<VV> Pin Offset - Description

Hugo Miller hugo at aruncoaches.co.uk
Tue Nov 5 17:33:05 EST 2019

Thanks - I understand the concept well enough - but I am amazed that I 
have never come across this in all the years I have been playing with 
engines. I have a sneaking feeling this is a US rather than a UK 
practice, and I intend to research this.
I found an article where an engne rebuilder had assembled a V8 engine 
with all the marks pointing towards the camshaft. You can see their 
logic, but all the marks should have been to the right (or left, I 
forget now). The fact that it subsequently rattled indicates that the 
few thou involved are enough to make a difference.
About forty years ago I had an Aston DB5 that had a horrible piston 
slap on one cylinder when cold. It was easy enough to identify the 
cylinder by pulling the plug leads off one by one, and I duly stripped 
the engine down. There was absolutely no visually identifiable reason 
why that piston would have made a noise, but I replaced it and the noise 
went away.
If you want something really weird, there is an ancient British car 
alled a Trojan. It has a sort of V twin arrangement, but one cylinder is 
an auxilliary pumping cylinder. The weird part is that there is only one 
conrod, cast in the form of a V. On each stroke, this has to bend a 
little. I refused to believe this when I first heard it, but I spoke to 
someone who owns one and he confirmed that it is correct!

On 2019-11-05 22:36, edward szuch via VirtualVairs wrote:
> Hugo;
> By pin offset, I mean that if you lay the piston and rod assembly on 
> a
> bench with the pin vertical to the world, it will not be centered in 
> the
> piston.  It is slightly offset side to side in the piston.  This 
> slight
> offset compensates for the gas pressure and resulting thrust and 
> cocking
> loads on the piston and piston skirt.  You're right, it is not a 
> large
> amount but cylinder pressures are high enough to move the piston 
> around in
> the bore within the very tight tolerances that are available and 
> create a
> tick or slapping sound as they are pushed down the cylinder.  I know 
> that
> may boggle the mind the first time you hear about it but there are 
> similar
> issues with piston skirt profiles.  Modern skirt profiles go through 
> a lot
> of development in terms of computer simulation and dyno/vehicle 
> development
> to determine the best profile that minimizes noise yet produces long 
> life
> without scuffing.  An operating piston has to be quiet when cold and 
> run
> without scuffing (micro-weld to bore wall) when hot.  Pin offset and 
> skirt
> profiles are two of the tools used in their design.  It's amazing how 
> just
> a thou or two of clearance can affect it all.
> Back to the pin offset.  The reason I brought it up is that I wasn't 
> sure
> if the after market piston was marked sufficiently or the re-builder 
> paid
> attention to the marks.  The Chevy Service Manual assumes you're 
> using
> factory pistons and rods where pin offset will be correct if the 
> arrow or
> piston mark is pointing forward and the rod numbers are up when 
> installed
> in the crankcase.  That's probably the reason it's not discussed more 
> often
> during engine discussions.
> Gary Szuch
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