[vidguy]

8) With the "notch" facing forward on the #1 piston and the piston having a slight offset towards the major thrust, it will be happy. The major thrust side of the cylinder bore will always be on the left side of the cylinder (drivers side) when looking from the flywheel.

Thanks Tom!
That was the clarification that turned on my light bulb.

ps The first set of piston pictures is correct. The second set is wrong.

And that clarification caps it.
I think NOW By Jove! I finally got it!
So can be one piston made for the motor.....
Much appreciated..

[Tom Vaught]

If the piston has the offset built in you need two sets of valve reliefs (4) to make it work.

If the piston has no offset then the following would happen:

Lay out the pistons like this:

-O1O -O2O -O3O -O4O
-O5O -O6O -O7O -O8O

The dash - you see is a magic marker mark on the pistons.
The O is a valve relief and the 5 is the number of the piston.

The left bank would look like this:

Front <--- -O1O -O3O -O5O -O7O

The right bank would look like this:

-O8O -O6O -O4O -O2O --> Front

All of the pistons could be the same but half would have the
Magic marker mark facing the rear of the engine (only two valve
reliefs and the valve reliefs would be the same size.)

If you had a large and a small valve relief you would need 4 and 4 pistons.

Hope this helps.

Tom V.

[Jack Gifford]

Looks like everything's falling into place pretty well now.
As long as we're beating this subject to death- a couple more points-
> The "major thrust" side of the bores is the passenger side. I tend to think backwards on this- viewing the engine from the front and picturing it rotating CW, my mind wants to imagine the "more CW" walls (driver's side) as major thrust- wrong!
> Every aftermarket Pontiac piston I've owned had the wrist pin holes centered, although there are apparently some with the 1/16" offset.
> Pontiac literature describes the early-rod spurt holes as providing oil only for opposite-bank cylinder walls, pistons, rings, and wrist pins- not the cam.
> The purpose of the slight fore/aft offset of connecting rod big/small ends is debatable. Some say it has a design-purpose, to influence the location of rod side clearance in a running engine. My guess is that it's merely a "fallout" of other engine dimensions.

[gtofreek]

Here is a good article that explains piston pin offset.

http://www.motorcycleproject.com/mot...ws-offset.html


Here's another oft-misunderstood bit of techie trivia -- what is piston pin offset for? That is, what purpose does it serve? Let's examine that one.
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. Make mine well done, please.

[Tom Vaught]

Very Good Post, Paul.

We are saying the same thing Jack just talking about it in different ways. You are thinking about the cylinder bore walls and the forces there and I am discussing piston dynamics as Paul mentioned. The discussion was about the pistons right? Piston slap will be higher with ANY centered piston (aftermarket) vs an oem part designed to reduce piston slap.

The second thing is getting people on the same page on how they are looking at the problem.

Remember, you were the expert on the roots blowers and still learned something.

Tom V.



Tom V.

[dreambird]

Sorry to dig up an old post such as this to kick it around a bit, but when thumbing through my factory service manual and comparing VIDGUY's diagram to the factory service manual, I can help but be a bit confused on the offset diagram. My factory service manual states that the offset of the pin in the piston is 1/16" from the centerline of the piston to the drivers side of the engine as you are looking at the back of the motor. This puts the pin closer to the drivers side of the bore in relation to the centerline of the piston. This jives with what Tom said in his post:

6) Wrist pin offset is related to rotation. The offset is designed to direct the angular resistance of the rod on the power stroke more closely to the center of the piston as the rod moves through the stroke. Usually, the piston pin is offset slightly toward the major thrust face to reduce. piston slap against the cylinder wall.

7) With the "notch" facing forward on the #2 piston and the piston having a slight offset towards the major thrust, it will be happy.

8) With the "notch" facing forward on the #1 piston and the piston having a slight offset towards the major thrust, it will be happy. The major thrust side of the cylinder bore will always be on the left side of the cylinder (drivers side) when looking from the flywheel.

VidGuy's drawing shows the offset towards the passenger side. Not to pick apart such a nice drawing, but is this right?

[vtxman]

Originally Posted by gtofreek

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. Make mine well done, please.

Here is another question I have.......If the offset in the piston serves two positive purposes, why are the aftermarket performance pistons manufactured without the offset?

[Tom Vaught]

Quote:

"Here is another question I have.......If the offset in the piston serves two positive purposes, why are the aftermarket performance pistons manufactured without the offset?"

1) Race guys don't care about offset (or piston slap noise as the car's exhaust and valvetrain is usually louder. LOL!

2) OEM manufacturers have different opinions on how much offset is needed vs the specs on the bottom end of the engine vs NVH allowed
(Noise, Vibration, Harshness).

Tom V.

[Skip Fix]

But Tom what about the guys that flip the offset on the pistons on purpose?

[vidguy]

Pick it apart please!

