offset wrist pins

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bentvalves
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offset wrist pins

Post by bentvalves »

I have a v8 engine that came in already disassembled (mopar)

the wrist pins are offset about .080" in the piston from center

4 rods were hung one way, 4 rods hung the other.

I do not know which way this offset should be oriented in the V during assembly.

is this offset/orientation relationship the same for all V style engines?
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Re: offset wrist pins

Post by SupStk »

As installed by the OEMs the pin offset is toward the major thrust side.
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Re: offset wrist pins

Post by hoodeng »

As per SupStk, In a petrol engine the offset goes to the thrust side of the bore. [opposite to the cranks rotation, in a ccw the right is the thrust face.]

If there are unequal piston reliefs, that will dictate orientation, but it is highly unlikely this would be contradictory.

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Re: offset wrist pins

Post by modok »

Yep
Short side is the thrust for the power stroke.
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Re: offset wrist pins

Post by bentvalves »

thanks fellows.

also, per an article in engine professional - Mike Mavrigian

The pin offset is biased toward the major thrust side of the piston. In a clockwise-rotating engine, the major thrust side is at the intake side on the left (driver) bank and the exhaust side on the right (passenger) bank. In a reverse-rotation engine, the thrust sides are opposite: the major thrust side will now be at the exhaust side of the left bank and the intake side of the right bank.
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Re: offset wrist pins

Post by 6sally6 »

The same as SBF engines?!
Old trick was to swap pistons from side-to-side to improve angle of piston-to-crank pin.
Pick up a few HP with the cost of a little more noise on cold crank up. (no big deal when running headers!)
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Re: offset wrist pins

Post by MadBill »

hoodeng wrote: Fri Jul 09, 2021 9:06 pm As per SupStk, In a petrol engine the offset goes to the thrust side of the bore. [opposite to the cranks rotation, in a ccw the right is the thrust face.]
Not the best example; the vast majority of car engines rotate CW as seen from the non-flywheel end, making the left side of the bores the thrust side.
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Re: offset wrist pins

Post by rebelyell »

to lessen any confusion re major thrust face etc, OE-type piston makers usually permanently mark each piston crown's edge with a dimple, F, notch, arrow etc to denote that side of piston should face FRONT of motor ... installed accordingly, pin offset will be to major thrust side & correct per OE. afaik OE purpose is to lessen slap.

reversing offset was also a smokey yunick thing (in restricted motors which required OE-type pistons) ... more piston dwell thus restricted cam behaves as though it has a tiny bit more duration.
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Re: offset wrist pins

Post by Dan Timberlake »

Reversing offset was also a fairly standard practice in the MOPAR Performance/Direct Connection tech papers for all engine families.
A little power at the expense of noise.

I'd be prepared for the possibility of some risk of un-intended consequences with an early days unsophisticated knock sensor.
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