Theoretical Question re: Wrist pin offset

Locksmith

Silver Supporter
#1
There has been a question that has bugged me since I was a young motor head!
Regarding the offset placement of the wrist pin in a "V" type gasoline engine;
In relation to the piston and connecting rod, is it offset to place more pressure on one side of the wall to prevent chattering and wear?
Or is it offset to balance the piston's diagonal travel on the four cycles of intake, compression, power, and exhaust? I have spoken to many engine builders, and no one agrees.
I hope I can get the answer before I get cremated, and finally will that my ashes be mixed with racing oil in a Chevy V-8 just for one last thrill! LOL!
 

littleblazer

Gold Supporter
#2
There has been a question that has bugged me since I was a young motor head!
Regarding the offset placement of the wrist pin in a "V" type gasoline engine;
In relation to the piston and connecting rod, is it offset to place more pressure on one side of the wall to prevent chattering and wear?
Or is it offset to balance the piston's diagonal travel on the four cycles of intake, compression, power, and exhaust? I have spoken to many engine builders, and no one agrees.
I hope I can get the answer before I get cremated, and finally will that my ashes be mixed with racing oil in a Chevy V-8 just for one last thrill! LOL!
Provides one fluid motion, here you go. You can get away with it on an inline because it'll rock around no matter what.
Page46.jpg
 

MRRSM

Lifetime VIP Supporter
#3
"In a Nut Shell,
It's Piston Dwell."

... This from an Engine Builders Technology Site:

https://www.eng-tips.com/viewthread.cfm?qid=86266

"In order to achieve the earlier discussed "piston dwell" of 0.001" piston movement at TDC for 13 degrees of crankshaft rotation, the wrist pin position is offset approximately 0.060" in the direction in which thrust is applied to the piston from the diametric center of the piston to accommodate a rod length of 6.5". Additionally, a piston radial clearance of 0.006" was selected to provide a small amount of piston "rock" which adds to the piston dwell. The combined effects of the offset and clearance permit thrust forces to offset the piston as the rod connected crank journal passes over dead center resulting in a closer spacing of piston top to journal axis than in the case with conventional construction. After the journal passes dead center the thrust forces are relieved and the piston centers itself. This centering action has a movement vector away from the journal and therefore it assists in maintaining the piston near top-dead-center."
 
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Locksmith

Locksmith

Silver Supporter
#5
Thanks MRRSM. My nutshell mind is gonna implode. How is that translated in layman's language? I grasped 75% of it. So am I to understand that in an inline engine, it has no offset, and on a V-8, it is offset so that on the power stroke, the piston and skirts are thrust towards the outer "V"?
 

MRRSM

Lifetime VIP Supporter
#6
Sorry for all of the Technical Gibberish... but to boil this idea down to its essence... The Goal of the Piston is to Compress Air and Fuel enough to magnify the resultant Power of Combustion...but also ... to do so in a way that does NOT cause the Piston to Lock in an Upright Position and stop or impede the Clockwise Rotational of the Crankshaft Force Vector in a manner that @littleblazer clarified as "smoothness".

In order to achieve this Maximum, Symmetrical Force.... the Piston(s) MUST remain still at Top Dead Center LONG enough to get as much of the seriously compressed Fuel Air Mixture to burn efficiently and create enough force to push the Piston straight back down inside of the rigid cylinder. So there has to be a little bit of a "Cheat" in order to ensure that the Direction of Rotation is enhanced as the piston heads back downwards...and having the Wrist Pin positioned slightly off center ... but leaning in the Clockwise direction of Connecting Rod is a MUST if this is to be as smooth a transition in the Force Vector as possible. I hope that explanation makes a little more sense.
 
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Locksmith

Locksmith

Silver Supporter
#7
Enlightened! TY! Now to figure out what non ash forming oil to add my ashes to.......
 

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