Press Pins and Balance
Moderator: Team
-
- New Member
- Posts: 38
- Joined: Mon Mar 29, 2010 9:22 am
- Location:
Press Pins and Balance
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
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
-
- Guru
- Posts: 1756
- Joined: Wed Aug 26, 2009 8:30 pm
- Location: Beautiful Southern Maryland
- Contact:
Re: Press Pins and Balance
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.
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.
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
www.enginerepairshop.com
Advanced Automotive Machine
Waldorf Md
www.enginerepairshop.com
Re: Press Pins and Balance
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.
Monty Frerichs
B&M Machine
B&M Machine
-
- Guru
- Posts: 1564
- Joined: Thu Sep 16, 2010 10:01 am
- Location: Virginia
- Contact:
Re: Press Pins and Balance
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.
You do not have the required permissions to view the files attached to this post.
"do it right or do it right away"
http://www.motionmachineinc.com/
https://www.youtube.com/watch?v=ZgIZbveknDs
http://www.motionmachineinc.com/
https://www.youtube.com/watch?v=ZgIZbveknDs
-
- New Member
- Posts: 38
- Joined: Mon Mar 29, 2010 9:22 am
- Location:
-
- Guru
- Posts: 1029
- Joined: Mon Sep 10, 2012 1:55 pm
- Location: Anaheim, CA
Re: Press Pins and Balance
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.
Re: Press Pins and Balance
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.
Just take the time to make a jig to level the rod center-to-center and to hold the piston assembly square.
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Studebaker-Packard V8 Limited
Obsolete Engineering
-
- New Member
- Posts: 38
- Joined: Mon Mar 29, 2010 9:22 am
- Location:
Re: Press Pins and Balance
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
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
-
- New Member
- Posts: 38
- Joined: Mon Mar 29, 2010 9:22 am
- Location:
Re: Press Pins and Balance
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...
Re: Press Pins and Balance
Are you anywhere near the Armstrong Bros? I'm sure they could help.hotrod1968 wrote: ↑Fri Jun 04, 2021 3:24 pm 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...
Re: Press Pins and Balance
Its easily done, and this is how you do it.hotrod1968 wrote: ↑Fri Jun 04, 2021 5:40 am 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
You do not have the required permissions to view the files attached to this post.
Racing a NA NHRA stocker should be mandatory before any posting.
- GuysMonteSS
- HotPass
- Posts: 298
- Joined: Mon Nov 19, 2007 5:35 pm
- Location: Nova Scotia Canada
Re: Press Pins and Balance
Yes I'm sure that they certainly could help with that.DCal wrote: ↑Sun Jun 06, 2021 8:47 pmAre you anywhere near the Armstrong Bros? I'm sure they could help.hotrod1968 wrote: ↑Fri Jun 04, 2021 3:24 pm 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...
They are the best in the region !!
And they have lots of nice gear.
Guy
'86 Monte SS,513 BBC,AFR heads,Bullet solid roller cam,Doug Nash 4+1 5 speed,Hurst Inline Shifter,Ford 9 inch Rearend.
- GuysMonteSS
- HotPass
- Posts: 298
- Joined: Mon Nov 19, 2007 5:35 pm
- Location: Nova Scotia Canada
Re: Press Pins and Balance
A short article on the Armstrong brothers.
Guy
https://saltwire.pressreader.com/@Reade ... m4Ghoq3zfQ
Guy
https://saltwire.pressreader.com/@Reade ... m4Ghoq3zfQ
'86 Monte SS,513 BBC,AFR heads,Bullet solid roller cam,Doug Nash 4+1 5 speed,Hurst Inline Shifter,Ford 9 inch Rearend.
Re: Press Pins and Balance
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.
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.
-
- New Member
- Posts: 38
- Joined: Mon Mar 29, 2010 9:22 am
- Location:
Re: Press Pins and Balance
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
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