[vidguy]

I guess Im a thread killer...

I'm reposting this in it's own thread to see if someone can clarify or confirm my understanding of the way to tell if your Pontiac pistons and rods are mounted properly. This relates to the orientation of the Rod, Any Rod chamfer and the bearing tangs taking into account the Offset on the Pontiac Piston wrist pin.

In an effort to help ME understand this and to create a VISUAL reference of this Rod / Piston / Bearing issue.. I created this little Flash gallery of Pontiac Rod pics that I marked up.. Please review this and let me know if this is CORRECT..

http://www.vidguy.com/carstuff/pistons/PontiacRods.html

[DiamondJim]

Nice presentation! Without seeing the valve releifs it is hard to tell on the pistons, the only notch I have ever seen on pistons was meant to point to the front. The rods are correct. -Jim

[Jagtec1]

On a Pontiac, when holding the piston/rod assy in your hand with valve reliefs at top, the chamfer should be to the right, always...for both banks, doesn't matter if it's DS or PS. This will ensure the proper rod to journal relationship. Simple, easy.

[HWYSTR455]

I've heard this debut before, as to the relationship of the tangs on the rods pointing away from the cam, and honestly, why bother? The theory USED to be that in an all-out race engine, on a particular stroke, the load would be in an area that has more bearing material, or at least that's my understanding. (If I'm mistaken, please, someone correct me). How much of a difference is that really going to make?

I believe that what matters is to ensure the chamfer on the rod big end match the radius on the crank journal. If you wanted to change the relationship of the bearing tang in relation to the "cam", pointing away, then you would either have to purchase rods that are left-hand & right-hand (no one I know of makes those, unless custom), or you would have to have the rod chamfered on the opposite side. That brings up the question of what (and how much) of an effect that will have on rod side-clearances, cooling due to more oil retention, and overall strength.

Valve relief orientation needs to be correct too, and that is effected/determined by how it was installed on the rod, in relation to the chamfer.

[gtofreek]

With the chamfer against the crank radius, the bearing tangs will be facing in towards the cam. On older rods the oil hole will be on the tang side also. The rods are slightly offset so it does matter how they go in besides just the chamfer and tangs. You cannot turn them around or they will bind. The pistons have 4 valve reliefs so as long as the notch is forward, the piston offset will be correct.

[HWYSTR455]

I thought only some factory, and some 'cheap' replacement pistons had 4 reliefs? My Ross don't have 4, that's for sure. Hm, I haven't used any TRWs lately, can't remember if the 'L's' have 4 or not. If it does have 4, they can be used on either side, and if it's a press pin, it can be offset to either side, right?

i've seen factory rods that have the slight offset you speak of, but it's always been to the correct side regarding the chamfer on the ones that I recall. Aftermarket rods don't have this offset, or at least the Crower Billets and Eagles I've seen don't. Anyway, your saying they would bind against each other if the offsets face together?

[vidguy]

Thanks for the added clarification guys..

So it sounds like the TANG/ Bearing orientation is not so critical (unless the bearing has a chamfer) as the ROD chamfer orientation is. The Sealed Power bearings I have here to use in my build do not have a chamfer, for instance.

If there is a "Correct" orientation, and that orientation is as described here, in looking at my pistons and rods, another way to state the "correct" rod orientation is to say that the bearing tang side of the rod gets installed to the side of the piston with the larger offset between the skirt and the wristpin.

I had read a couple of opinions about this orientation, some have said that swapping the Piston/rods side to side makes the rod act LONGER due to the offset and that some folks have run them like this succesfully albeit with noise.

Are the Pontiac RODS offset or are the Pistons wrist pin holes offset?

Also why not a chamfer on both sides of the rod as hwystr455 mentioned? Is this simply bad engineering for oiling purposes? Does it lessen the lubricated surface on the bearing adding to heat? Or is it a rod to rod issue on the crank?

[Mr. P-Body]

Guys... Guys...

Too much debate over a engineering issue... For assembly purposes, it's really quite simple.

