1964-1990 Gotchas



425 pistons are not fungible with 455 pistons due to a different distance from pin center to top of piston ("compression height"). The compression height for the 425 is 1.595" and the 455 is 1.720". Compression height is the distance from the center of the piston pin bore to the top of the piston.

Compression height refers to the placement of the wrist pin. With any stroke and rod length combination, the pin has be in the right place to complete the picture for a block of any given height. If the stroke is increased the engineer has to either shorten the rod, move the wrist pin location closer to the piston crown by ½ of the increase, or make the block taller.

If there is room they usually leave the rod alone and change the piston. This is probably cheaper and makes a better rod angle which cuts down on piston and cylinder wall wear. Sometimes there isn't room below the rings and other places, so it's easier to shorten the rod. Either of the other two options are cheaper than making the block taller because this changes lots of other things, such as intake width, pushrod length, location of the brackets, ect.

The 1965-67 400 block will take a 350 piston with a .057" overbore. The 350 pistons are readily available in a variety of compression ratios.

The diesel rod is slightly shorter than the gasoline rod, for the increased ring-land necessary for a diesel, I'm assuming.

[ Thanks to Chris Witt, Walter, Steve, Bob Barry for this information ]