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Was trying to decide on head gaskets so I checked piston out of the hole to get quench set up and found that they all fell between .012 and .014.
Which isn't an issue in an of itself as a .051 gasket sets my quench in a great area at .037-.039.
My question has to do with the deck height that is listed on the build sheet. It's at 9.234.
The stackup measures at 4/2 + 6.125 + 1.115 = 9.240. So by this, piston out of the hole should be .006, which is where most of them fall.
So I guess I'm confused why I'm measuring .012-.014 if the deck height truly is 9.234.
Am I missing something here? I measured by simply bringing the piston to TDC with a dial indicator, then measuring with a straight edge/feeler gauge in line with the wrist pin so as to take a lot of the rock out of the equation - so I think the measurements are pretty close. I've got a deck bridge coming to verify the numbers but I don't see them changing much.
Yes, It appears that it has, which could account for what I'm seeing. However, the spec they give on the sheet is 9.234", my understanding was that this was the spec after decking. Which is why I'm confused.
Isn't stock deck height 9.240? If it was decked .006 it would make 9.234.
Stock setups the piston is around .006 above deck.
Sounds like it adds up to me unless I've misunderstood something. I'm no expert.
I measured tonight using a deck bridge, and here are the results. I'm a bit concerned here as I don't see how the quoted 9.234" deck height is accurate. Also, with a .051 gasket, this puts my quench in an uncomfortable area at .031-.032. I'd really prefer not running a thicker gasket to compensate for this, but it seems like for whatever reason the block had/needed a bunch shaved off?
All measurements are between .016-.020 (just shy).
I'm looking for opinions I suppose as I'm not a machinist, or an engine builder.
I pulled a view of another block part number to compare, my block has been shaved enough to start getting into the part number on the block, so that also seems to be a decent indication that it didn't just have a couple .001's taken off for clean up/levelling.
What do you guys think? Again, I appreciate the input.
Try rocking the piston in the bore 12oclock and 6oclock. Maybe your using an offset pin piston, which is common with LS slugs. Rock it slowly and observe your numbers. This is why I use two dial indicators on my bridge, so I can rock the piston and determine when it’s top is parallel to my deck.
Try rocking the piston in the bore 12oclock and 6oclock. Maybe your using an offset pin piston, which is common with LS slugs. Rock it slowly and observe your numbers. This is why I use two dial indicators on my bridge, so I can rock the piston and determine when it’s top is parallel to my deck.
Fair point.
I just ran out and did that and it did help slightly, but was still up around .0175" and oddly enough, the amount at 12 and 6 varied quite a bit hole to hole when stopping on the upstroke.
Well at this point it is what it is. You have a couple options here. You can pull it all back down and have your piston tops milled, or you can go with a thicker head gasket. Nothing wrong with either option. The gasket route will likely be cheaper and won’t affect your assembly balance, although milling the pistons won’t affect the balance enough to matter, unless your OCD like I am, and everything matters. I ended up with a .056” Cometic head gasket, which is a 5 layer gasket, to get mine right. I had one piston .0215” out, hence the .056” gasket. It happens.
Well at this point it is what it is. You have a couple options here. You can pull it all back down and have your piston tops milled, or you can go with a thicker head gasket. Nothing wrong with either option. The gasket route will likely be cheaper and won’t affect your assembly balance, although milling the pistons won’t affect the balance enough to matter, unless your OCD like I am, and everything matters. I ended up with a .056” Cometic head gasket, which is a 5 layer gasket, to get mine right. I had one piston .0215” out, hence the .056” gasket. It happens.
Yeah, I suppose what bothers me is that the engine was blueprinted, and documented as having a deck height of 9.234", which I do not see at all how that's possible at this point. So I guess I just wonder what else isn't correct? Just a little worrisome when I consider how much I spent to have the shortblock done...
Also, my engine builder STRONGLY advises against Cometic gaskets, but at this point, there's no GM gasket that will comfortably work either.
