To Zero Deck Or Not?

Have been researching piston height to deck clearance for my new 383 build to try and acheive a nominal .040 quench distance and based upon available head gasket thicknesses and bore I am wondering what most of you do to acheive your quench clearances and desired cylinder volumes.

First, for a stock 9.025 deck ht, 3.75 stroke, 5.7" rod length and a KB135 18cc D-Cup .030 overbore piston with a compression height of 1.433, I calculate a nominal deck clearance of .017" - probably slightly less after leveling the deck. Gasket thicknesses - most with significantly more than .030 overbore - range from .015 thick to well over .040 thick.

So my questions are;

1) Is it normal or desirable to use a head gasket with up to say a 4.1" bore with a 4.030 cylinder bore", or would you try to use a head gasket that more closely matches the cylinder bore?

2) Would you zero deck the block (zero piston to deck clearance) and use a .040 thick gasket, or just mill the deck enough to match a given gasket thickness to acheive an .040 quench?

My confusion comes from what appears to me to be a head gasket mis-match between available bores and thicknesses, in that I can acheive my nominal .040 quench ht without completely zeroing the deck ht if I do not concern myself with a significant overbore on the head gasket. Am I missing something? What would you do?




Thanks for any advice!

1. when the machine shop bores and hones the bores, they will leave a slight chamfer at the top of the bore anyway to help start the rings down into the bore when you assemble the motor. Check with the shop that's going to do your work and coordinate the finished o.d. at the top of the bore with the o.d. of the gasket you wish to use. Just don't allow them to chamfer too much so that you have a ledge of gasket overhanging the bore. That could be the source of a hot spot that would initiate preignition.

2. Mix and match gasket thickness with deck height to achieve the piston to head clearance you want. Check several sources for gaskets including GM parts. Take note though, that some GM gaskets such as 10105117, 12557236 and possibly others have a bore of 4.000", so you wouldn't want to use them in a bored 350.
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confusion. Do not concern yourself with the gasket bore as long as you address it with the machine shop . The smallest gasket bore that would work is the Mr. Gasket 3130G at 4.060", 0.040" compressed. Fel-Pro makes several thicknesses in the 4.100" bore size....
0.015" #1094
0.016" #7733SH1
0.040" #1142

I'm sure you know how to proceed, but I'll outline it for the newbies...
The block goes to the shop to be magnafluxed so we can find out if it's cracked anywhere and also where all the meat is in the cylinder walls. In this case, we have determined that we will bore the motor 0.030" and use a 3.750" stroke crank to make a 383. We determine the pistons we're going to use along with the rings and we order them. The pistons will set the theme for the entire motor because they will determine the compression ratio and that will determine the cam used. We take the pistons to the machine shop and have them bore and hone the block to the pistons. Do not have the deck cut at this time. Pick up the block, one piston/rod assembly, all main bearings and one set of rod bearings. You won't need the rings. At home, assemble the crank in the block with all the oiled main bearings and assemble the one piston/rod assembly into #1 cylinder bore. Using a dial indicator, find TDC. Using a depth micrometer, measure the distance from the block deck to the piston crown at the 3 o'clock and 9 o'clock positions as you're standing at the side of the block. Record these measurements. Move the piston/rod and bearings to the #2 cylinder and do the same thing, then the #7 and #8 cylinders. When you are through, you'll know the deck height of the block at all four corners and you'll be able to communicate this info to your machinist so that he can cut the decks in accordance with the gaskets you're going to use in order to establish your piston to head clearance (squish). Of course, you can have the shop do this if you want, it just costs more money than doing it yourself.

I would deck it to .015" deck clearance and then use a .028" head gasket offered by Mr. Gasket for a total clearance of .043". This way, if you damage you're deck, you still have .015" of material to cut into to straighten things out. If zero decked, any damage to the deck means you have to find a new block.

if you O deck it and hurt it there are thing you can do befor it is junk? if it gets hurt go to a thicker head gasket like mls like comtic or you can mill the tops of the pistons .

Hey thanks Tech, I forgot all about the bore chamfer - been 25 years since I built anything.

76GMC, It looks as though the MRG-1134 head gasket with a compressed thickness of .028 could get me to a dead on quench distance of .040" with a finished deck height of 9.020", if I went for a piston to deck clearance of .012".

Is it likely that a nominal .005" off the deck will square things up (I don't know how far out a new bare block is likely to be)?

Do you guys think the 4.130 bore of this gasket is OK? It would yield a nominal .050" bore edge clearance - sound like enough room for the bore chamfer with no gasket overhang?

well. i would not cut the block before you have mock it up. with all parts and the block you use . i have seen the decks way off and if you tru-deck it off the crank and cam center line thay can be way out of square

Thanks Pat.

I do intend to mock it up first. I'm just trying to get my plan in order to see if it is possible to acheive an .040" quench with the parts available (head gasket thickness's).

I intend to use a new GMPP 383 bare block. Using the nominal dimensions of my rotating assembly, it would be nice if the block will square up with what calculates out to be about five thousands off the standard 9.025 deck height. Just nominal dimensions mind you as I understand it will all depend on the actual installed piston height.

Thus I ask those of you with more experience than I; Is it likely the new block will square up in .005" or are they typically further out of tolerance than that?

