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Old 07-22-2007, 10:22 AM
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piston to valve clearence/quench question

I am helping a friend put together a 350.Right now the block is at the machinist and is almost done.The only problem we have is that we told the machinist that we wanted the block zero decked.He has measured the piston to deck height and told us that the pistons were about 32 thousandths in the hole.From what I understand this is a pretty bad place to be considering what we what to do with this engine.The machinist told us that if he decked the block that there is a chance of the valves hitting the pistons.I don't think that the cam is all that radical.It has 488/488 lift.My question is how much should we tell him to take off the deck to get the ideal quench using a standard 40 thousandths compressed head gasket?I really could use an answer fast because he is ready to send us the block and I just don't want to put it together like this. I will list the rest of the combo if it will help.Sleeved 350 block,summit flat top pistons,summit 488/488 lift 234/234 duration cam,882 heads milled 30 thousandths.4 speed manual,dual exhaust headers.

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Old 07-22-2007, 10:55 AM
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You want a finished .040" clearance between the piston head including the gasket. So you'd want to 0 deck the block.
Althou the valves will be moved closer to the pistons by decking and shaving the head. You're likely going to be ok with that cam. Usually you run into problems once the cam duration gets around 260@.050" And you've done lots of decking and head shaving. Things get a little close.
You should pick a cam with a tighter lobe separation. like 108 to make power with those heads. They need all the help they can get.
Check out Isky cams Hydraulics (Mega Cam) on the summit site.

I'd go with something like this
I'd degree it in on a 104 intake C/L. Much more snotty but simular rpm range. ( the .050" specs is the same.)
You can check valve to piston clearance. I check everything over 250@.050".
I trust you'll be porting the &^% out of those lame 882's for power.
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Old 07-22-2007, 02:48 PM
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piston/valve clearance

babygene, f-bird is right on the money, o deck your block. sam-missle
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Old 07-22-2007, 03:16 PM
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Thanks for the quick replies guys.I will tell the machinist to mill 30 thousandths off the deck of the block.That will put us right where we need to be right?The 882's won't be on there long because he is going to buy some edelbrock 170cc aluminum heads.Any other comments or remarks about this combo are appreciated.
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Old 07-22-2007, 03:59 PM
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Hold on a minute here, what kind of pistons are you using? if these are the pistons that have .020 off the compression height then you`re removing alot of soft cast iron off the decks. If the piston does have .020 off the tops it`s going to cause problems in more ways than one. if you deck it this far then there`s a much better chance when you torque the head bolts the deck will crack, seen this happen on more than one small block. then if you plan to rebuild it later and use the correct compression height pistons they`ll sit higher than the deck. Decking it this far will also create misalighnment of the intake so it will have to milled as well to be corrected then if you want to use the intake on a different engine later it won`t seal up. it`s also going to create valve train geometry problems that you`ll have to check and correct. If the pistons are the correct compression height, then I would have the block decked .020 then use a mr.gasket head gasket that has a .028 compressed thickness, this will give you the same .040 quench distance. However, you`ll still have to do a mockup and check all the alignment and the valve train geometry. My belief has always been, don`t remove no more than you have to from the decks, just remove enough to get the quench correct.
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Old 07-22-2007, 05:03 PM
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Another way to do it is to deck the block 0.010" and use a 0.016" steel shim gasket. That'll put the squish at 0.038"
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Old 07-22-2007, 08:56 PM
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piston/valve clearance

