New Piston Heavier Than Original Piston

Nessism1 said:
Nice work on the piston.

Are you going to run the original pin? As long as it's the same length, you should be okay.
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Yes sir. Using original wrist pin due to new wrist pin being 6 grams heavier. Exact same length. Thanks for the assist (y)
 
Hohn said:
It seems to me that “hole” is the location of material removal for factory balance.
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I wasn't sure so I duplicated the hole on the new piston. The amount removed to drill that hole was about 1/10 of a gram.
 
Hohn said:
Precisely my point. Someone is asserting the pistons do not comply with a spec that doesn’t exist.
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In the Factory Service Manual for this engine, it is clearly listed as well as limits. I was able to reduce the new piston weight to 439g.
Unfortunately, the new wrist pin made the problem worse because it was 6g heavier than the original. I used the lighter original wrist pin.

PistonSpecs.webp
 
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My problem with the EngineTech brand set of pistons is that they do not comply with the published weight limits for their replacement pistons. Their original weight is 445g - 2g over manufacturer limits. A very small amount, but precision is the difference between amateur engine building and professional engine building.
 
CheezWhiz said:
well somewhere along the way an engineer probably specified the weight of the pistons ... along with a lot of other things that didn't make it into the service manual.
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Weight isn’t specified at the piston level because by itself the piston weight means nothing. The reciprocating *assembly* is what is balanced, not individual parts of it. Pistons are only ever balanced with rings, pins pin locks, etc and typically half the weight of the rod as reciprocating weight, plus some allotment for oil.

Pistons have fairly tightly controlled dimensions (mostly) and combined with requirement for uniform density produces a fairly narrow weight range by default. Piston weight is a “driven parameter” and not a “driving parameter” if that distinction makes sense to you. It’s not controlled directly because 1) other parameters somewhat control it, and 2) by itself it doesn’t matter because of how its used in the overall machine.
 
Hohn said:
Weight isn’t specified at the piston level because by itself the piston weight means nothing. The reciprocating *assembly* is what is balanced, not individual parts of it. Pistons are only ever balanced with rings, pins pin locks, etc and typically half the weight of the rod as reciprocating weight, plus some allotment for oil.

Pistons have fairly tightly controlled dimensions (mostly) and combined with requirement for uniform density produces a fairly narrow weight range by default. Piston weight is a “driven parameter” and not a “driving parameter” if that distinction makes sense to you. It’s not controlled directly because 1) other parameters somewhat control it, and 2) by itself it doesn’t matter because of how its used in the overall machine.
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definitely not the craziest thing i have heard on here, thx.
 
MaximumMini said:
In the Factory Service Manual for this engine, it is clearly listed as well as limits. I was able to reduce the new piston weight to 439g.
Unfortunately, the new wrist pin made the problem worse because it was 6g heavier than the original. I used the lighter original wrist pin.

View attachment 278201
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I stand corrected. Perhaps in a service context it makes to specify a weight limit just to ensure parts interchange, and I’m sure that’s the context here.

Being in the engineering side, we’d never specify weight, we’re of course already making it as light as is practical.

This spec seems to be just an assessment of the balance limits for the assembly. It’s assuming certain things about the other parts.
 
MaximumMini said:
My problem with the EngineTech brand set of pistons is that they do not comply with the published weight limits for their replacement pistons. Their original weight is 445g - 2g over manufacturer limits. A very small amount, but precision is the difference between amateur engine building and professional engine building.
Click to expand...

I don’t understand on the one hand shooting for precision in individual piston weight and other the other having no idea where the rest of the pistons are for weight. If the factory limits 438 + 5 grams, you frankly have no idea where the other pistons are within that distribution. There’s no point is striving to precisely miss a target.

I might have a hyper accurate sniper rifle, but if I don’t know where to aim, I will miss just as badly as if I had a terrible rifle.

I think the point of the tolerance limit is to allow one to entirely skip all precision considerations and have an “easy button” where it’s either in or out. It won’t precise but it’s not intended to be. It’s intended to be good enough.

A precise balance can only happen with the entire assembly balanced.
 
Hohn said:
Weight isn’t specified at the piston level because by itself the piston weight means nothing. The reciprocating *assembly* is what is balanced, not individual parts of it. Pistons are only ever balanced with rings, pins pin locks, etc and typically half the weight of the rod as reciprocating weight, plus some allotment for oil.

Pistons have fairly tightly controlled dimensions (mostly) and combined with requirement for uniform density produces a fairly narrow weight range by default. Piston weight is a “driven parameter” and not a “driving parameter” if that distinction makes sense to you. It’s not controlled directly because 1) other parameters somewhat control it, and 2) by itself it doesn’t matter because of how its used in the overall machine.
Click to expand...

