Page 201 of Larry Shepard's "Magnum Engines" book
says that the piston pin is offset for lower noise,
but that piston/con rods can be swapped
left to right bank
to get lower internal engine friction
and more torque.

I think Tom Hoover used to cover this is his traveling tech talks too.

Does anyone know how much in inches
the factory Magnum
5.2 and 5.9 pistons are offset ?

Does anyone know what previous
LA and A engine series
pistons were offset?

Does anyone know the numbers
for any offsets of
GM or Ford pistons ?

Does anyone know,
or have a weblink to,
how to calculate the maximum offset of a piston pin
before 'bad things' happen
besides more noise?

I know that on the original Honda Insight 3 Cylinder
the centerline of the crankshaft was ofset
rather than the piston pin.
Is this approach better?
Better rod angle?
Even more noise?

Thanks in advance for any guidance

I reverse mounted some cheap cast badger pistons(sb) for a circle track eng but I havent used it. It is my understanding that the rod squirt holes throw too much oil onto the cyls & alot of bearing dont even have the groove for oil to reach the squirt holes I was wondering if there'd be a problem if rods were reversed.

I have heard someone claimed reversing the BB rods and getting 32MPG on a 440 in an E body

By right you reverse the piston on the rod. the pin location on the piston is slightly offset. It is/was done to reduce a tendency for piston slap. There has been evidence of a slight increase in power in stock applications using stock pistons. After market pistons do not have the off set pin bores I have been told.

I just put Keith Black KB745 forged pistons in my stroker small block and the pins were not offset.

thanks for replies so far

I have heard from a retired GM engineer that he thought their past factory pistons were offset 0.03 to 0.06 inches in the direction of less noise,
and when the present 5.3L V8 went less in the other direction to get a bit lower friction,
that is when the Silverado pickup customers began to sue for 'piston slap' noise.

what was on the internet:
===========

"Offset Pins

Some of our pistons are supplied with offset gudgeon pins. This is to help
reduce piston "slap" and have a pin that is offset to the thrust side.
Although all piston crowns are marked accordingly, additional machining
processes can inadvertently remove these markings. Offset pin pistons fitted
incorrectly will result in mechanical noise and reduce efficiency. Race
pistons defer almost exclusively to a central pin design, a necessity for
engine speeds over 10,000 rpm.
Accralite Performance Forged Pistons "

"Most engines today utilize pistons with an offset pin bore. That is, the pin
bore is "moved" a specific distance from the centerline of the piston. In gas
engines the offset is always to the major thrust side of the piston. The
piston thrust side is the part of the piston perpendicular to the pin bore
that carries the majority of side loading during the power stroke.
The primary reason for pin offset is to prevent the piston from slamming into
the cylinder bore after the connecting rod journal passes top dead center.
This problem is referred to as piston slap. The desired scenario for piston
movement is to gently rock from side to side within the bore. This rocking
motion eliminates slap. "
by Hunter Betts

"
Modern powersports engine pistons are not mounted on their connecting rods
exactly on center. They are offset radially, that is, perpendicular to their
rotational axis. In other words, in a vertical, single-cylinder engine, the
piston is offset toward the exhaust side of the cylinder. That is, the piston
pin hole is bored off center toward the intake side, usually less than 1
millimeter. It is frequently stated with a sort of bored complacency that the
purpose of this offset is to reduce piston slap. The truth is, while piston
slap is reduced through piston pin offset, but that is not the purpose of the
technology.
The piston engine has three major parts: crankshaft, connecting rod, and
piston. Each has a different job to do. The reciprocating part -- the
piston -- makes the crankshaft -- the rotating part, uhh, rotate. The
connecting rod is simply the part in the middle. It translates the piston's
recip motion into the crankshaft rotary motion. The neat thing is that in the
process, it shares in the motion of both. That is, the connecting rod is both
a reciprocating part and a rotational part, at the same time. (In fact, when
balancing an engine, it is common to divide the connecting rod's weight in
two, thus permitting half its weight to be calculated as recip and half as
rotation.) The point is, the upper half of the connecting rod reciprocates
with the piston, the lower half rotates with the crankshaft, and this is
important to understanding the stresses on all three parts, but especially
those on the connecting rod.
The piston and its half of the connecting rod stops twice per crankshaft
revolution, even though the crankshaft continues to turn. This means the
piston and top of the rod also start back up twice. This stopping and starting
imposes stresses on all three of the parts, stresses that increase with
crankshaft rpm. To reduce these loads, the piston is mounted to the connecting
rod slightly offset. This causes the piston to reach top dead center at a
different time than the connecting rod,
effectively spreading the shock loading over a greater number of crankshaft
degrees. In short, the real reason for piston pin offset is that it softens
reciprocal loading, permitting lighter more power-efficient parts to be used,
and the engines to be capable of higher rpm.
However, there is another phenomenon at work also, a kind of side benefit.
Because the connecting rod spends most of its time in the engine at an angle,
the piston engine has what is called minor and major thrust. Major thrust
refers to the downward-stroking piston's force against the cylinder wall
during combustion, due to the rod being angled in that direction. Minor thrust
is the piston's thrust against the opposite cylinder wall during compression,
because the rod's angle is opposite also. These thrust forces push the piston
firmly against the cylinder wall. The important thing is that at TDC, they
flip-flop. Major thrust turns into minor thrust, and visa-versa. In older
engines, this flip-flop caused the piston smack the cylinder, resulting in a
noise. Fortunately, the piston pin offset in today's engines, besides reducing
inertia stresses, does two things that reduce this noise. First, because the
piston is mounted off center, the transition from major to minor thrust is
less sudden. There is less impact. Second, instead of a sudden lateral shift,
the piston actually rolls from major to minor thrust. That is, the piston
shifts first at the skirt, then gradually the rest of the piston makes
contact, instead of all of the piston at once.
To summarize, piston pin offset is the manufacturer's way of reducing stress
on reciprocating parts. It permits these parts to be lighter, which results in
more efficient manufacture and less power loss in the engine, as well as
higher rpm. A complementary result of piston pin offset is reduced piston slap
due to the more gradual shift from major to minor thrust. "
by Mike Nixon

My original 340 pistons had offset pins from the factory so this is not a "modern" concept.

