Which headgasket for my application?

[DeNoZZo]

My machinist tells me my head is decked to the factory limit and the block was decked just barely. What headgasket would be best for my goal of a 350hp-ish capable motor? I was planning on going with the arp headstud/cometic headgasket from extremepsi.com, but didn't realize until I tried to order a few minutes ago that they don't have the standard bore size. I just had my cylinders honed. What are my options? What do you guys suggest? I have had the cometic hg recommended to me a few times.

 

[My1GLaser]

At that power level you can get an OEM composite an run ARPs and be fine, or an MLS if the head and block were decked. I personally like the Mitsu MLS gasket better and have heard unpleasant things about the Cometic sealing poorly.

I have a question though. Are the piston flush with the deck or are they above or below the surface? When they say factory limit for the head do they mean .008"? Mine has def been decked more than that and you can make up the difference with the thickness HG you choose.

The reason I ask is b/c of what is called "quench pad." It is the distance from the top of the piston to the bottom of the head at TDC. .040" is ideal from the research I did when I built my motor 2 years ago.

For instance, my pistons were .010" above the block deck surface. If I would have run an OEM composite gasket which is .042" then I would only have .032" quench and run the risk of pistons hitting valves at high rpms when rods stretch slightly. You also don't want your pistons to be .010" below the deck surface and run a .054" thick gasket and end up with .064" quench. I chose a .054" thick Mitsu MLS gasket and I have a quench of .044" which is close enough.

Quench is important to get a violent swirl action of the mixture to reduce the motors tendency to knock. Some will say its not important on a 4g63 b/c we have dished pistons and others think its very important. I am of the persuasion that while you have it apart, its probably worth the extra effort to get the correct thickness HG. Ask your machinist what they think about quench pad.

 

[DeNoZZo]

At that power level you can get an OEM composite an run ARPs and be fine, or an MLS if the head and block were decked. I personally like the Mitsu MLS gasket better and have heard unpleasant things about the Cometic sealing poorly.

I have a question though. Are the piston flush with the deck or are they above or below the surface? When they say factory limit for the head do they mean .008"? Mine has def been decked more than that and you can make up the difference with the thickness HG you choose.

The reason I ask is b/c of what is called "quench pad." It is the distance from the top of the piston to the bottom of the head at TDC. .040" is ideal from the research I did when I built my motor 2 years ago.

For instance, my pistons were .010" above the block deck surface. If I would have run an OEM composite gasket which is .042" then I would only have .032" quench and run the risk of pistons hitting valves at high rpms when rods stretch slightly. You also don't want your pistons to be .010" below the deck surface and run a .054" thick gasket and end up with .064" quench. I chose a .054" thick Mitsu MLS gasket and I have a quench of .044" which is close enough.

Quench is important to get a violent swirl action of the mixture to reduce the motors tendency to knock. Some will say its not important on a 4g63 b/c we have dished pistons and others think its very important. I am of the persuasion that while you have it apart, its probably worth the extra effort to get the correct thickness HG. Ask your machinist what they think about quench pad.

I had the block decked .005 and the head .012. Well my recipe says surface head .009 and then surface head .012. So I'm guessing he did it once and it wasn't enough. The pistons are perfectly flush to the touch with the block at tdc. So an OEM composite would work fine? Where can I order one to a certain thickness?

After looking at a vendor couple websites, I think I can use the 85.5 cometic headgasket with a standard bore. Can anyone confirm this? How can I choose thickness?

 

[My1GLaser]

An OEM composite with ARPs would work fine up to probably 25-30 psi.

The std Cometic is .051 thick according to their website. Since the pistons sit flush and the head was cut .012", this will put everything back pretty close to stock. You can go to the Cometic website directly, find the std bore part number and see if one of these vendors carries it or can order it.

Its hard to get it perfect and we have such little quench area that its hard to make good use of it anyway, so I wouldn't fret too much over finding a specific thickness. OEM composite is .042, Mitsu MLS is .054, Cometic is .051 so there are enough thickness choices already if you know what you need IMO.

I just wanted to mention it in case your specs were way out of whack. Because the other problem with machining the block and head and changing HG is that it will throw off your timing marks if it gets too far out spec. Without adj cam gears to compensate your cams could be a few deg off. I never knew about all this until I built my motor, and as far as I know you should be fine with what you have there with the std Cometic.

 

[DeNoZZo]

Alright I think I'm going to go with the cometic/arp stud combo.

Thanks a lot for your help!

I I did the math right, my head should be .008 closer to the block because of the milling and after the .051 thick headgasket. With .008 being the service limit, I shouldn't need adjustable cam gears right?