons were .020 down in the hole---so I used Victor .026 head gaskets to better the quench.
 

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For stock engines, I've heard a gasket thickness of .040" is used. I also read that stock height is 9.025". Assuming this is actually true and mine is 9.005", would I need to use a .060" head gasket? Or would the recommendation be to stick with stock .040" and happily accept the small increase in CR?

I guess I am looking for confirmation from experts that a 0.040" gasket thickness is typical for stock, and that 9.025" is the stock "0" height for decks and if a change in gasket thickness is necessary when you deviate from this.

Thanks
Mark
TYPICALLY, stock GM pistons are in the hole .020-.035 on small blocks. Not sure if it was explained to you, but when a block deck is cut , its normally cut parallel to the crank centerlines, from there you determine your rod lengths, then piston compression heights.
If you are looking to increase compression and what most race builders mean by " blue printing the block" this is what is considered.

If you add a thicker head gasket , like a .060, you are basically lowering your static compression. If you are doing all of this on your own math , you will be fine running a thinner gasket like a .039 or the GM type steel shim of .017-.019, but remember, you do your homework, trust your math . If you are already at .005 in the hole and you go for the thinner gasket say .025, I feel thats cutting it too close for a street car. Best to err on the high side of your piston to head clearances.
Another factor is to pay attention to what your cam lifts are as they play a part in this game and I don't think you are in it , yet.

Here is one compression calculator, but there are many out on the internet. https://www.yourmechanic.com/article/how-to-calculate-compression-ratio
If you use the .060 gasket and your known in the hole is .005, you can safely say you are running a .065 piston to head clearance.
This does not take in account of piston rocking in the bore so as I mentioned before, do your math , if you want to increase compression in a simple way.

Its simple if you can understand it. Here is another calculator tool. http://www.diamondracing.net/tools/

I might add that to use some of these calculators, you need to know about the piston and its specs as wells as rings, bore, etc. Enjoy.
 

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TYPICALLY, stock GM pistons are in the hole .020-.035 on small blocks. Not sure if it was explained to you, but when a block deck is cut , its normally cut parallel to the crank centerlines, from there you determine your rod lengths, then piston compression heights.
If you are looking to increase compression and what most race builders mean by " blue printing the block" this is what is considered.

If you add a thicker head gasket , like a .060, you are basically lowering your static compression. If you are doing all of this on your own math , you will be fine running a thinner gasket like a .039 or the GM type steel shim of .017-.019, but remember, you do your homework, trust your math . If you are already at .005 in the hole and you go for the thinner gasket say .025, I feel thats cutting it too close for a street car. Best to err on the high side of your piston to head clearances.
Another factor is to pay attention to what your cam lifts are as they play a part in this game and I don't think you are in it , yet.

Here is one compression calculator, but there are many out on the internet. https://www.yourmechanic.com/article/how-to-calculate-compression-ratio
If you use the .060 gasket and your known in the hole is .005, you can safely say you are running a .065 piston to head clearance.
This does not take in account of piston rocking in the bore so as I mentioned before, do your math , if you want to increase compression in a simple way.

Its simple if you can understand it. Here is another calculator tool. Compression Ratio Calculator | Tools | Diamond Racing Pistons

I might add that to use some of these calculators, you need to know about the piston and its specs as wells as rings, bore, etc. Enjoy.
Good info.

Until you measure you are only guessing where the piston is in the hole.

The difference between a .060 and .040 gasket only makes about .25 diff in CR on a 327. You should also consider ideal quench to minimize detonation

To determine the correct head gasket you need to know piston dome and head volume cc. You should also consider ideal quench to minimize detonation.

That said if you just want to wing it use a .040 and check PTV clearance and you'll be fine.

btw when I build a performance engine I zero deck the block.
 

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Discussion Starter #9
Good info.

Until you measure you are only guessing where the piston is in the hole.

The difference between a .060 and .040 gasket only makes about .25 diff in CR on a 327. You should also consider ideal quench to minimize detonation

To determine the correct head gasket you need to know piston dome and head volume cc. You should also consider ideal quench to minimize detonation.