My 434” is my first build with a Cometic and so far so good. I was reluctant but Tony assured me it would be fine. He uses Cometic a lot. I get what your saying about your stated vs. actual deck heights. New question for you...have you mocked up PTV yet? You need to check that on the cylinder that’s out of the hole the most, or trust yourself to do the math correctly. Steel rods in this build, correct?
Thompson assures me that there will be no issue and quench down at .031-.032 won't cause me any issues either. They have a fantastic reputation around here, so I'm assuming they are right.
Still don't really understand the deck height issue as it was recorded, but I suppose it's just what it is at this point.
Well at this point it is what it is. You have a couple options here. You can pull it all back down and have your piston tops milled, or you can go with a thicker head gasket. Nothing wrong with either option. The gasket route will likely be cheaper and won’t affect your assembly balance, although milling the pistons won’t affect the balance enough to matter, unless your OCD like I am, and everything matters. I ended up with a .056” Cometic head gasket, which is a 5 layer gasket, to get mine right. I had one piston .0215” out, hence the .056” gasket. It happens.
What quench is preferred on a NA setup like a 434ci LSX that will see as much as 300 shot of N2O? I made a calculator in excel to play around with the numbers but now I'm hearing that not everyone likes Cometic and that you shouldn't go too thick on the gasket. Is 0.056" about the thickest you would run?
What quench is preferred on a NA setup like a 434ci LSX that will see as much as 300 lbs of boost? I made a calculator in excel to play around with the numbers but now I'm hearing that not everyone likes Cometic and that you shouldn't go too thick on the gasket. Is 0.056" about the thickest you would run?
I can’t give you an honest answer regarding boost, as I have never built an LS engine for boost, but used to mess around with big block Chevy supercharged off shore engines. I’m sure the quench numbers would be vastly different between the two.
Concerning Cometic, I had my reservations based solely on reviews and internet threads...which can be taken with a grain of salt...I prefer to have personal experience. Tony assured me that the Cometics would behave just fine, as he has sold gobs of them and built many, many engines as well using cometics. I was backed into a corner, due to my goofy out of hole numbers, and this is a max effort build so quinch had to be right. Cometic offered what I needed, in a .056”, so that’s why I’m running that gasket. I certainly didn’t want that much gasket, due to there being a greater chance for gasket failure, as you go taller/thicker. Next time engine gets torn down, the block deck will be cut square, as it should have been the first time, and my gasket dimension will shrink significantly. Hope this helps...
I can’t give you an honest answer regarding boost, as I have never built an LS engine for boost, but used to mess around with big block Chevy supercharged off shore engines. I’m sure the quench numbers would be vastly different between the two.
Concerning Cometic, I had my reservations based solely on reviews and internet threads...which can be taken with a grain of salt...I prefer to have personal experience. Tony assured me that the Cometics would behave just fine, as he has sold gobs of them and built many, many engines as well using cometics. I was backed into a corner, due to my goofy out of hole numbers, and this is a max effort build so quinch had to be right. Cometic offered what I needed, in a .056”, so that’s why I’m running that gasket. I certainly didn’t want that much gasket, due to there being a greater chance for gasket failure, as you go taller/thicker. Next time engine gets torn down, the block deck will be cut square, as it should have been the first time, and my gasket dimension will shrink significantly. Hope this helps...
not 300# boost, meant 300 shot of N2O. Bad typo on my part. I read 0.040-0.050 is good for quench on that setup. Cometic seems to offer a good assortment of options which certainly come in handy than having to shell out for custom pistons.
Last edited by 5.7stroker; 03-15-2019 at 10:15 PM.
For big boosted and heavy nitrous engines, a lot of quench is a good thing. By a lot, I mean over .100”. It tends to slow the burn down and give the tuner a wider margin of safety. For a street car with a 300 shot, I’d stay in the .040”ish range.
03-13-2019, 08:13 PM
Deck height/piston deck clearance question. 