First, both the rod and the piston are "offset". That is, the center of the rod, in relation to the bores, is off jut a bit, to the INSIDE. That makes the chamfers opposite on each rod per journal. The piston pin is offset to the non-thrust side, to reduce thrust force during reciprocation.

The purpose of the chamfer and offset, is to allow the oil to escape the rod bearing. The rods are chamfered, stock. When resized, the chamfer is "re-cut". Sometimes, the machinist can get a little over exhuberent, chamfering both sides equally. You MUST use the tangs as a reference. The tang "locates" the bearing, which is also offset in the housing bore.

If you look at a Pontiac block, you will notice it is more like a Ford than any GM engine. It's "laid out" like the Ford (furthest cylinder forward is on the right, not the left), but numbered like GM. Some Ford engines also have the "tangs in".

Yes, Speed Pro (TRW) pistons have 4 reliefs. They also are marked "front", to put the offset in the right direction. Pistons with only two reliefs ALWAYS have the reliefs toward the top, and have no offset. With the two-relief pistons, the bearing tangs line up with the reliefs.

All in all, it simply isn't as complex as some are trying to make it.

FWIW

Jim

[vidguy]

Jim-
Thanks for all the insight. All of that which you described is very good background and answers many of the WHY questions.
Much I didn't know, and had never read or heard explained, but I'm a novice here..


For instance now i understand what DiamondJim meant about the Valve Reliefs. I kept wondering how you could orient them so they were not "UP"?? I just did not know there were 2 relief pistons.

When you are about to assemble your first Pontiac engine, it's great to know WHY something is as it is, so that when you are not sure about something you can make an educated assumption based on that knowledge..

Thanks again. This thread is a a keeper for me now.

[gtofreek]

Originally Posted by HWYSTR455

I thought only some factory, and some 'cheap' replacement pistons had 4 reliefs? My Ross don't have 4, that's for sure. Hm, I haven't used any TRWs lately, can't remember if the 'L's' have 4 or not. If it does have 4, they can be used on either side, and if it's a press pin, it can be offset to either side, right?

i've seen factory rods that have the slight offset you speak of, but it's always been to the correct side regarding the chamfer on the ones that I recall. Aftermarket rods don't have this offset, or at least the Crower Billets and Eagles I've seen don't. Anyway, your saying they would bind against each other if the offsets face together?

OK, let me clarify. The pictures I saw in the presentation were stock Pontiac rods and pistons. Most ALL Pontiacs have 4 valve reliefs[ever taken apart a 301,326,350,389,400,421,428,455? They all have them. The only Pontiac I have personally seen without 4 valve reliefs is the SD-455 which has 1 valve trough. Ross pistons and expensive forged aftermarket pistons have only 2 valve reliefs because they are trying to keep compression up and plus they probably figure since your spending a lot on pistons, they may as well make them better for performance. Yes, the TRW "L"'s have 4 reliefs. Besides how many valve reliefs the piston has is meaningless in this thread as long as they are oriented correctly on the rod. With 4 valve reliefs the manufacturer only needs to make 1 piston for 1 engine. With intake and ex. valve reliefs cut into the piston with a pin offset, the manufacturer needs to make 4 different pistons for 1 engine.

You need to take another look at your aftermarket rods. THEY ARE OFFSET! Almost ALL V-8 rods are OFFSET! They have to be or the small end will be up against one side of the piston[cocking the piston in it's bore in severe cases] and you will have little to no rod side clearence. Inline engine rods are centered. V engine rods are offset. Some may only be offset by .060" or so but they are still offset.

Think about what you said;
"i've seen factory rods that have the slight offset you speak of, but it's always been to the correct side regarding the chamfer on the ones that I recall. Aftermarket rods don't have this offset, or at least the Crower Billets and Eagles I've seen don't."
Now I ask you this; if the factory rods were offset, how in the world would an aftermarket rod work if it were not offset? Answer, IT WON'T!

