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Old 04-24-2012, 09:16 PM
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Head gasket thickness, Gasket or Shim?

I did a little searching on this subject, but didn't find quickly what I was looking for.

In regards to headgaskets or steel shims, I understand the importance of achieving a good quench or squish height of around .040. I am in the process of building a motor which will likely result in decking the block. I have the choice of barely touching it and using something like a .015 shim or cutting it down almost to a zero deck and using a .041 gasket, both results would produce the same compression ratio and meet the desired quench height after taking into account the piston to deck height.

So my question for you, is if all other factors remain the same, is it better to use a gasket or a shim?.

Along those same lines, what compressed thicknesses are readily available?
I'm getting ready to take my block to the shop for cylinder boring and would like to instruct them to deck the block at the same time. I just don't know yet what height to go for.

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Old 04-24-2012, 09:35 PM
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Your cylinder head material, and perhaps other factors will play a role in this decision.

Pat
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Old 04-24-2012, 09:39 PM
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Originally Posted by PatM
Your cylinder head material, and perhaps other factors will play a role in this decision.Pat
My plan is to purchase brand new cast iron vortec heads. Ranging from genuine GM to various aftermarket versions. Still researching the possibilities.
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Old 04-24-2012, 09:49 PM
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Steel shims are incredibly strong. They are good for forced induction applications. They can also be comfortably run on normally aspirated engines as well. However, they require a flat surface on the deck and heads. Not a problem if you get the block decked. However, it will add to the cost of your head work. BUT there is one other gotcha with steel shims. And that is it has to be a smooth surface finish. Most machine shops will not/are not capable of creating a smooth enough surface to accommodate a steel shim. When you get a block decked you will see circular machine marks on the deck (kinda like how a circular saw makes circular grooves on wood). I also suspect that if your engine ever overheats and it warps the deck/heads, you will get leaks (compression and/or coolant), but this is speculation on my part, perhaps someone else will correct me.

Go back and reread post #19 by Cobalt (Vortec 350 Build )

Composition head gaskets are not picky at all. And from what I have seen of your project you are not going forced induction. They will be a good choice.

Another option to consider are graphite head gaskets. Check out this thread I started: Graphite head gaskets?

If you go to SummitRacing.com and do a search you will see there are a lot of options for compressed thickness. I think the trick is to not put your actual bore size, a lot of head gaskets will be punched out to 4.125" to accommodate a lot of bore sizes. This will slightly reduce your compression, but not by much and still allow you to achieve your quench.

Also bear in mind that deck can reduce the strength of the block too and cause cracks. I just had my block decked and it destroyed the head bolt threads on cyl #7.

So again run some numbers, figure out the volume of your heads, pick out some pistons and take into account the compression height, and then work out your deck and head gasket numbers.
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Old 04-25-2012, 12:06 AM
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Here is a page on head gaskets, unfortunately not that much on shim types, but that will be fixed. Painting the shim HG w/aluminum paint is mentioned (gold can also be used). This is still done by some builders, as well as using the copper coat spray. The best advancement to the shim HG is the coating that is applied to some of them. The thin rubber coating will be somewhat more forgiving than a bare or painted shim HG.

The deck thickness of a production SBC block is none too great so using a thinner HG to keep the deck thickness isn't a bad idea.
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Old 04-25-2012, 12:30 PM
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I don't have a strong opinion either way, but a thinner gasket limits the dead zone between the head and block decks. If you think it's a good idea to run the top ring as high on the piston as possible, then you'll want to limit the gap between the decks.
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Old 04-25-2012, 03:15 PM
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Quote:
Originally Posted by rollie715
I did a little searching on this subject, but didn't find quickly what I was looking for.

In regards to head gaskets or steel shims, I understand the importance of achieving a good quench or squish height of around .040. I am in the process of building a motor which will likely result in decking the block. I have the choice of barely touching it and using something like a .015 shim or cutting it down almost to a zero deck and using a .041 gasket, both results would produce the same compression ratio and meet the desired quench height after taking into account the piston to deck height.

So my question for you, is if all other factors remain the same, is it better to use a gasket or a shim?.

Along those same lines, what compressed thicknesses are readily available?
I'm getting ready to take my block to the shop for cylinder boring and would like to instruct them to deck the block at the same time. I just don't know yet what height to go for.
I'm of the school that minimizes material removal. What we are pleased call modern thin will casting got its start on the original Winsdor Ford 221 in 1962, by the early 1970's everybody was cutting wall thicknesses. While this makes for engines that are light in weight it reduces the number of times that decks can be milled or cylinders rebored before strength is compromised.

So unless there is some good overriding reason to zero deck a block or bore more than .030 I just don't do it. That's not to say I don't do it at all, but I just don't start a fresh build by boring and milling to the limit unless its a banzai engine that is expected to win races while it lives and then is scrapped when it can't anymore.

In that vein, I recommend not removing any more material than is necessary to get a a flat surface which is parallel with the length of the crankshaft and square to it's centerline. You can still get to the .040 or so squish/quench with a thinner gasket. The shim style works very well when sandwiching cast iron the OEMs use this almost exclusively. The elastomer coated shim seems to work OK with an aluminum to iron combination as well. MLS gaskets aren't a lot thicker and also provide excellent service between aluminum and iron but are pricey. Copper with an embedded stainless O ring also work on a high output engine that would or could use O rings and a conventional copper gasket again these are costly. Composite gaskets work well with aluminum to iron or iron to iron but they tend to be thicker which means either a compromise on the squish/quench or cutting more from the deck and /or head to reduce this distance. It is also quite acceptable to stack shim gaskets which kind of makes a home brew MLS gasket.

So if you want some trouble free miles I'd recommend no more material removal than what it takes to make things flat, parallel and square. Since Vortec heads are iron I'd go with a steel shim head gasket. If you can get away without needing to zero deck the block you shouldn't have any problems with this set up. Your machinist should be able to tell you how much material, if any, needs to come off the deck, then calculate what it takes to get the desired squish/quench clearance from that point to choose the gasket thickness needed.

Bogie
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