Offsetting the pin towards the major thrust side increases the effectivel rod length on the power/intake stroke and reduces it on the compression/exhaust stroke. The worse the rod ratio is the more benefit seen from reversing OE style offset pistons. Ford 289/302 guys claim they pick up around 10hp reversing a .060 offset piston. I remember some of the engine masters guys did it as well. Some custom piston companies charge about $50 to offset piston pins .050 to .062. Some will do it for no additional charge (they usually go from zero offset to .050 away from major thrust to reduce noise like OEM pistons). If you dont mind the noise I think you will see a small incremental power increase (my guess: 4-8hp reversing an OEM piston, more in low rod/stroke ratio applcations).

thanks for replies

i am primarily interested in how much friction reduction there would be in the 1500 to 2500 rpm range on a 3.58 stroke

a bit more info:

http://staff.washington.edu/ryanlee/Engine/UofWindsorManual/Piston%20Design.htm

sample quote

"Some Briggs & Stratton engines use a barrel-shaped piston skirt. The barrel shape provides a smoother transition during directional changes of the piston. The piston rolls into the cylinder wall when changing direction at the end of a stroke. This reduces noise, spreads the force of the directional change across a greater surface, and reduces side loading on the piston skirt. Some piston designs have the piston pin offset from center in the piston. The proper orientation of the piston pin offset is marked by a notch or an arrow on the piston head. The mark on all Briggs & Stratton pistons should be facing or closest to the flywheel on all one- and two-cylinder engines. The offset piston pin design offers a quieter running engine by reducing piston wobble and related noise.

in a related area
Does anyone have any information
or experience
about how much
Noise increases
with full floating piston pin set-ups
over factory pins pressed into the connecting rod?

Any info (dyno tests?)
on whether there is less friction
with the full floating pins?

thanks in advance for any replies

360, the benefits of this type of change are so minute... One wants to ask why...lol. They were offset for noise and other concerns, none of which added power. No aftermarket piston I know of (even stock tyep replacements) even come that way anymore. I am sure you could order them, but the "RH" and
"LH" pistons were for valve reliefs, not pin offset. The circle trackers used to swap them in stock applications... but again, not sure whyat this point unless you are class limited why you might want to. It's akin to worrying about the increased frictional loss the 340 main bearing width has vs the 360 main bearing width.

In terms of noise... when a forged piston with full floating pin has expanded, and assuming the bore clearances were set properly, they will be no noiseier than a cast or hyper piston. They may be noisey when started cold. And it sounds like the rattle the pins in my Neon, and the pins in the 2.2L/2.5L I-4s had.

I agree the gains or losses are small.

However, when you realize that gains
in reduced engine friction
or fuel economy
are usually less than
1% per year
committing to learn
every little detail
is worthwhile.

How many percent of HP
separates first place from second?

I applaud your initiatives. Looking at every detail does have it's rewards. Have you also looked into smaller (narrower)ring stacks as they are MUCH more of a "drag" on the rotating assembly. And coatings... And oil control in the crankcase. Those three will also yield good results for fairly small effort.

there is definitely a trend toward
thinner and thinner rings

man, the 1/16th rings look so thin to me
that i admit I am uneasy about using them

I have been thinking about swapping
bank to bank on an old
Magnum 5.9 V8
to see if the piston pin offset
friction reduction results in any measurable
fuel economy difference.

One thing that I have also considered
is doing a 'Total Seal' type ring replacement
at the same time,
but I am uneasy about doing two changes at the same time

quote from the engine section of the Ram MPG FAQ:

"Rings and Friction

There has been a fairly clear trend over the last few decades to go to
thinner rings in order to create less friction as the piston goes up and
down in the bore.
1971 to 1992 LA 360 V8 top two rings were 0.0781 inches in thickness.
1994 to 2003 Magnum 5.9V8 rings were 0.061 (1.54 mm)
2003+ 5.7 V8 Hemi piston rings are 0.0582 inches (1.481 mm)
2008 408 ci strokers with custom pistons from Diamond are being built with 0.043 inches (1.09 mm )

There are lower friction coatings to use on bores to reduce friction even
further, but the most common coating called NikaSil can be damaged
by high sulfur in gasoline, as BMW found out in an expensive way:

http://www.bmwworld.com/engines/nikasil.htm

From what I have learned through experience and from others Total Seal rings are fine in race motor but not something that is going to be run on a regular basis.There is a lubrication issue with extended run time.

perhaps shot peening the factory piston skirts with the right size balls might be a better bet.
From a Honda press release about the pistons in the 3-cyl Insight's engine:


LOW-FRICTION PISTONS
In the interest of friction reduction, Honda engineers specified a new
lightweight aluminum-alloy piston design for the IMA engine. The pistons have
a minimal skirt area and the surface of the skirt has been shot-peened.
Shot-peening is a process in which a metal part, such as a piston or
connecting rod, is blasted with shot-like particles, creating uniform,
microscopic dimples on the surface. This dimpled surface is better able to
retain a lubricating oil film. Shot-peening the IMA engine's pistons accounts
for another 1.5- to 2.0-percent reduction in internal friction.