Thread: BBC crank application

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Roger touched on "stack". Let me embellish that a little.....
Pistons have what is called a compression height, the distance from the centerline of the wrist pin to the crown of the piston (the crown is the flat part of the piston that is adjacent to the cylinder wall, just above the top ring). Depending on the crank stroke of the motor, pistons are made with a different compression height so that they will end up with the crown at the top of the bore at top dead center.

The BBC block deck height (measurement from the centerline of the main bearing bore to the block decks where the heads bolt on) is 9.800", so we must choose components that add up to close to that figure in order to have a working motor.
http://jamisonequipment.com/sites/de...s/BHM-24-1.gif

If the radius (half the stroke) of a 454 crankshaft is 2.000" and the 454 connecting rod is 6.135", then we must use a piston with a compression height of around 1.640". Adding those 3 values together would yield a stack of 9.775", leaving the piston crown 0.025" below the top of the block with the piston at top dead center. This 0.025" piston deck height added to the head gasket thickness comprises "squish". A squish of 0.035" to 0.045" is considered by many to be ideal, so in this case, we might add a shim head gasket measuring 0.020". The piston deck height of 0.025" added to the gasket thickness of 0.020" would give us a 0.045" squish.

Now, if a fellow were to use aluminum heads, they don't much like solid shim steel gaskets, preferring a thicker composition gasket so that they can move around a little without cracking or fretting. Problem is, with a piston deck height of 0.025" and a ~0.040 composition gasket, the squish is 0.065", too wide to be effective in controlling detonation on pump gas. What we might do in this case is to cut the block decks down by 0.020", creating a new block deck height of 9.780" and a new squish of 0.045".

Using a 396 crank in your 454 block, as Roger and Glenn said, would make a 427. The crank radius would be 1.880", the rod would be 6.135" and you would use a piston with a compression height of around 1.765" to make a stack of 9.780". Many fellows will cut the block decks to "zero", meaning that the crown of the piston comes exactly even with the block decks with the piston at top dead center and there is no piston deck height. In that case, you would use a composition gasket and the thickness of the gasket would become the squish figure. If you used a 0.040" gasket thickness, then the squish would be 0.040".

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