Thread: Changing Deck height

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So the std. deck height on sbc 400 is 9.025 right? How much Could the deck be shortened... but with using a 3.48 crank with 6.00'' rods? I'm jut curious about the tolerances between the pistons and heads/valves. Or should I stick with the standard deck height? Thanks

well there only way to know is to first check your block so your say your block is 9.025 now? you know this as it was checked ? not just asume it is 9.025 i deck the engine base off the rod lenth 1/2 the crank stroke and the piston CH that tell me were i can deck the block to many time the blocks are never true by the time this is correct with the BHJ tru deck fixture if way off the block decks can be at O deck your realy should have a game plan should talk it over with the guy doing your machine work then it is easy for him to do the math and have him deck it i have two mill s that i deck blocks with built only to cut engines. block. head .intakes

Performance building with a SBC would include setting the squish (the dimension from the piston crown to the underside of the cylinder head, with the piston at top dead center and the head gasket in place.) Most everyone will agree that the squish on a SBC should be between 0.035" and 0.045", so you measure your "stack" of parts before you begin your build. You use the radius of the crank stroke (half the stroke), so on a 3.48" 350 crank, the radius would be 1.740". Add that to your 6.000" rod length and you have a 7.740" stack so far. If we agree that the nominal block deck height of a SBC block is ~9.025", then subtracting 7.740" from 9.025" tells us that we can use a maximum piston compression height of 1.285" before we begin pushing the piston crown out of the hole with a stack that is too tall for the available block deck height. Now, don't misunderstand, it is acceptable to pop the piston crown out of the hole by a few thousandths if you need to make up for a very thick gasket and reach a desired squish. Let's say for instance that the block had been surfaced before you got it and measures a block deck height of 8.990", but you need to use your 9.000" stack of parts in the block. Assemble the motor with the piston popped up out of the bore by 0.010" and use a gasket thickness of 0.050". This will put the squish at 0.040" and everyone's happy.

Back to the 7.740" stack that we had. If we were going to build a high-perf motor, we might choose a forged, flat-top piston such as this Ross example, with a compression height of 1.268".
Ross Racing Forged Pistons 95465 - SummitRacing.com
Adding that figure to the partial stack we had earlier will make a completed stack of 9.008". Again, with a block deck height of 9.025", that leaves us with a piston deck height of 0.017" (with the piston at top dead center, it lacks reaching the top of the bore by 0.017".) Now, with any other SBC block, there is a variety of head gasket thicknesses available, but with a 400 block, composition gaskets start at 0.040" and go thicker. If we are to have a nominal 0.040" squish, then we must cut the block decks to zero deck, the sum of the stack, which is 9.008".
With a block deck height of 9.008", a stack of 9.008" and a gasket of 0.040", we will have a 0.040" squish and all is right with the world.

As it is normal for all 400's,you mag and sonic check the block before spending any money on machining.

The chances of you finding a useable stock bore size on a 40yr old 400 block
is unlikely.So the sonic check is for wall thickness on thin wall 400's and the bore/hone should be done with machine shop having the pistons in hand and with torque plates.

A great writeup, Tech! Simple and to the point, explaining the steps to getting it right. Ch3vy, if you follow Tech's advice you'll get where you're wanting to be. He knows the subject and explains things in plain language, no BS.

no B.S i deck them all the time .here is a tip from a guy that builds them in a shop check line bore first . if engine needs a line bore crank can go up in the block by .005 or more if hurt .were the piston sits in the block has much to do the piston CD Comp distance or called C H Comp heigth of built piston get you pistons first . i have seen them not all to be the same mahle come to mine on this . like gary said it over 20 years old less it,s a stock engine with STD bore i would bet that has been deck evey engine i do get decked even the new block .first you start knowing what you have deck height in a prefect world you would have a block that .s 9.025 and decks with a good finish from GM and is true to cam and crank center line this this should be under line as you can find blocks off side seen this many times . so true block is far from the case this is the block you build your house off of so to speak every thing from side to side is off it will affect to push rods and how the heads sit on the block if you have any question on how machine work is done or how it will effect your build just ask Jerry if still on here or me will tell you the no BS were the only ones that have the machines and deck blocks on here as for the deck and for quench may want to think about your head gasket they can be up to .051 thick with the seam hole to 053 thick you did not say with CR your build here or if your going to spray it but the mild build ROL are good head gasket s hg31250ht 046 or the felpro f1014 on builds were power adders are use or hi CR i use Cometic they come in 040...051..060..thick the finish need to be very smooth were you may ask the shop you use if they can get the finish with there mill like a CBN or carbide broach machine can do i have both

Originally Posted by pat mccarthy

no B.S i deck them all the time.....

Pat, I wasn't saying you're BS'ing, or anything about your post at all. I was simply pointing out that, in my opinion, Tech's advice about squish is dead on accurate and very easy to understand. He's only explaining "squish", and I think he did it in a way that makes it very clear how the component parts, including the head gasket, need to be considered in establishing deck height. Your points about line boring to set the foundation for the build are dead on, too.

Originally Posted by rspears

Pat, I wasn't saying you're BS'ing, or anything about your post at all. I was simply pointing out that, in my opinion, Tech's advice about squish is dead on accurate and very easy to understand. He's only explaining "squish", and I think he did it in a way that makes it very clear how the component parts, including the head gasket, need to be considered in establishing deck height. Your points about line boring to set the foundation for the build are dead on, too.

HEY i did got think that but he never asked about squish ?? more about PV??? the guy was asking about the deck the piston CH /CD tell were you put the deck at the point if you have a block thats not way off from good old GM then you can be 005 or 0.010 in the hole . but when you start to square the block you can not move it so some times head gaskets are what you end up with on paper is sound geat run to the shop tell them just cut 010 off then i be at 9015 call it a day just saying a old block it will not always work that way less you have 20 to pick threw your going to dance with the girl you took to the dance ????