piston pin offsets - too much coffee?

[David J. Campbell Jr.]

when reassembling my engine, I read all the books and followed recommended
procedures. BUT, my engine sat for ten, maybe 20 years prior to the
overhaul, and the acids in the oil (or whatever), after time, etched and
slightly pitted the cylinder walls, and that etching, or staining, was
mostly, but not entirely, removed by the honing. Purists and łstrict
interpretationists˛ may not want to read further -

In order to łfuture equalize˛ the situation somewhat, Iąd like to łrotate˛
the entire pistons and cylinders 180 degrees on my engine. In other words,
piston and cylinder one would go to position three, and position three
stuff to position one. łOld˛ piston and cyl number two would go to
position four, and łold˛ piston assembly four would now occupy cylinder
number two position. I am -not- talking about any łX exchanges˛ or
anything like that here. The net effect would be all piston arrows would
-still- face the flywheel, and their respective pistons would all -still-
be in the same bores they came out of, with their same rings, with the
same side of each piston and itsą bore łforward˛ as before, but what
-were- the łskyward side˛ of each piston & cyl assembly would now be their
łstreetward side˛, or bottoms.

And thatąs where the łpiston pin offset˛ question comes in...

Iąve stared at the diagram in the Bentley manual concerning łpiston pin
offset˛, and spoken to meachanic pals about this, and nobody seems to have
a complete answer. The diagram seems to indicate the wrist pin bore in
each piston is offset toward the side of each piston slightly. I assume
that when they make pistons, they make them all alike, with the offset
toward the same side (in relation to the arrow pointing to the flywheel).
I also kind of assume the łnarrow˛ side of each piston should be either
down on all pistons, OR up, but NOT łeither way, as long as the arrow
faces the flywheel˛. But if no attention is paid to łpin offset˛, and
pistons are all made alike, the piston pins will be offset upward on one
bank and downward on the opposite bank. Is that how it is? If not, then
the question is:

are the pistons made in TWO configurations, łoffset correctly for pistons
one and two˛ and łoffset correctly for pistons three and four˛? I mean,
itąs hard to imagine the offset being different, with each side of engine
being opposite the way the other side is, -but- none of the books I have
seem to indicate any -other- neccessary łpistion placement criteria˛ is
important other than the arrow, which, by ITSELF, isnąt enough information
to indicate to which SIDE of the piston the pin is offset toward (toward
the top or toward the bottom) ....

Iąm beginning to think it was designed so that 1 and 2 have the pin offset
upward (or downward, doesnąt matter for this discussion) and cylinders
three and four have their pin offset the opposite way, all this having to
do with with wether the crankshaft is łrising˛ or łfalling˛ on that side
of the engine...is that about it?

or do I need to drink less coffee?

thanks,

dave

[Jammin' Jeff]

David J. Campbell Jr. wrote:
>
big snip....
> .....as long as the arrow
> faces the flywheelē. But if no attention is paid to ģpin offsetē, and

> pistons are all made alike, the piston pins will be offset upward on one
> bank and downward on the opposite bank. Is that how it is?

That's how she is!!

> is that about it?
> or do I need to drink less coffee?
>
> thanks, dave

Drink more coffee, because you're thinkig too hard. Keep the pistons in
the same jugs and point the arrows forward (to the flywheel).

hope it works for you, Jammim' Jeff

[Rob Boardman]

David J. Campbell Jr. wrote:
>

> when reassembling my engine, I read all the books and followed recommended
> procedures. BUT, my engine sat for ten, maybe 20 years prior to the
> overhaul, and the acids in the oil (or whatever), after time, etched and
> slightly pitted the cylinder walls, and that etching, or staining, was

> mostly, but not entirely, removed by the honing. Purists and ³strict

> interpretationists² may not want to read further -
>

> In order to ³future equalize² the situation somewhat, I¹d like to ³rotate²

> the entire pistons and cylinders 180 degrees on my engine. In other words,
> piston and cylinder one would go to position three, and position three
> stuff to position one.

clip
> And that¹s where the ³piston pin offset² question comes in...
clip
> dave

In a 'description of changes year by year' list I have, it says
"1950 - Offset wrist pin to reduce piston slap".

I'd love to know exactly what 'piston slap' is - skyward/earthward
or fore/aft movement of the piston in the cylinder perhaps?

There was obviously a problem with the early engines, and my
guess is that they continued with the offset pins to reduce the
problem, so altering it might make for interesting motoring.
--
Rob
R.Boardman'at'bom'dot'gov'dot'au
replace 'at' with @
replace 'dot' with .
Use this address - incorrect address above to fool auto mailers
(1970 1500sp, one owner, 237,500 miles on it's original engine)

[Dave Hall]

You reached the right answer yourself, David. The push on each piston due
to the explosion will be the same side relative to the con-rod position, so
they are all working in the same way.
Piston slap is when the piston skirt hits the cylinder walls.
Dave.
--
Da...@spam.me.nothallvw.clara.co.uk
Please delete "spam.me.not" when replying!
Check out the VW Type 3 & 4 Club website
at http://home.clara.net/hallvw/
for great pics, news and technical advice.
Jammin' Jeff wrote in message <34952E...@concentric.net>...

