Re: [Amc-list] piston pin offset

[adh@xxxxxxxxxxxxxxxxx (Sandwich Maker)]

" From: Doug Dornbos <dornbos@xxxxxxxxxxx>
" 
" Hi all
" 
" the Jeep Engines Book says on p. 140 (pertaining to the 4.0):
" "Factory pistons have the pin offset to reduce piston slap. By
" reversing the offset (reversing the piston), you can reduce engine
" friction."
" 
" and then on page 142 (pertaining to the 4.0):
" "It's commonly known that you can turn pistons around. The piston,
" when originally made, has an offset pin. This offset pin makes the
" engine run quieter. By reversing the piston (the arrow on the top of
" the piston now points to the rear of the engine instead of the front
" ('stock' installation position), you reverse the centerline placement
" of the pin. This makes the engine noisier, but it will also produce a
" little more torque."
" 
" and then on page 212 (pertaining to the 4.2):
" "The piston pin boss is "offset" from the piston centerline to place it 
" nearer the thrust side of the piston. To ensure correct installation of 
" the piston in the bore, a notch is cast into the top perimeter of the 
" piston head. The notch must face forward."
" 
" here are my questions (specifically in regards to my 4.2):
" 1. does it change the effective stroke and/or the effective
" compression ratio to turn the piston around?

it might have a minuscule effect on c/r; i'd be surprised if it was
measurable.  it does not change effective stroke.

" 2. if I switch to Chevy pistons, will they have an offset pin? if not,
" what are the implications of using a non-offset piston in this
" application (258.)

engines that don't have offset pins are very rare.

data point: gm tried zero or 'backward' pin offset on one of their
recent crate motors [zz4?] but got so many complaints about the noise
that they gave up trying to explain and just changed the offset back
to 'normal'.

" 3. can I just simply take the pistons & rods out of the hole, turn
" them around backwards and put them back in? or is there more to it
" than that?

if the dish is symmetrical you can.  this is an old hotrodder's trick.
iirc you leave the rods alone and reverse the slugs on them.

" 4. does anyone have experience with this?

old hotrodders...
________________________________________________________________________
Andrew Hay                                  the genius nature
internet rambler                            is to see what all have seen
adh@xxxxxxxxxxxxxxxxx                       and think what none thought
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