Piston weights - different inside

[Bryan]

Working on other stuff besides the block. Taking off pistons from the rods. 

Noticed near the piston pins there were different thicknesses. Pistons came from same engine.

Checked 3 different ones out.  Weighed them as they were, with rings, carbon, etc. Two were almost the same weight, 3rd was about 3 grams off.

 

 

[desoto1939]

not sure if the 3 grams will make that much of a difference but not an expert.  Are all 6 piston the same manufacturer. i noticed that one had the DPCD stamped logo on the inside.  Might be that at some point they just did a top end on this old engine and could be different manufacturer of the piston.

 

Just my thought on the question.

 

Rich Hartung

[Sniper]

Odds are those are balance pads, note the thinner one is also the lightest one.  3 grams might be production tolerances, high performance ones usually shoot for 1 gram or less.  I wouldn't worry too much about it but if you had a mill and wanted to even them all up, you could.

[Bryan]

26 minutes ago, desoto1939 said:

not sure if the 3 grams will make that much of a difference but not an expert.  Are all 6 piston the same manufacturer. i noticed that one had the DPCD stamped logo on the inside.  Might be that at some point they just did a top end on this old engine and could be different manufacturer of the piston.

 

Just my thought on the question.

 

Rich Hartung

Wasn't paying attention to that but when I noticed the different pad thicknesses I grabbed the 3 unlike ones to see how they compared.

[Bryan]

18 minutes ago, Sniper said:

Odds are those are balance pads, note the thinner one is also the lightest one.  3 grams might be production tolerances, high performance ones usually shoot for 1 gram or less.  I wouldn't worry too much about it but if you had a mill and wanted to even them all up, you could.

When I get those NOS .030 pistons from VPW I'll compare all of them and match them up.  Just wondering if adding rings later would change the weights much. I'll probably compare the rods also.  I got a very accurate scale. Watch me shoot for 1/2 a gram.  ?  

[Sniper]

Rods ends are weighed and matched separately.  As in all big ends should weight the same, all small ends should weight the same.  Rings are weighted separately, but frankly there is little you can do to match weights, just ensure the gaps are good.  Pistons are weighed separately and matched. 

 

If you really dig into the process you can find that a lot of material can be taken off the pistons to lighten them.  But that's just another snowball waiting to roll.

[FarmerJon]

If you don't have a mill, you can use a large end mill or drill bit with a shallow angle tip in a drill press to remove material from the underside. I balance pistons myself and aim for 0 to 1/2 gram, whatever I have patience for. I write beginning weight on the crown in permanent marker, then write final weight when done. If you find a set of rings weights more than the rest, match them with the piston that will make up the difference. Same as wrist pins.

You can practice on your junk pistons, so you get a good idea of how much to remove to get a certain weight.

Not worth paying a machine shop to do, but only costing your time and effort, may as well be particular.

 

If you ever do a performance import engine build, you will find that they are usually VERY well balanced from the factory. I think that is one of several reasons they have such a good reputation for reliability.

 

Edited May 13, 2022 by FarmerJon

[Bryan]

1 hour ago, Sniper said:

Rods ends are weighed and matched separately.  As in all big ends should weight the same, all small ends should weight the same.  Rings are weighted separately, but frankly there is little you can do to match weights, just ensure the gaps are good.  Pistons are weighed separately and matched. 

 

If you really dig into the process you can find that a lot of material can be taken off the pistons to lighten them.  But that's just another snowball waiting to roll.

How would one weigh a rod end separately?  You'd have to find the same mid point but then trying to get a rod end on a scale? 

[Bryan]

I'll play with mixing and matching wrist pins and rings with the old ones. Good idea.

[Sniper]

there is a special jig they use to measure rod end weight, as an example.

 

https://www.speedwaymotors.com/Connecting-Rod-Balancing-Stand,2646.html

[Bryan]

32 minutes ago, Sniper said:

there is a special jig they use to measure rod end weight, as an example.

 

https://www.speedwaymotors.com/Connecting-Rod-Balancing-Stand,2646.html

Dang, think I'll let the machine shop do that..

[Sniper]

lol. if you have a scale you can probably rig something up to hold the rod level as you weigh the ends.

[Dodgeb4ya]

A $100.00 bill weighs a gram..piston weights should not differ by more than a $1.00♟

[Bryan]

2 hours ago, Dodgeb4ya said:

A $100.00 bill weighs a gram..piston weights should not differ by more than a $1.00♟

Namely, also a gram.

[rallyace]

Similar to the fixture that Sniper indicated, when I did my rebuild I went cheap but effective. I took a piece of coathanger wire and a couple of pieces of scrap lumber and made a similar fixture.  I was astounded how well balanced the stock rods were.