If that assertion were true, every vehicle could have different weights for each crankshaft, each rod and each piston. This would make rebuilding or even repairing (much less manufacturing replacement parts) a nightmare. As you can clearly see here, the weight is indeed specified at the piston level.

I would assume that since a piston weight range limitation was indeed posted for this motor, then their factory likely is very precise about controlling the weights and balances of all reciprocating parts. Meaning all crankshafts, all rods and all pistons must fall within tight tolerances....and match.
I would bet that if I took any random identical motor apart it's components would be within these specifications.

When the manufacturing process is this tightly controlled, this is how they can specify weight range tolerances for individual parts. They are using some God awful expensive machinery to mass produce these parts. it is well within their capability to be that precise in the manufacturing process. It is all done by robotics. These massive crankshaft turning machines are able to precisely balance thousands of crankshafts so that their reciprocating balances are identical in a few seconds. No guess work. I do not believe Chrysler takes the time to assemble the entire reciprocating assembly, balance it individually, then disassemble it again for installation, for mass produced, lower rpm engines.

I can see that being done for high performance and or racing engines, but not for minivan engines.

Can you show me where Chrysler does that for this motor? I would be interested.
 
I think you guys are sort of saying the same thing from two totally opposite directions. Hohn is speaking from a standpoint of a totally new engine build, MM is speaking from a component piece replacement only. It gets to the same result in a awkward way. MM is trying to re-create that multi- component balance as it was from the factory. I think.

MM - did you try to find the lightest piston and wrist pin of the six you bought? That's where I would have started. But I would have tried to assemble the same weighted piston group with rings, wristpin, & pin retainers. That's all the same mass that the connecting rod has seen. If there is any missing material from the original piston, all bets are off.

Either way, I wish you good luck and am interested to hear of your results!
 
SubieRubyRoo said:
There's no way in Hades I'd put that old wrist pin back into an engine looking the way it does. It's one thing when it's in there and you don't know the condition; it's another thing when you're knowingly shoving that thing into new parts IMO.
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How does it look?
Can you explain what you mean?
Every measurement I took showed a very healthy motor with very little wear.

I don't ever recall hearing of a wrist pin failure in a less than 5k rpm, 200hp motor?
I would personally feel that there is no risk reusing the old wrist pin.
If you know something I don't about this please share it.

The wrist pin moves freely in the piston. It only is pressed into the rod end. Maybe you thought it pressed into the piston?
Basically, the wrist pin and rod become one. I checked the diameter of the old wrist pin vs the new and those measurements were both precisely 0.9000in

In all my years of engine building I have never had or seen a wrist pin break. What I have seen is turbo charged, nitrous assisted motors I have built completely hand grenade. Even through the carnage, the wrist pins were fine.
 
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crashz said:
I think you guys are sort of saying the same thing from two totally opposite directions. Hohn is speaking from a standpoint of a totally new engine build, MM is speaking from a component piece replacement only. It gets to the same result in a awkward way. MM is trying to re-create that multi- component balance as it was from the factory. I think.

MM - did you try to find the lightest piston and wrist pin of the six you bought? That's where I would have started. But I would have tried to assemble the same weighted piston group with rings, wristpin, & pin retainers. That's all the same mass that the connecting rod has seen. If there is any missing material from the original piston, all bets are off.

Either way, I wish you good luck and am interested to hear of your results!
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All the pistons in the new set were the same weight within 1g. All but one was 445g like the one I lightened. All the wrist pins were the same weight. Replacing them all was not chosen since the compression on the rest is great and the wear on the cylinder bore on the one I check was well within new specs.
 
Don't cross the street looking for trouble. Do a complete rebuild or the equivalent of a pad slap by putting the one piston in as is. Anything else is begging for trouble.
 
RooflessVW said:
Leave it. You may have pulled the lightest piston.
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Fer shur. You are very keen to have picked up on that. No one else did.
The one im installing is only 439grams. ALL the rest are 445grams. Odd, right? it's magic I tell ya.
 
Does it matter WHERE you remove weight from the piston? Will removing it from the wrong place cause it to wobble and score the cylinder?
 
tcp71 said:
Does it matter WHERE you remove weight from the piston? Will removing it from the wrong place cause it to wobble and score the cylinder?
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That's a good question imo.
The TOTAL weight removed was 6 grams. That's just slightly less than what a quarter weighs.
I took amounts from different locations on the piston. There was no manual to tell exactly how to do this so it's mostly just intuitive.
I doubt that removing 6 grams in a manner that is equal around the piston, near the center of mass will cause any problems.

But then I'm only an engine ear. ;)
 
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