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AMC v8 block/crank/rod/piston Specs

Contributed By: Michael Baxter

AMC deck height = 9.218" (SB Chevy 9.025" for S & Gs)

360 stroke = 3.44"
390 stroke = 3.57"
401 stroke = 3.68"

360 rod = 5.875"
390/401 rod = 5.858"

360 bore = 4.08"
390/401 bore = 4.165"

360 piston compression height = 1.601"
390 piston compression height = 1.56"
401 piston compression height = 1.508"

Keep in mind rods become a little shorter when they're rebuilt.

DH = deck height
CH = compression height
DC = deck clearance (above or below deck clearance of the top of the piston
at TDC)

Formula:

       stroke
DH  -  ------  -  rod length  +/-  DC  =  CH
         2

1st, if you're going to have custom or off-the-shelf flat top pistons fly-cut for valve notches and dish volume, you might as well set-up good quench. Quench is when the flat area of the pistons corresponds to the flat area of the heads. It does all sorts of good things. You would want the quench clearance to be in the .035-.040" range. Most head gaskets have a compressed height of ~.040" We'll assume .040" for this example.

So, if we want .035" or piston to head clearance (quench) and the gasket is .040" thick, we better get the pistons out of the block .005". Deck clearance would be + .005":

           3.57"
9.218"  -  -----  -  5.858"  +  .005"  =   1.58"
             2

1.58" is the ultimate CH for your pistons plus any loss of rod length from rebuilding and if you mill any material from the block's decks, you need to subtract that as well. In fact, you have to measure the subject block's DH to be on the safe side. The numbers I quote above are nominal figures. Plus, you never know if any milling was previously done (even at the factory during original production).

360's max. recommended over-bore by AMC is .045".

360 piston pin diameter = .931"
390/401 pin diameter = 1.001"
SB Chevy pin diameter = .9272"

Remember you can not use a 360 rod on a 390 crank. The 390/401's rod journals are 2.247" while the 360's are 2.094". If you're going to put a 390/401 crank in a 360 block, the 390/401 rods have to come along for the ride.

There are a lot of options at this point. You could have custom pistons made. You could sleeve the block to a 4.00" bore (lots more piston availability). You could find some off-the-shelf pistons which are close and work from there. The later probably being the cheapest unless you have to get into offset grinding the crank for a little different stroke.

If you punch the block out to 4.125" and re-bush the little end of the rods to the Chevy pin, I found a SB Chevy 400 piston with a 1.56" CH. You would need to mill ~.020 from the decks.

Unfortunately it is not a flat-top piston. It has a 12cc "D" shaped dish which is probably not shaped correctly. It might be possible to order a set without the dish so the appropriate quench, notches and dish could be cut.  It also might be possible to raise the pin height 0.20" as well.

The bottom line is I think it can be done with relative ease in the case of the 390 crank. The whole reciprocating assy. would need to be balanced by a machine shop, though.

 -- Michael Baxter, MBaxter@Compuserve.com

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