That said if you just want to wing it use a .040 and check PTV clearance and you'll be fine.

btw when I build a performance engine I zero deck the block.
Thanks for all the advice. By "Zero Deck the Block", what do you mean exactly? Does that mean you machine the deck so that the piston at TDC is flush with the deck? I am planning to use Powerforged pistons that are rated at 10.4:1 with a 64cc head (which I am planning on using). Here are the specs:

327ci Stock Type Piston, .040'' Overbore
•Piston Dia: 4.040''
•Piston Top: .125'' Dome with 2-Valve Reliefs
•Pressed Pin
•Comp Ratio: 10.40:1 w/64cc Head
•Ring Grooves: 5/64'', 5/64'', 3/16''
 

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Randy, when you asked the question on what gasket thickness to use, you also mentioned that your machine shop gave your the .005 piston in the hole or deck. You answered your question as it means just that. Your machine shop cleaned up the block deck for some reason , it may have been your instructions, from here , I can't tell other that what you're telling us.

If the information your shop is correct and you trust him, then ask him what he feels your gasket thickness should be .
Based on what information you are providing, I calculated from the Diamond engines calculator and it reads below.

Calculate
Total Volume
>

>

756.5
Cylinder Volume
>
Total cylinder volume in CC's =
>

682.4
Clearance Volume
>
All effective vol. above block deck @ TDC in CC's =
>

74.1
Gasket Volume
>
Volume of your head gasket in CC's =
>

8.43
Top Ring Volume
>
Total volume above the top ring @ TDC in CC's =
>

0.49
Deck Volume
>
Volume of area above the pistons @ TDC in CC's =
>

1.05
Piston Top Land
>
Piston top land diameter for volume calculations =
>

4.001
1/2 Stroke
>
1/2 stroke of your engine's crankshaft in inches =
>

1.625
Compression Ht.
>
Ctr. of piston pin to top of piston (FLAT) in inches =
>

1.675
Cubic Inches
>
Cubic inch displacement of your engine =
>

333.13
For the given dimensions, your compression ratio =
>

10.21

I don't have all your pistons data on hand but I did calculate your compression at 10.21:1 using the felpro 1010 .039 thick gasket.
At this point , I would not recommend going to a thinner gasket. As you can see from the necessary input of information on the Diamond calculator, they do factor in final bore and finished bore.
I would not use the .060 gasket mainly as you will loose some compression. I believe John calculated that loss may be a 1/4 of a point or .021:1.

My suggestion is to discuss this with your shop. Its important that you know all of the variables when you get into something like this or trust someone to assist you.
I would be glad to assist in any way I can and I am sure many of us here on this site will do the same.

In closing, the term piston deck is referring to the piston flats, just below the valve reliefs in relation to your block, when looking at final assembly. I feel you will be just fine using the felpro gasket in putting this motor back together. We all strive for utmost compression but outside enviroments such as cly head combustion chamber volumes, valve sizes, camshaft profiles, head material to name a few and even un-removed carbon in the head chamber as well as fuel , have impacts on detonation.
Any other questions , let us know.
 

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I think if his deck height is 9.005---a .040 head gasket would be fine----I remember on my 327 with the pistons .020 down in the hole, I was using a .041 head gasket. quench was .061, after I found out how far the pistons were down, I changed to the .026 gasket, now I have .046 quench, alot better than it was. My pistons also have a 1.671 compression height. I came out to 9.6 cr with 64cc vortec heads
 

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Your machinist didn't do you any favors by leaving the piston deck at -0.005". I assume he didn't zero deck the block (he has done all of the physical labor of set-up and cutting already) because he didn't have your rotating assembly in hand to determine your actual measured compression height.

With a zero deck your Quench (aka Squish) is determined by your compressed head gasket thickness (listed in the sale catalog). With a SBC that has a pent roof combustion chamber (before the Vortec and before the fast burn combustion chamber) you want 0.040" of an inch of space to force the gas and air mixture out from beneath the head and to mix up the air fuel in the combustion chamber to optimize combustion. If you visualize this happening you can see where the term "squish" comes from. This is of increasing concern as your combustion chamber increases in volume.