babygene, decking your block .030 does not create missalignment for your manifold, it does indeed increase the "v" . use the thick mr. gasket intake gaskets and use silicone on the end rails and you will not have any problems. missalignment will happen if you angle mill. most chevy blocks require more than .010 off the decks to square the decks to the center line of the crank. i always recommend using head studs instead of bolts if your budget permits.
sam-missle
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Old 07-23-2007, 01:10 PM
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Quote:
Originally Posted by babygene41042
I am helping a friend put together a 350.Right now the block is at the machinist and is almost done.The only problem we have is that we told the machinist that we wanted the block zero decked.He has measured the piston to deck height and told us that the pistons were about 32 thousandths in the hole.From what I understand this is a pretty bad place to be considering what we what to do with this engine.The machinist told us that if he decked the block that there is a chance of the valves hitting the pistons.I don't think that the cam is all that radical.It has 488/488 lift.My question is how much should we tell him to take off the deck to get the ideal quench using a standard 40 thousandths compressed head gasket?I really could use an answer fast because he is ready to send us the block and I just don't want to put it together like this. I will list the rest of the combo if it will help.Sleeved 350 block,summit flat top pistons,summit 488/488 lift 234/234 duration cam,882 heads milled 30 thousandths.4 speed manual,dual exhaust headers.
Whose pistons, some replacement pistons come with a lower crown to compensate for deck milling. The .032 sounds excessive given the factory specifies .025 and hardly ever has that much. If these are low crown pistons, milling the deck to zero will be taking too much material off making the deck structurally weak. This will lead to leaking head gaskets especially when combined with a milled head which now also has a weakened gasket surface. After all this milling it's unlikely the intake will fit properly without additional milling, you machinist will have a chart to tell him the ratio that comes off the side and base to compensate for head and deck milling. Fel Pro sells alternate thickness intake gasket sets to help in getting a vacuum and oil tight seal, see their web site. The distributor must be checked for mounting height. All this milling drops it deeper into the oil pump drive and may obstruct the right side lifter oil passage. Shims are available to space it up to regain these clearances.

The piston valve clearance is always checked with a build that uses more than a factory (weak duration, low lift cam; say 160 to 200 degrees and .36 to .42 lift). Where head milling, non stock pistons and deck milling has taken place it is imperative to check the valve to piston clearance. Additionally, the push rod length needs to be checked to insure that geometry at the valve to rocker is correct, that the rocker doesn't hit its stud, and that the push rod doesn't bind in its hole/passage nor against the guide if one is used.

The squish/quench distance can be dialed in with head gasket selection. .060 to .1 is plenty good. You get this first, then work the piston to valve clearance. That clearance is adjusted by milling the piston's valve reliefs, one has to be careful not to weaken this area, a serious hot rod piston will have sufficient material here. If you're using rebuilder or claimer pistons, you can get into trouble. Having a manual transmission REALLY means you've got to stay on top of these numbers as a missed shift can spell disaster if these clearances aren't right on the money.

Yeah, that probably means the engine goes together and comes apart a couple times before it gets put together for good.

Bogie
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Old 07-23-2007, 05:02 PM
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Thanks for all the good advice guys.The pistons are not rebuilder pistons and summit says they will give you a 9.6 compression ratio with a 64 cc head.From what I have read and heard that it is normal for the piston to be 30 to 40 thousandths in the hole on a common sbc.
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Old 07-23-2007, 06:24 PM
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I`ve seen some that were further in the hole than that, and less in the hole than that, and it`s certainly true most decks are tipped. Just to say though, I sure wouldn`t have that much removed, as noted, when the deck gets too thin it`ll cause head gasket problems as well as the risk the deck will crack when torqueing the bolts. My bro in law was building a customers 400 small block that had been decked more than few times, and was torqueing the bolts down and "CRACK!" thinking the socket had just slipped off the bolt, he tried to tighten it and realized it wouldn`t, he removed the head and found it had cracked from the bolt hole all the way down past the lower section of the bore and half way around, with 2 more cracks going into each bore. He got that sickening feeling as the rest of us did. One time when I was inspecting a friends 350 4 bolt "509" block, these blocks are still sought after. Anyways, I was cleaning the deck and noticed the same thing, a crack going inbetween the bores, and 2 other cracks going into each bore. and the block had never been decked, made him sick to have to scrap a otherwise mint 509 block.
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