Do new blocks need have the mains align bored too?

if you O deck it and go off the old deck it may. but if you tru-deck it it may. not there is a lot of head gaskets out there you need to find the gaskets you are going to use but if it will not go .005 you will have to find a thicker head gasket and cut it for that head gaskets an i do not think it will need the mains bored out i try like hell not have this done this will move the crank up in the block and if need s to be move a lot a new timming set is needed for this .a good new block will not need this done but should be check

My machinist said my core was one of the best he had seen for straightness. It was off by about .005"-.007" across the block. Because of budget constraints, I did not have it square decked off of the crank. I just had it parallel milled which means it's still off by that much. If you're building a low compression motor, that's no big deal. I might have a cylinder at 9.25:1 and another at 9.4:1. On a high compression motor that is very borderline for fuel, you'll have one cylinder at 10.5:1 which is ok for pump gas, but another might be at 11:1+ which will be way into detonation and you won't know about it because all of the other cylinders run fine. Oh yeah, I didn't even need to have the mains line honed. The rear main set up a little looser than the rest at .0025" while the others were .002" for oil film clearance.

Quote:

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Originally posted by MainCap
Have been researching piston height to deck clearance for my new 383 build to try and acheive a nominal .040 quench distance and based upon available head gasket thicknesses and bore I am wondering what most of you do to acheive your quench clearances and desired cylinder volumes.

First, for a stock 9.025 deck ht, 3.75 stroke, 5.7" rod length and a KB135 18cc D-Cup .030 overbore piston with a compression height of 1.433, I calculate a nominal deck clearance of .017" - probably slightly less after leveling the deck. Gasket thicknesses - most with significantly more than .030 overbore - range from .015 thick to well over .040 thick.

So my questions are;

1) Is it normal or desirable to use a head gasket with up to say a 4.1" bore with a 4.030 cylinder bore", or would you try to use a head gasket that more closely matches the cylinder bore?

2) Would you zero deck the block (zero piston to deck clearance) and use a .040 thick gasket, or just mill the deck enough to match a given gasket thickness to acheive an .040 quench?

My confusion comes from what appears to me to be a head gasket mis-match between available bores and thicknesses, in that I can acheive my nominal .040 quench ht without completely zeroing the deck ht if I do not concern myself with a significant overbore on the head gasket. Am I missing something? What would you do?




Thanks for any advice!

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IN A STREET ENGINE I WOULD NEVER BUILD A TRUE (ZERO DECK) ENGINE.WHAT HAPPENS IF ON THE NEXT REBUILD YOU HAVE TO DECK THE BLOCK BECAUSE IT IS NOT SQUARE THEN YOU WOULD HAVE A PISTON THAT IS (POSITIVE).I WOULD TRY AND STICK THE PISTON DOWN IN THE HOLE ABOUT .010 AND CALL IT GOOD.NEXT TIME AROUND YOU STILL HAVE A USABLE BLOCK NOT A DOOR STOP.

yes i would keep it 0.010 and call it good to. but if some thing happens 0.010 will not help much. and i guess it this point i will take all the door stops out there. there is a easy fix for them like i said :whacked:

OK

If the block will square up close enough with a .005 cut the piston will still be .012 below deck.

What kind of damage to the deck is likely to happen anyway? This is going to be a 5000rpm max towing motor for a Subuuuuuuuurban.

Quote:

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Originally posted by MainCap
OK

If the block will square up close enough with a .005 cut the piston will still be .012 below deck.

What kind of damage to the deck is likely to happen anyway? This is going to be a 5000rpm max towing motor for a Subuuuuuuuurban.

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not much but if you lose a head gaskets and burn the deck and it will it will take more than that 0.010to fix it .i just fixed a big block that was crack on both sid of the block up by the deck and it took 0.016 to clean it up when done the welding pull on the deck . this was fixed because this was a $2000 block

I went with a zero deck and use FelPro 1003 gaskets. They torque up at .040-.039. Good for bores up to 4.060. I am using Vortec heads and pistons with a slight "D" cup. Compression is about 9.2 or 9.3. Runs all day on 89 octaine.

No matter how you set your quench, getting it right it is well worth the effort.

Quote:

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Originally posted by Bib_Overalls
I went with a zero deck and use FelPro 1003 gaskets. They torque up at .040-.039. Good for bores up to 4.060. I am using Vortec heads and pistons with a slight "D" cup. Compression is about 9.2 or 9.3. Runs all day on 89 octaine.

No matter how you set your quench, getting it right it is well worth the effort.

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TRUE WORDS OF WISDOM


I have found that for some reason the quench works better with a zero deck height. I think a lot of it has to do with the way the air/fuel is pushed into the cylinder wall. The purpose of the quench is to act like a wedge and direct the mixture out from under the flat side of the head and into the chamber. This rapid movement air/fuel atomizes better to create a better mixture and help eliminate detonation and enhance power just because it burns better.

--divide stroke in half
--add rod lenth
--add piston compression height

Usually comes out to 9.0, get block decked to that height and run a Fel-pro .039 head gasket Part No. 1010 I don't remember the bore size of it, but I know the part no. and thickness in my head. I have never had a problem with that set up.

In my experience and opinion, if you have a problem that ends up you needing to deck the block again, chances are, That is the least of your worries. That's street use. Track use, those problems are expected sooner or later so leaving some extra deck height is a good idea on a $2000+ race block.