240 / 300 Pistons
240 / 300 Pistons
Cross-Country
Joined: Jan 2005
Posts: 83
Likes: 3
piston pin placement
These engines used many different pistons due to application and compression ratio.
The wrist pin diameters changed as follows:240 cid '68= .9121", '69-'72= .9751".
300cid <'73=.9121", '73>=.9751". I once bored a 300 .050" and installed 390 pistons.
It gave me 308 cid and a 10.7:1 c/r. Had to run 15% methanol and retard the timing
5deg atc. Machined the "bowl" from the tops giving me 9.4 c/r and it ran much better.
As for the piston pin placement, the rods are .040" longer on the 240 so changing pistons would significantly change the C/R. There once was a racer who used 240 rods on a 300 crank but could only use a 2 ring piston
The wrist pin diameters changed as follows:240 cid '68= .9121", '69-'72= .9751".
300cid <'73=.9121", '73>=.9751". I once bored a 300 .050" and installed 390 pistons.
It gave me 308 cid and a 10.7:1 c/r. Had to run 15% methanol and retard the timing
5deg atc. Machined the "bowl" from the tops giving me 9.4 c/r and it ran much better.
As for the piston pin placement, the rods are .040" longer on the 240 so changing pistons would significantly change the C/R. There once was a racer who used 240 rods on a 300 crank but could only use a 2 ring piston
Last edited by belchfire; Jan 14, 2005 at 10:30 AM. Reason: more info
New User
Joined: Dec 2004
Posts: 7
Likes: 0
Try a piston for a 350 with 6" rods to fit the 300 with 240 rods.You will have to trim the top about .020" for a .005 above deck with a .040 thick head gasket.Also bore and hone the rod small end for the chevy pin. The pistons I bought had 1/16 comp rings and weighed only 400 gm w/ forged J E pistons. Good for controlling cylinder wall stress on overbored blocks and reduced piston speed at high RPMs.