The heart shaped (I think a kidney bean is a better description) combustion chamber of the "Fast Burn" style late model Vortec heads is smaller (in the case of the 305 only 58 cc in size) to create even more turbulence for better mixing of the air and fuel. I feel the combustion chamber's shape is where most of the 40 horsepower gain most experience when using these heads comes from though the straighter shot at the valve with raised ports does contribute to cylinder filling.

With the Otto cycle just about 95% of all of the power of the power stroke occurs within 15 degrees of top dead center; as there is very little added cylinder pressure contributed by further combustion (heat) as the piston is rapidly pushed down the bore (increasing the volume which lowers the pressure). So tiny details such as Quench, dome volume (or lack of it) all affect the propagation of the flame front across the combustion chamber.

Modern technology in terms of increased power to the plug with HEI or aftermarket ignition boxes with CD technology; as well as improvements in head and cam technology (thanks to computer simulation of what is occurring in the engine) is why modern cars today are making more power than their muscle car era counterparts. I do not feel that this can be attributed to EFI which still can not beat a carb in peak power or in handling a rough idling cam (though others may argue this). So you need to embrace as much of modern technology as you can to optimize power with out sacrificing your ability to enjoy your ride.

Big Dave
 

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I owe someone an apology, I started the last post with Randy when I thought I was addressing Mark, this is almost like keeping track of my grandkids , sometimes. So, My apologies to both.
Dave brought up a point here, In order to determine actually necessary deck, the block has to be mocked up and measured, Since Mark did not say that, the shop may have just guessed as more times over, small blocks are normally at .20-.030 deck with piston being in hole.

I recall long ago when I will wrenching Hemi motors, I saw the machinest bring out a huge outside caliper, resembling the old ice block hooks. They had it marked on the caliper just what Hemi blocks were suppose to be , from the mains to the decks. Old days of SS/AA mopars.

Guess its siesta time.
 

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its fine Don, I knew who you were answering Mark
 

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Echoing Dave stating that the block is 9.05 does not mean anything with regard to piston in the hole. It could be .20 down or .10 up.

And all the calculations posted above are meaningless without actually measuring with the rotating assembly installed.

Squish is the space between the flat of the head outside the combustion chamber and the flat of the piston.
 
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Echoing Dave stating that the block is 9.05 does not mean anything with regard to piston in the hole. It could be .20 down or .10 up.

And all the calculations posted above are meaningless without actually measuring with the rotating assembly installed.

Squish is the space between the flat of the head outside the combustion chamber and the flat of the piston.
If this deck proves to be correct, he can still build off it as if it was 0, select his proper quench and put it together providing block square with crank. Be fine.
 

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If it's .05 in the hole I would use a .035 head gasket
 

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I'm going through all of this right now with my 494ci. I think I'd be more worried about piston to valve than piston to head clearance if it's .05" piston to deck. It sounds like the OP isn't positive about that even. No mention of cam lift etc? OP, this isn't where you want to spit ball things. Measure correctly or cry about it later. If you don't know how to do it, start with Google and YouTube. You're going to need some special tools that add up if you don't have them.
 

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Discussion Starter #20
No problem Don, I knew you were referring to me on your response.

Thanks for everyone's input on this. The machine shop told me they decked the block to 9.005". To me, that means the distance from the crank centerline to the deck surface. And assuming the virgin deck height is 9.025", I was assuming that I needed to use a head gasket that was .02" thicker than the stock - compensating for the loss of deck height. But thanks to you all I am understanding there is much more to it.

The crank, rods and pistons will also dictate where the piston is relative to the deck so I am now realizing I need to put it all together to measure. The pistons I am using are rated for 10.4:1 CR based on 64cc heads. But when they calculate this CR for the piston, what assumptions are being made for the piston-to-deck measurement, head gasket thickness? In order for me to truly achieve 10.4 CR, wouldn't I need to know this and use the head gasket thickness as my only variable?

(Really great discussion here guys, this is awesome stuff!!! Thanks for your input.)
 
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