Press Pins and Balance

[hotrod1968]

I’m curious to know the press fit on OEM GM wrist pins, and if it is realistic to think I could remove pistons without destroying them. It has been my understanding that pistons are sacrificial during this operation, I assume overcoming interference fit distorts the pistons. I have TRW forged pistons L2252AF30 on OEM rods. I machined the domes to adjust my compression ratio, and I need to balance my assembly. These forged pistons are 64g lighter than the OEM pistons, and I’m using the OEM crank.
I have some tool steel cradles I’d made the last time I did a press pin thing in a 351W, but I discarded the pistons that trip. I have an idea I can support the pin bosses, but if I’m doomed to distort the piston no matter how careful I am, i guess that’s life.
These assemblies I’m fooling with now are for myself, a 350 sbc. I bought them used and they were already together. I wanted to be throrough and disassmbled them to balance the pistons to themselves, then weigh big end, small end, all that and show up to the shop with a target bobweight.
My other scheme was to leave them assembled, and see what my options are to provide a small end weight somehow with the piston assembled. I understand the fulcrum changes and the piston is an overhung load when using the pin as a center balance point . I’m sure this is a very unconventional means of measuring the small end, just wanted to see if anyone has done it this way. I know to do it right the parts should never be together, but I’m trying to do the right thing with the wrong budget.
Thanks for any insight

[BillK]

HR,
The press fit is around .001- .0015 On my Sunnen pin press the gauge usually goes up to about 3-4 tons before they break loose.

When I have a set I need to take off for the purpose you are describing, I give them to a friend who has the OTC fixture that goes inside the piston and supports the rod itself so there is no pressure put on the piston. Goodson makes one also. Maybe you can ask around and see if somebody close to you has one ? Where are you located.

The biggest problem with pressing them off is the fact that the inside of the pin boss is not usually perfectly "square" to the rod so when you try to press the piston off the rod is not hitting all the way around and distorts the piston.

I have actually weighed assemblies like that by supporting the piston on a small box or something similar to get it perfectly level. Then I weigh the big end and subtract its weight from the total to get the reciprocating weight. I have probably done 4 or 5 that way over the years and have never heard of any bad results.

Also, I would only do one assembly to get a weight. The others will be close enough for 99% of engines.

[SupStk]

Kent-Moore also makes a pin press that supports the rod eye during R/R. Just about all GM dealers had them. A little heat on the rod will ease removal.

[MotionMachine]

I concur with Bill's numbers on press fit and pressure amount before a pin breaks loose. This is the OTC tool that he refers to. Some of those TRW forged pistons had very little space between the rod small end and the piston boss so getting the adapter in there might be tricky. I've ground the skinniest one down (#6) to fit some tight apps but you lose some support and I've bent them before.

DSC01291 (Small).JPG

DSC01292 (Small).JPG

[hotrod1968]

That’s perfect, thank you for taking the time to reply.

[piston guy]

I have an older "shadow graph" style holding fixture. I can weigh the rod and piston assembly together and come up with a bob weight.

[PackardV8]

If one thinks about it, weighing assemblies is more efficient than weighing piston, rings, pins, small end and adding them to get a bobweight total.

Just take the time to make a jig to level the rod center-to-center and to hold the piston assembly square.

[hotrod1968]

I appreciate the knowledge on the fit and method to gently separate the pins from the rods. I do like the idea of not disturbing them and weighing them as assembled, I can manage a means of balancing the small end square and level. Is the center of the pin still my datum or target? It doesn’t shift due the overhung weight of the piston, rings etc. does it?

I used to moonlight at a shop and did a bit of everything, but only balanced a few assemblies. Is the big end measured with bearings and just counted once? While the small end is measured factoring the piston, small end and all the parts associated and multiplied by 2? Excuse my ignorance, I’ll be taking this to a great shop, I’d just like to have it as prepared for them as possible, and save them fussing with trivial stuff.
Thanks

[hotrod1968]

Sorry BillK I missed the question, I’m in NS Canada. I do have a friendly shop who would likely have the gear, just didn’t want to bother them. Not that I want to bother you all with questions about it...

