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Old 03-25-2012, 09:44 PM
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Originally Posted by White85Camaro
Alright guys, I'll make sure to beware of the extra costs that come along and try not to get too ahead of myself, Lol. And I'll definitely have to get a dyno done. There aren't a whole lot of shops around here that are reputable for high performance applications.
I'm thinking of picking up a second 400 block that I can have all the machining done. Try to get that ready, rather then take my engine out and not be able to drive it while I'm building an engine.
When I take it to have it machined, what do I need to have done exactly? This is one place I can't quite understand. I know it's going to need to be bored, honed and decked. Is this all that needs to be done? How do I know what specs I need to have everything done to? I believe I understand boring. But I'm not totally sure about decking. Is there anyone that can dumb this down a bit for me?
Thanks everyone. The knowledge is always appreciated!
First of all, stick with the stock crank. They are reliable and should be considered high quality and a bench mark to grade other cranks. Unless you spend big bucks on a high quality aftermarket crank (which is NOT Eagle or Scat), then you are pressing your luck.

What to do with the block?

Yes, bore and hone to get you +.040".

You should strongly consider align honing your block. This ensures the main bearing caps/bores are perfectly in line and straight and centered under the cylinder bores. This is not really a big deal for 400-ish HP engines, but as desired power levels increase this becomes necessary.

Decking is taking material off the top of the block (where the heads sit). If you just tell the machine shop, "oh yeah, deck the block too" they will probably just take .010" off to ensure the surface is nice and flat. Most of the guys here like to get the block to what is called "zero deck". In other words the pistons at TDC come up exactly to the deck (they are not down in the hole or coming out of the top). From there you chose the proper head gasket to achieve 2 things: (a) desired compression ratio (b) proper squish and quench (aim for around .040-.045").

To get the block at zero deck height takes extra time (read MONEY) by the machinist as he will have to assemble the short block and take measurements at cylinders 1,2,7 and 8 (aka each corner) to figure out how much material to cut. The added benefit here is that now you get a flat surface AND a straight/level surface (where as blindly milling off .010" is flat, but is it even with the bore of the mains? This leads to some cylinders with higher or lower compression ratios).

Keep in mind that if you get your crank reground and polished it probably won't be exactly 3.75" stroke. Also if you recondition your rods they are also likely to not be 5.565" in length. So again, all the more reason why the machinist must take the effort to assemble the shortblock to take measurements.

GM knew a thing or two about squish. Engines left the factory with around .040-.046" squish (.025" deck clearance + .015-.021" head gasket). There is a ton of articles about this topic, so I will let you google that. Here are some numbers though.

Deck clearance = Block Height - Reciprocating Height

Stock block height of a SBC is 9.025" (measured from center of main bearing to deck)

To get total height of your rotating assembly (measured from center of main bearing to top of piston -> does not include dish/dome):
Reciprocating Height = Crank Stroke Radius + Rod Length + Compression Height of Piston

Here are some examples:

350SBC Example:
Crank Stroke Radius: 3.48" / 2 = 1.74"
Rod Length: 5.7"
Compression Height of Piston: 1.560" (rebuilder special pistons are often 1.540" to allow more leeway for decking and head gasket selection)
1.74" + 5.7" + 1.560" = 9.000" Reciprocating Height
9.025" - 9.000" = .025" deck clearance

400SBC Example:
Crank Stroke Radius: 3.75" / 2 = 1.875"
Rod Length: 5.565"
Compression Height of Piston: 1.560" (rebuilder special pistons are often 1.540" to allow more leeway for decking and head gasket selection)
1.875" + 5.565" + 1.560" = 9.000" Reciprocating Height
9.025" - 9.000" = .025" deck clearance
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