I created it so that we could have a visual reference and as you can see I really wanted to understand. The intent was to have those that understand, explain it.

What is confusing might be the way that the offset is described:
Is the offset,describes as the side of the piston where the most MASS is?
(this is the opposite of the Pin offset....)

You have to consider which way the Crank is turning and how that creates the thrust.

Tom's response was:
that the TOP drawing is correct - in a Pontiac Engine.

Piston notches to the front
Rod tangs always up and hopefully rod chamfers ground correctly.

Tom V.

ps The first set of piston pictures is correct. The second set is wrong.

The two drawings I created represent the two possible ways the offset could be used.

The Top Drawing = Picture of piston with SAME offset on either bank
(ONE PISTON IS MADE FOR THE MOTOR)
-----------------------------------------

The Bottom Drawing = Picture of piston with MIRRORED offset on either bank.
(ONE PISTON IS MADE FOR THE MOTOR)
-----------------------------------------

I'm glad you brought it back up, and if I need to clarify with a new drawing, I will!

[dreambird]

The only reason why I caught it was that I was reading my factory service manual and I was a bit confused on the drawing in the book so I searched the forums and came up with this post. If I can scan the picture at work tomorrow and attach it to the post I will. My service manual shows the center line of the piston and the center line of the pin. when looking at the picture of the 2 assemblies (left bank on the left and right bank on the right), the centerline of the piston pin is offset to the left of the centerline of the piston by 1/16" for both banks, putting the pin further to the left of the piston centerline. So, according to my service manual, if you drew the picture using two right side pistons from the first drawing for both left and right, that would be just like my chassis manual. In other words, a mirror immage of the offset asymmetrical drawing. That is assuming that my chassis manual is correct.

[vidguy]

We definitely want the correct info up here. So let me know. I'll look at my 67 service manual too.....

I just looked, and the drawing in my manual is the same one you're looking at I bet.. Fig 6-80... The digram says Right bank.(on the left) and Left bank (on the right) This means that the view is from the front, I think...

On page 6-75 of my 67 Manual, it refers to that diagram and it states:
"Piston pins are offset 1/16" towards the thrust side (Right-hand Side) to provide a gradual change in thrust pressure against the cylinder wall as the piston travels its path."

So I think that my Diagram reflects that. The manuals viewpoint is different then mine. They are looking from the Front, Mine looks from the Rear. That is confusing in this context.
Again I think this is correct. If I'm wrong let me know. Thanks!

[dreambird]

LOL...Our diagrams look the same except for the fact that mine are labled left on the left and right on the right. The manual states that the left bank cylinders are numbered 1-3-5-7 , etc. Manual than states that the piston pins are offset 1/16" toward thrust side (right hand side) to provide gradual change in thrust pressure against cylinder wall as the piston travels it's path. Based on what you have shown, I think your offset assymetrical is right. My diagram in the book is somewhat misleading.

Thanks for helping out a newbie such as me!

[vidguy]

Now aint THAT confusing...
Heck I wonder who's manual is correct....! Dang. And what they consider left and right?

The old timers here are gonna start shaking their heads wondering why in the world we are still talking about this..

It started as a question about how the Rods get installed on the piston, and their proper installation on the crank journals, and why. And evolved into the theory of the need for piston offset.

I learned a lot from that, and the theory and background is interesting!

So one last time, If this is the right diagram, I'll let it rest.
Someone please clear this up for us dummies.
----------
Is this the way the offset is machined into the piston (on a Pontiac V8 anyhow), assuming Tangs are UP on the rods.
Thanks to all of you knowledgeable fella's that have taken the time to discuss and help explain this!

It IS appreciated.

[badlefthand]

Dug it up again. Just as clear as mud. Whenever I want to be confused -I just read this thread. On older pontiac large boss on left bank goes fwd-small boss on right side goes fwd or is it backword. I got a headache. Last year for the first time in 50 years I bought a short block from flightline engines.60 389 Never trusted anyone to put my engines together,Started to examine assm and just reinforsed my convictions rods and pistons are installed backwards inside out and every way but up side down that is why I am useing the thread.Somehow I think I got what I deserved for takeing a short cut.Got to rave a bit. Dumb. Thanks

[gtofreek]

The easiest way to remember it is like this:

The bearing tangs face in towards the camshaft on both banks and the notch in the top of the piston faces forward on each bank. If you have pistons with only 2 valve reliefs, the valve reliefs must face in towards cam. If the rods have oil squirter holes in the side of the big end[they will be on the bearing tang side and later rods don't have them] then the squirter holes need to face in towards the cam.

Also remember that on Pontiacs the number 1 cylinder is on the left bank[drivers side] which is BEHIND #2 cylinder. One of the few engines that is numbered like this. Shared only with the Y-block Fords[272,292,312], as far as I know of anyway.