[gtofreek]

AMEN to Mr. P-Body. Something so simple is blown way out of proportion. Just face the bearing tangs in towards the cam and be done with it. The chamfers and everything else will fall into place. It really is not rocket science!

[PONTIAC DUDE]

Originally Posted by gtofreek

With the chamfer against the crank radius, the bearing tangs will be facing in towards the cam. On older rods the oil hole will be on the tang side also. The rods are slightly offset so it does matter how they go in besides just the chamfer and tangs. You cannot turn them around or they will bind. The pistons have 4 valve reliefs so as long as the notch is forward, the piston offset will be correct.

Ditto.

I have had rods with chamfers pretty radial on both sides of the rods. So the tangs on the same side as race 2 relief is the best way to do it.

As stated above. With 4 reliefs, notch or arrow forward, tangs up.

[HWYSTR455]

Ok, you have a point gtofreek about why wouldn't a aftermarket rod be offset and a factory one is, so I physically looked, and yes, they are offset, just never looked that hard at them or didn't pay attention to it in the past. Not that I doubted it, just never looked since it was the chamfer I was more concerned about.

That brings up another point though, like if the rod's offset, and the chamfer is to the 'long' side, and they only make one rod for both sides, how could one mix them up? I must still be missing something here, or it's just another way of saying the same thing when one mentions the tang orientation? I would think you'd want to look at the chamfer & offset for orientation, and would think who cares about the tang?

On the pistons, that does clarify, thank you. It's been so long since I've even SEEN a factory piston, and have only used one set or TRW/SpeedPros in the last number of years, so I just plain couldn't remember. I'm still confused on the pin offset though. If they have 4 reliefs they only have to make one piston? Then how could the pins be offset? I do remember seeing some with arrows that say 'front' on them, but if you flip that to the other bank, it would be facing the other way, right? The only reason the reliefs were a question were because of the pin offset thing, think that's why it came up. When I used press fit pin pistons, like the TRWs, that had an arrow, I always had the machine shop press them on, and they numbered the rods as well, so, I guess I never paid attention to that stuff during assembly. (Guess I should have). I just looked at the job cards for 3 sets of Ross pistons I have, and none have offset pins. (Or at least it's not mentioned on the job cards). Why then would some be offset and others not? If they are offset, darn, maybe I've just been either lucky over the years or the offset would be something that would particularly make the engine come apart!

I must have taken a lot of the parameters for granted, since these things were generally taken care of by my machinist. Assembly, as from that point on, beyond the rod/piston assembly, were what I felt common sense items, to an extent. Learn something every day!

[gtofreek]

On the piston pin offset with 4 valve reliefs, since all the pistons are basically going the same direction the offset doesn't matter as long as the notch is forward. This will set the offset to the correct direction. The cylinders themselves will have thrust on opposite side from bank to bank. With the notchs of all pistons facing forward, 4 rods will be mounted forward and 4 backwards.

As far as your Ross pistons go, most aftermarket forged "race" pistons do not run any offset unless you order them with offset. The reason one would want to offset the pin would be to either cut down on piston slap noise[why the manufacturers do it] or to gain performance for different reasons.

1. If you offset the pin against direction of rotation[if you are looking at the front of the engine offset to the left] the piston will have less dwell time at TDC and the rod will have a bit of angle in it and will yank the piston down off TDC faster[making for a harder suck on the head] and will have a straighter rod angle at BDC when piston starts to come back up.

2. If you offset the pin to the right[in direction of rotation] you will have more dwell time for the piston at TDC and have a straighter rod angle on combustion but more rod angle at BDC when the piston comes back up.

Both have their advantages and disadvantages. Offsetting the pin can effectively change the rod length in the way the engine reacts.

[vidguy]

Ok that was a good clarification, I think..
I was and maybe still am not clear on the thrust concept. help me out just so I can understand.