>David J. Campbell Jr. wrote:
>>

>big snip....
>> .....as long as the arrow

>> faces the flywheel². But if no attention is paid to ³pin offset², and

[John Connolly]

> >> faces the flywheel². But if no attention is paid to ³pin offset², and
> >> pistons are all made alike, the piston pins will be offset upward on one
> >> bank and downward on the opposite bank. Is that how it is?
> >

With respect to the pistons, the crankshaft is rotating the SAME
direction when mounted in this manner.

As Dave said, the offset prevents excessive piston slap. Racing pistons
do NOT have this; more power is provided with a centered pin (AND
manufacurers are lazy since offset pins require more WORK to make :)
), even though they are slightly louder and ring life is reduced
slightly.

John

[DesertAuto]

In article <3494AD...@nospamgov.bom.au>, Rob Boardman
<R.Boa...@nospamgov.bom.au> writes:

>
In a 'description of changes year by year' list I have, it says
"1950 -
>Offset wrist pin to reduce piston slap".

I'd love to know exactly what
>'piston slap' is - skyward/earthward
or fore/aft movement of the piston in
>the cylinder perhaps?

Piston slap is the movement up and down in the cylinder. As the piston comes
back towards the case on the power stroke, it is pushingand will go whichever
way it can. This would be towards the ground on #1 and #2 and up on #3 and #4.
As it goes back toward the head, it will push the other way. When the piston
becomes to worn, there will be excessive slap. The offset wrist pin allows the

piston to be stronger on the side that is receiving the most abuse.

darryl
Desert AutoHaus
16574 D Street
Victorville, CA 92392
760-243-1868

[jd...@lehigh.edu]

I agree with everything Conman wrote except for the statement that centered
pins make more power. Actually offset pins make more power when they are
installed backwards from the factory position. Formula vee engine builders
discovered this many years ago, of course there is a downside, additional
noise and wear.

John Caffrey
http://www.win.net/~letsfly/4stroke/vw.htm
96 Protoform/JC Formula Vee

[John Connolly]

I forgot to add "than correctly installed offset pistons" ....

so flog me...

that trick has been around for years, but most of the advice here is for
street cars. We don't want to give some wannabe-engineer some ideas....

John :)

[con...@sisna.com]

For clarification, I consider piston slap a motion that is likened to
"cocking" in the cylinder. (if that makes any sense)

A rotation about the wrist pin axis.

John

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[Mike]

Wrist pin off-set is indeed intended to reduce piston slap, and most
engines use it. When pistons and cylinders wear, the pistons rock in the
bores when the motor is cold and produces and audible knock. What the
offset does is to produce a "pre-load" of sorts to minimize the knock. It
also produces a bit more internal friction and it's common racing practice
to switch the piston around to make the offset opposite of the way it was
designed to reduce this friction and thus produce a smidgen more
horsepower. (Drag racing trick in the stock classes where modifications are
prohibited...sort of).

At any rate it doesn't really matter where you put the pistons. But for
longevities sake and to keep 'er from knockin'. put 'em where they belong.
The pitting won't care where it is.

[Melissa & Jim]

d...@gdn.net (David J. Campbell Jr.) wrote:

>Išm beginning to think it was designed so that 1 and 2 have the pin offset
>upward (or downward, doesnšt matter for this discussion) and cylinders

>three and four have their pin offset the opposite way, all this having to
>do with with wether the crankshaft is łrising˛ or łfalling˛ on that side
>of the engine...is that about it?

You've obviously spent a lot of time thinking about this, but once you
get your engine apart you can measure and see that all 4 pistons are
exactly alike. The offset is in relation to the direction of crank
rotation, so the offset is up on one side of the engine and down on
the other.

I can see no problem with "rotating" your P/Cs as you suggested, but
there should be no real advantage either.

Jim
-------------------------------------------------------------------
Melissa Kepner Jim Adney
Laura Kepner-Adney
mgke...@facstaff.wisc.edu
Madison, Wisconsin
-------------------------------------------------------------------

[con...@sisna.com]

Just a side note to this discussion:

Changing the wrist pin offset ALSO changes the relationship between the
crankshaft and the pistons! So, this changes your cam degree index, and
your ignition timing!

The change is slight (probably only 1-3 degrees), but IS there! I haven't
thought about it much, but this is probably part of the reason for the
increase in power!

THINK ABOUT IT, it's sort of like changing your rod length (in a somewhat
related way). Pretend your pin is offset 1 inch or so.

At "TDC" the piston will NOT be at TDC! We have that offset to make up
for! Now, the crank will not have to move another inch, since the stroke
of the piston drops while it moves sideways. Somewhere between the two is
where the piston will "stall", effectively becoming TDC!

Hope this makes sense; if you don't understand, don't worry about it.
It's just something interesting to help people better understand the
internal combustion engine.

[JUSA14]

i get it!!!
juan