New Piston Heavier Than Original Piston

[MaximumMini]

Hello

I am replacing the original piston in my 3.8L Dodge Caravan engine and there is a difference in weight between the old and new.

The original wrist pin weighs 134 g
The new wrist pin weighs 140 g
The Original piston with wrist pin weighed 568 grams total
The New piston and wrist pin weighs 585 grams

For a total difference of 17 grams heavier for the new replacement piston. I read that 10 grams is the maximum difference you should have between piston assembly weights on an engine. Being that this is a relatively low RPM engine (4200rpm max) it may not matter?

I am inclined to carefully remove at least 10 grams from the new piston to bring them closer.
I will begin by drilling the hole seen in the original piston in the new piston shown at the yellow arrow.
Then remove weight from the areas indicated by the red arrow until the weights match.
2 Quarters weigh 11 grams.

My options are
1). Do Nothing, ignore the difference
2). Modify the piston to reduce the weight
3). Get a lighter wrist pin or have the new wrist pin machined to lose weight.
4). Get the wrist pin machined AND reduce the piston weight
5). Use the Old wrist pin to reduce total mass by 6 grams (Both wrist pins measure precisely 0.9000in) (I will likely do this)

Opinions?
PS - Does that hole in original piston serve a purpose?

 

[Nessism1]

Are you changing one piston, or all? If changing all, I wouldn't worry about the weight difference.

 

[MaximumMini]

Are you changing one piston, or all? If changing all, I wouldn't worry about the weight difference.

Just one.

 

[JavierH19]

Hmm a 2.53% weight increase on one piston. That might be enough to cause a touch of additional vibration but I doubt it would be bad for engine life.

Edit: I forgot to add the pins. It goes to 2.99% but I don't think that'll hurt engine life maybe just add a touch of vibration mostly at idle at worst. There's no way a 3% weight gain on one cylinder on a v6 will make it shake or vibrate enough to be worrisome.

 

[MaximumMini]

Hmm a 2.53% weight increase on one piston. That might be enough to cause a touch of additional vibration but I doubt it would be bad for engine life.

Thanks. I have a slight concern about a slight increase in stress on that one rod bearing and that any additional vibration could lead to another eventual ring or ring land failure? It's pretty easy for me to remove 6 grams from the new piston with my dremel grinding wheel.
I'm considering removing metal at the locations shown by the red arrows in the photo in the OP.

 

[Shel_B]

I would think additional vibration could affect engine life. My non-existant engineering degree supports my contention

Quick question: How much might the deposits that have accumulated on the old piston weigh? Could they be enought to effect the piston weight?

 

[Shel_B]

Thanks. I have a slight concern about a slight increase in stress on that one rod bearing and that any additional vibration could lead to another eventual ring or ring land failure? It's pretty easy for me to remove 6 grams from the new piston with my dremel grinding wheel.
I'm considering removing metal at the locations shown by the red arrows in the photo in the OP.

Might removing smaller amounts from different locations around the piston be a better (more balanced) solution? Trying to equalize the piston weights is a smart move in any case.

 

[JavierH19]

I would think additional vibration could affect engine life. My non-existant engineering degree supports my contention

Quick question: How much might the deposits that have accumulated on the old piston weigh? Could they be enought to effect the piston weight?

One would think but some of the most long lasting reliable engines mainly very old 4 and 6 cylinders are buzzy and vibrate a bunch. My buzzy shaky 2.5 ford went over 400k before the trans blew and only burned a touch. There are some engines where deleting the balancing shaft is preferred as it makes it last longer. I'm thinking of a vw/audi engine but can't remember which one.

 

[MaximumMini]

I would think additional vibration could affect engine life. My non-existant engineering degree supports my contention

Quick question: How much might the deposits that have accumulated on the old piston weigh? Could they be enought to effect the piston weight?

Hello Shel. Love your reply
The deposits would have made the problem worse, since removing any deposits could only make the original piston lighter right?
The original piston is clean enough that I would estimate less than 1 gram is possible in deposits. Probably much less like 1/15th.

 

[OVERKILL]

I think it's pretty obvious where material was removed on the factory pistons, I'd remove material from the same area. I'd check the wrist pin for material removed from the centre.

 

[JavierH19]

If you have a drill press even if a basic one try to carefully remove some material.

 

[Shel_B]

One would think but some of the most long lasting reliable engines mainly very old 4 and 6 cylinders are buzzy and vibrate a bunch. My buzzy shaky 2.5 ford went over 400k before the trans went and only burned a touch. There are some engines where deleting the balancing shaft is preferred as it makes it last longer. I'm thinking of a vw/audi engine but can't remember which one.

Imagine how much longer it could have gone were it smooth and balanced ...

Actually, it my not make a lot of difference, but it will make some difference. Will it be enough to affect longevity or the driving experience? Do either of us really know? Is there a downside to having a better balanced engine? I think we know the answer to that question.

 

[Nessism1]

A rotary tool with carbide burr would allow you to blend rough edges inside the piston, and strategically remove material. The burr will clog with aluminum, though. Maybe a cheap set from Amazon?

Of bigger concern than weight, is why only replace one piston. How much does a set cost? I'd be inclined to replace all.

 

[MaximumMini]

A rotary tool with carbide burr would allow you to blend rough edges inside the piston, and strategically remove material. The burr will clog with aluminum, though. Maybe a cheap set from Amazon?

Of bigger concern than weight, is why only replace one piston. How much does a set cost? I'd be inclined to replace all.

Hello,
I was originally planning a total rebuild. But after taking the head off and measuring cylinder bore for wear and rod journal it looks very healthy other than this one piston that is likely to have been damaged by detonation due to a failed fuel injector.
The engine has 157k miles. Relatively young.
All other cylinders seem 100% (compression)
The engine is not out of the vehicle.

If the mileage was over 250k and there was signs of abuse or wear but there isn't.
I have an entire set. But if only one piston needs replacing.......

 

[Shel_B]

Of bigger concern than weight, is why only replace one piston. How much does a set cost? I'd be inclined to replace all.

Here's a complete set of 6 from Summit: $246.99
https://www.summitracing.com/parts/mop-04666129aa/make/dodge

The complete set includes connecting rods. To the OP: did you consider the weight of the rods in your calculations?

 

[BC1]

The oil piston is cracked and missing material in the lands right? Did you weigh another old one to compare?

Summit actually thinks someone will buy a single piston/rod for $250? $1500 for a set?

 

[MaximumMini]

Here's a complete set of 6 from Summit: $246.99
https://www.summitracing.com/parts/mop-04666129aa/make/dodge

The complete set includes connecting rods. To the OP: did you consider the weight of the rods in your calculations?

That is for ONE cylinder
No. I'm only considering the weight of the old piston vs the weight of the new piston. I "trust" that the factory got the original rods the same weight or well within tolerances.

The set of 6 pistons I bought from RA is EngineTech, P5043(6)STD brand.
I did notify RockAuto of the piston weight being out of factory specs.

 

[andyd]

Please post a follow up. I once replaced a ruined piston with a used piston/rod from a junked motor. Re-used original rod inserts drove the Datsun another 50k miles.

 

[CheezWhiz]

seems like a lot of work with high risk of failure... just bite the bullet and rebuild the engine.

 

[JRay’07]

I would leave the new piston alone. You don’t know if there were design updates and improvements implemented and you could cause more harm than good.