<VV> Piston Slap - Pin Offset

[James Davis]

Yes it makes a great difference for power and slap.  All pistons with a pin
offset (99%) will have a notch in the piston crown or an arrow showing the
orientation of the piston in relation to the flywheel.  The amount of
offset is determined by the angle of the connecting rod where maximum
chamber pressure is developed.  The object is to reduce the piston's side
load as it pivots on the piston pin.  Installing the pistons backwards
increases noise ad the skirt of the piston moves to the side with great
force.  This causes increases piston skirt wear and a reduction in power..
Jim Davis

On Mon, Nov 4, 2019 at 4:29 AM Hugo Miller via VirtualVairs <
virtualvairs at corvair.org> wrote:

> Interesting - you mean offset across the engine, not in line with the
> crank? I've never heard of such a thing. Not saying you're wrong, just
> that I've never heard of it. I did hear of one engine (Cadillac maybe?)
> where at some point in mid-production they reversed the pistons as this
> made them quieter. But I have a feeling that this was a split-skirt
> piston. Frankly I'd be surprised if a fifty or sixty thou offset mould
> make much difference to anything, but that's just my opinion of course.
> Most pistons in my encounter have an arrow or similar denotng which way
> round they should be fitted.
> How would you even notice such a small offset I wonder?
>
>
> On 2019-11-04 03:27, edward szuch via VirtualVairs wrote:
> > Regarding the piston slap discussion.......
> > When you installed the new forged pistons, was there a way to
> > determine if
> > there was any piston pin offset in the piston and if so, did you take
> > this
> > into account when installing the piston on the rod?
> > The Chevy shop manual makes no mention of this because they always
> > refer to
> > keeping rod, pin and piston as a complete set from the factory, which
> > would
> > have the correct pin offset.
> > However, pin offset is very important for noise in particular and I
> > know of
> > no engine that doesn't have some amount.  Pin offset tends to
> > transmit
> > combustion loads more gradually to the skirt (cocking in the bore)
> > and
> > minimize slap.
> > If you have a copy of Bill Fisher's "How to Hot Rod Corvair Engines"
> > you'll
> > see a couple of paragraphs on the subject on page 58 (revision III).
> > It
> > says there is a 0.055" - 0.065" pin offset on the stock Corvair
> > piston.  It
> > gives a brief sketch and instructions for ensuring the correct
> > rod/pin
> > installation.  I'm not sure what your forged piston has for offset
> > but it
> > should have some.
> > It only takes one of them to make noise and since I saw no mention of
> > it in
> > your e-mail, I thought it might be worth consideration.
> > If you don't have the Fisher book, I could try scanning the page and
> > e-mailing it to you.
> > Hope this helps.
> > Gary Szuch
> > 248-600-7473
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