[DCal]

Sorry BillK I missed the question, I’m in NS Canada. I do have a friendly shop who would likely have the gear, just didn’t want to bother them. Not that I want to bother you all with questions about it...

Are you anywhere near the Armstrong Bros? I'm sure they could help.

[vortecpro]

I’m curious to know the press fit on OEM GM wrist pins, and if it is realistic to think I could remove pistons without destroying them. It has been my understanding that pistons are sacrificial during this operation, I assume overcoming interference fit distorts the pistons. I have TRW forged pistons L2252AF30 on OEM rods. I machined the domes to adjust my compression ratio, and I need to balance my assembly. These forged pistons are 64g lighter than the OEM pistons, and I’m using the OEM crank.
I have some tool steel cradles I’d made the last time I did a press pin thing in a 351W, but I discarded the pistons that trip. I have an idea I can support the pin bosses, but if I’m doomed to distort the piston no matter how careful I am, i guess that’s life.
These assemblies I’m fooling with now are for myself, a 350 sbc. I bought them used and they were already together. I wanted to be throrough and disassmbled them to balance the pistons to themselves, then weigh big end, small end, all that and show up to the shop with a target bobweight.
My other scheme was to leave them assembled, and see what my options are to provide a small end weight somehow with the piston assembled. I understand the fulcrum changes and the piston is an overhung load when using the pin as a center balance point . I’m sure this is a very unconventional means of measuring the small end, just wanted to see if anyone has done it this way. I know to do it right the parts should never be together, but I’m trying to do the right thing with the wrong budget.
Thanks for any insight

Its easily done, and this is how you do it.

[GuysMonteSS]

Sorry BillK I missed the question, I’m in NS Canada. I do have a friendly shop who would likely have the gear, just didn’t want to bother them. Not that I want to bother you all with questions about it...

Are you anywhere near the Armstrong Bros? I'm sure they could help.

Yes I'm sure that they certainly could help with that.
They are the best in the region !!
And they have lots of nice gear.
Guy

[GuysMonteSS]

A short article on the Armstrong brothers.
Guy

https://saltwire.pressreader.com/@Reade ... m4Ghoq3zfQ

[modok]

It should be possible to weigh the rod & piston "end for end" without taking it apart.
I'm totally game to try it but... nobody has ever asked to even do that, so I haven't got much experience trying it. Does not happen often.
I would probably hang the big end, and set the piston on the scale, should work. Could also be too much friction to get a good reading and maybe you'd have to solve that one way or another.

Using a special tool to support the rod while pressing out the pin is also in the same category.
Sometimes it works but it depends on the piston. Every piston design is a little different.
I have not seen any universal design that works for more than a few certain pistons.

[hotrod1968]

Thanks for the advice. I’ve taken measurements of the assembled pistons, I weighed the rod bearing and rings separate, but the pin rod and piston were together. The small end was easy to level and weigh. However, I needed 9g of paper towel jammed in the piston to keep it perpendicular when measuring the big end. Can that be subtracted somewhere in the equation? Or am I splitting hairs…

I provided some of the individual measured parts in these parentheses, but the summed big end and small end are further below.
————————————————————————————————————-
1st ring 15.26g
2nd ring 13.73g
Oil ring and rails 13.71g

(Ring pack 42.7g total)

rod brg 45.3g
Piston+sm end (no rings) 879.91g
Big end (no brgs) 418.87g
8.92g of paper towel to level piston when measuring big end.
————————————————————————————————————


(Big end w/brg 464.16g)
(Small end w/piston,pin,ring pack 922.61g)

I did not have the rod bearing installed or the rings when I weighed this up, they were weighed separate and added. Does this seem alright?

Thanks