I was a bit confused after reading the statement about making one Piston for a motor. Thats because I realized that the offset on the Piston would move from the Inboard side of the cylinder on the drivers side, to the outboard side of the cylinder on the passenger side. (Depending on what we are calling the offset of the pin.) Either way, the centerline of the piston pin in the bore moves in the same direction as you move the piston from bank to bank..
Correct? And that was not a Mirror image of the assembly from bank to bank.

But in considering the notion of which way is the rotation within the block, it then makes sense that the thrust pressure would be on different sides of the cylinders on each bank. Am I correct there?
Thanks

[gtofreek]

Originally Posted by vidguy

Ok that was a good clarification, I think..
I was and maybe still am not clear on the thrust concept. help me out just so I can understand.

I was a bit confused after reading the statement about making one Piston for a motor. Thats because I realized that the offset on the Piston would move from the Inboard side of the cylinder on the drivers side, to the outboard side of the cylinder on the passenger side. (Depending on what we are calling the offset of the pin.) Either way, the centerline of the piston pin in the bore moves in the same direction as you move the piston from bank to bank..
Correct? And that was not a Mirror image of the assembly from bank to bank.

But in considering the notion of which way is the rotation within the block, it then makes sense that the thrust pressure would be on different sides of the cylinders on each bank. Am I correct there?
Thanks

By Jove, I think he's got it!

[vidguy]

Super! And to further beat this issue to death, to make sure I never have to ask about this again..
As I understand it, the thrust sides on each bank of a V engine follows the rotation of the crank, so it's never;
Outside / Outside and never Inside / Inside
but rather
Outside / Inside or Inside / Outside...
Correct?

Don't you love it when the pieces of a puzzle fall into place?

Thanks a lot fellas, for extending my knowledge..

[Mr. P-Body]

The offset in the piston must be the same on each side. It must also be located the same. This is why the piston is marked "Front". The "thrust side" is the same in both banks, as the pressure on the rod/piston comes from the same direction during rotation. For the same reasons, pistons with only two reliefs have NO offset.

Jim

[vidguy]

Ok so Im back to square one on this...
Offset should be on the Inboard or outboard side on both banks? A mirror image?
Then there must be 2 pistons for the motor, if the pistons are offset, no?

One with the offset one way in relation to the arrow and the other is the opposite?

I am referring to the offset in the left/right orientation of the Piston pin which I measured, on the pistons.
Are you referring to the fore/aft orientation of the rods on the crank?
Sorry to confuse the issue. I just want to understand.

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



---Picture of piston with MIRRORED offset on either bank
(TWO PISTONS ARE MADE FOR THE MOTOR)
-------------------------------------------------

[Tom Vaught]

I will try on this deal:

1) All of the pistons have a notch that should face the front of the engine no matter which side it is installed on.

2) Pistons with "offsets" are made to eliminate "piston slap" noise

3) Looking at the engine from the flywheel side, the passenger side bank is on the right. The Driver's side bank is on the left.

4) The normal firing order for a pontiac is 18436572. If we move that
one cylinder event forward the numbers would be 21843657, ok?

5) We want to talk about the #2 piston and the #1 piston for our discussion.

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.

Does that make it clear now?

Connecting rods:

1) Even though we call the 1st cylinder on the drivers bank the #1 cylinder, the #2 cylinder is actually farther forward.

2) Lay a crankshaft on the ground with the flywheel flange closest to you. Flywheel flange at 6:00 and balancer snout at 12:00

3) Rotate the first rod journal on the crankshaft so it faces toward 3:00 and visualize two rods installed on the rod journal.

4) The rod closest to the 12:00 position will be the rod for the #2 cylinder and the rod chamfer will face 12:00 on the crank.

5) The second rod on that rod journal will be the #1 rod journal and that rod's chamfer will be facing 6:00

6) The rods are oriented that way for two reasons:
a) The chamfer will clear the crankshaft radius ground into the crankshaft
b) The Bearing tangs will face upward as the crankshaft rotates in the block. Old factory Pontiac rods had oil squirters on the tang side of the rods to cool the piston bottoms and lubricate the camshaft and cylinder walls.

So to answer your questions:

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.