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Piston wrist pin offset

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GPGoverMPG
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Piston wrist pin offset

Post #1 by GPGoverMPG » Fri May 03, 2019 9:26 am

So the pistons I purchased have an advertised pin offset of .006 and the tops are all the same. Does this mean I need to have two installed backwards? Also, which one are correct left or right bank? I am going to measure the offset today. The pistons have the notch that you point to the front so there is some offset just can't see the .006.

I'm hoping this leads to a good discussion. I searched for a good explanation here and in in-line tech but didn't find what I was looking for. Could be my search skills.

Thanks again
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CNC-Dude
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Re: Piston wrist pin offset

Post #2 by CNC-Dude » Fri May 03, 2019 9:48 am

Most piston offsets are in the range of .060", not .006" which would basically be zero offset. The offset will be to the same side of the piston when installed with the notches pointing the same direction. When installed in a V8 even with the notches in the same direction, the offset will be on different sides of the cylinder wall on right and left banks.
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Re: Piston wrist pin offset

Post #3 by pmuller9 » Fri May 03, 2019 9:59 am

Your piston tops are symmetrical with 4 valve reliefs so there aren't any left or right bank pistons.
You install them with the notch facing forward in all six cylinders.

The pin has an .060" offset which is noticeable from the bottom view.
You can see from the following view that the pin boss is higher than the center of the piston.
download/file.php?id=3368&mode=view

The wrist pin must be offset toward the passenger side of the block.
All six pistons installed with the notches facing forward should accomplish this.
Check it and see.
Last edited by pmuller9 on Fri May 03, 2019 12:18 pm, edited 3 times in total.

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THE FRENCHTOWN FLYER
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Re: Piston wrist pin offset

Post #4 by THE FRENCHTOWN FLYER » Fri May 03, 2019 10:04 am

I'm curious to see if the offset is really .006. I am not used to seeing that small of an offset. It would be more common to see a .060" offset, which makes me wonder if there may have been a typo on the printed spec. At any rate, I would not be concerned with a .006, offset, but my experience is a .060 offset installed backwards will give you piston slap. No worries, it is a lot like the cracked skirts often found in a 300 oil pan. They break, but rarely cause catastrophic failure. Your noisy engine with two backwards pistons will likely run a long time without any problem.
Optimum?
no
But if you don't mind some extra piston noise I'd live with it, like I'm doing on my Willys. Which gets abused regularly. [mine have domes, necessitating a couple installed backwards.]
You could use the pistons intended for a Ford V8 numbering system 5 - 8 and a couple from the right (V8) bank. All the notches facing forward. You said the tops are all the same. If they have no domes or directional dishes on them it will matter little. [An offset changes the effective rod length a leetle beet.]
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Re: Piston wrist pin offset

Post #5 by GPGoverMPG » Fri May 03, 2019 1:18 pm

So there is certainly offset. I used information from Summit or Jegs on the .006 number could have been mine or their error. I can't really find a good site for speed-pro piston information. The notches facing forward on all 8 pistons makes the most sense. I don't remember where I saw the run 2 with the notches pointing backwards. I didn't make sense at the time because offset should be based on direction of crank rotation. Thanks for clearing that up.
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
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Re: Piston wrist pin offset

Post #6 by THE FRENCHTOWN FLYER » Fri May 03, 2019 2:05 pm

You would only have to run two of the (V8) pistons backwards if there was a dome or asymmetric dish on the piston. Since most V8s use the same cylinder head left-to-right side the combustion chamber orientation vs the front of the engine changes you would have to reverse two of the domes [dishes] if you wanted to align six in a row, necessitating running two of the domed [dished] pistons backwards.

An easy way to check the amount of pin offset is to insert a pin in the piston and use a caliper to measure the distance from the pin OD to the skirt, on both sides of the piston and divide the difference in those measurements by two.
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Re: Piston wrist pin offset

Post #7 by GPGoverMPG » Fri May 03, 2019 4:27 pm

You know that sounds great. I tried the claliper thing in the wrist pin then turn the piston and remeasure. To inconsistent to know what the real offset is I know there is more than .006. I wish I could find better data like Keith black and silvo-light.
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Re: Piston wrist pin offset

Post #8 by CNC-Dude » Fri May 03, 2019 5:35 pm

One thing to consider and do some deductive reasoning with is that cast pistons are not typically made with little to no pin offset, this is nearly a "race only" type of thing because it does make them knock around in the cylinders more, and that type of extra skirt loading is not desireable for cast pistons. So I'd dismiss the .006" mentioned as a misprint intending to be .060" instead.
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Re: Piston wrist pin offset

Post #9 by pmuller9 » Fri May 03, 2019 8:21 pm

GPGoverMPG wrote:You know that sounds great. I tried the claliper thing in the wrist pin then turn the piston and remeasure. To inconsistent to know what the real offset is I know there is more than .006. I wish I could find better data like Keith black and silvo-light.

Do what FTF suggested and you will get the actual pin offset.

With the pin installed measure from the outside of the pin wall (from inside of the piston) to the outside of the skirt on the opposite side of the piston.
Then go from the other side of the pin to the other side of the piston.
Then divide the difference of those two measurements by two.

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Re: Piston wrist pin offset

Post #10 by Max_Effort » Fri May 03, 2019 9:51 pm

There are engine builders that have gained some power by installing all the pistons “backwards” , with the piston offset towards the minor thrust wall.

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Re: Piston wrist pin offset

Post #11 by pmuller9 » Fri May 03, 2019 10:27 pm

Max_Effort wrote:There are engine builders that have gained some power by installing all the pistons “backwards” , with the piston offset towards the minor thrust wall.

A desaxe engine works on that principle.

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Re: Piston wrist pin offset

Post #12 by GPGoverMPG » Sat May 04, 2019 7:40 am

I think I understand the measurement method now. I was inside the pin to the outside of the skirt and could rock my caliper too much for a good reading. I will try the inside the piston plan today.
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
2014 SHO

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GPGoverMPG
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Re: Piston wrist pin offset

Post #13 by GPGoverMPG » Sat May 04, 2019 2:12 pm

With my fat fingers on the caliper I measured .125 after subtraction. Split that in half and it's .06125 so basically .060. Everything is going great. Thanks
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
2014 SHO

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