Rod length versus piston compression height

-
dibbons

dibbons

Well-Known Member
Joined
Nov 6, 2009
Messages
5,723
Reaction score
3,786
Location
La Paz, B.C.S., Mexico
Thinking about stroking or de-stroking a 340 but this is a general question. When stroking or de-stroking a motor, is it customary to change only the rod length or both the rod length and the piston compression height? I'm new at this, so may not understand all of the technical aspects of the answer. Thank you.
 
Both. If you put a longer rod in the piston will hit the head.



Ray
 
It depends, most 4" stroker pistons are for a stock length 6.123" rod and adjust the pin height in the piston. On my 408 I ran a Chevy sized rod 6.125" long, 2.1" rod journal with a .927" pin. I ordered my crank with the 2.1" rod journal because it opens up more options for rods and bearings. My new build uses a 4.125" crank with 2.00" rod journals and a 6.200" rod, obviously the pin is closer to the top of the piston than a 4" stroke piston.
 
If you are stroking it us a piston the that is designed for the stroke. If you go over a 4" crank stroke it gets a little bit more tricky.

usually you don't change the rod length.
 
If I de-stroke the motor, i need a longer rod to put the piston back up to stock. In that case, I don't think the piston would strike anything.

340: stock stroke 3.310/1.655 (50%) 339.44 cubic inches
305: custom stroke 2.96/1.48 (50%) 303.55 cubic inches
305: custom stroke 2.94/1.47 (50%) 301.50 cubic inches

340: stock block deck height 9.6
340: stock piston above deck .018
340: stock connecting rod length 6.123

305: custom connecting rod length 6.25 (0.127 > than stock)
340: stock piston compression height 1.840
305: custom piston compression height 2.00 (0.16 > than stock)


340: stock rod length (6.123) + stock compression height (1.840) + 50% of stock stroke (1.6550) = 9.618
9.618 - 9.6 stock deck height = 0.018 piston above deck (factory spec)

305: custom rod length (6.25) + custom compression height (2.00) + 50% of stroke 1.48/1.47 = 9.730/9.720
9.730/9.720 - 9.6 deck height = 0.13/0.12 piston above deck

alternate combo #1: custom rod length (6.25) + stock compression height (1.840) + 50% of stroke 1.48/1.47 = 9.57/9.56
9.57 (9.57 - 9.6 deck height = 0.03 piston below deck)
9.56 (9.56 - 9.6 deck height = 0.04 piston below deck)

alternate combo #2: stock rod length (6.123) + custom compression height (2.00) + 50% of stroke 1.48/1.47 = 9.603/9.593
9.603 (9.603 - 9.6 deck height = 0.003 piston above deck
9.593 (9.593 - 9.6 deck height = 0.007 piston below deck
 
I destroked my engine, only because I got a good deal on a high dollar 3.25 crankshaft. I am going to use a 6.300 long rod and a set of custom pistons.

If I didn't have this crank I'd be doing a larger stroke engine forsure.
 
You must log in or register to reply here.
-

Back
Top