Piston Weights

[DSPTurtle]

My wife is out of town so my kitchen looks like a machine shop
I just finished weighing my pistons... by moving the heavy wrist pin and light piston together I am able to get two of the pistons to weigh at 599 grams. The third weighs only 597 grams (I am only doing the one side to replace a head stud).
Should I try to take 2 grams off of each of the wrist pins for the other two cylinders or am I straining at gnats?
If I do take off some material, do I take it out of the bore in the wrist pin? Or, I guess I could weld in bolt to make the one piston heavier
Any thoughts are greatly appreciated!!!

[304065]

Yeah, weld in a bolt or have a beaver gnaw some material off the pin

You know we spend too much time here when we get that joke.

Why not match the rods in the set and make everything weigh the same before you cut anything?

[DSPTurtle]

We don't have too many beavers down here in Florida... at least not the kind that builds dams... What is amazing is that it is the most read thread on the whole dang board! I am officially dumber for having followed that melee.
I am going to get flamed for this, but I just want to get back on the road... I did not pull the rods. So, my poor man's solution is to just get the pistons and wrist pins as balanced as possible.

[2.7RACER]

This can be some fun.
The piston and pin assembly is matched to the opposite set.
That is 1 is matched to 4, 2 is matched to 5, and 3 is matched to 6.
If you want your engine balanced, pull the other side apart and weigh those assemblies.
The heaviest set goes nearest the flywheel.

[notmytarga]

Since you are not into the other bank you are not really balancing anything by making changes. The Potential benefits will come with the risk of reducing balance. The factory may very well have had the opposing parts match but had a variation within the bank as you have found. That said, you are within a gram or two and will be fine either way.



As you can see my wife was out of town over the weekend too.




I had my parts balanced last week. I did not disagree with the machinist when he proposed taking the aluminum off the bottom of the piston crown with a mill. Wrist pins are all close and the steel makes them more difficult to mill and right where the stress is applied. Two pistons needed this much milling to balance - the others didn't. Now all within a gram with their assigned pins. Yes, I couldn't help myself - I polished the domes.

[DSPTurtle]

Well after adding the rings and moving the wrist pins around a little bit I am now at 636 (#4), 638 (#5), and 637 (#6). Not too bad for gov't work. Oh yeah, I think I took the sword maker guy a little too seriously about getting the piston tops polished A light 450 wet sand to get the machine marks out, a 2000 grit wet sand to smooth out the scratches and a little bit of time on the polishing wheel with a medium pad and polishing compound. To top it all off I used some Mothers mag wheel polish to get the glossy finish. I figure after the first 500 revolutions my 4 hours of work will be reduced to a pile of nice hard carbon

[304065]

Egad! Finless cylinders!

Seriously, those were tried by Porsche and they didn't work, the consensus is to replace with the full -fin variety. Particularly for a turbo-motor. Contact tom1394racing for help.

[DSPTurtle]

Those are pure stock Mahle pistons and cylinders. I measured everything in the workshop manual and it Wayne's book. Only downside was some ring gaps in the .04 range... over the .03 for new and under the .08 for wear limits.
The fins are on the other side

[aws]

I have used the "fin-less" turbo 97mm cylinders in several twin turbo race applications and in several atmo racing applications with no problems. One customer blew a fan belt off the alternator and kept going until the engine got so hot it caught fire. On tear down the JE pistons had deformed but the cylinders checked good. Porsche used those cylinders from 1978 to the early 90's and I have torn down high mileage cores that show no ill effect from the "fin-less" cylinders. When Porsche designed the 3.6 engine they increased the fin area on the cylinder head by 17-20% but if you look at he 100mm cylinders they have very little fin area, with the top of the cylinder barely having the outline of a fin. My opinion is that the pumping action of the pistons cools the cylinders internally quite effectively. I understand why people don't like the idea of a "fin-less" cylinder but I have had very good luck with them.

aws

[tom1394racing]

I agree. 1/2 finned are OK to re-use.

I have inspected and sold a half dozen sets of these in the past couple of years.

While these are more susceptible to becoming out of round, the key is to check the bore diameter at 90 deg intervals at several locations along the stroke. The factory spec limit for ovality is 0.04 mm.

[ChrisBennet]

Originally Posted by DSPTurtle

Well after adding the rings and moving the wrist pins around a little bit I am now at 636 (#4), 638 (#5), and 637 (#6). Not too bad for gov't work. Oh yeah, I think I took the sword maker guy a little too seriously about getting the piston tops polished A light 450 wet sand to get the machine marks out, a 2000 grit wet sand to smooth out the scratches and a little bit of time on the polishing wheel with a medium pad and polishing compound. To top it all off I used some Mothers mag wheel polish to get the glossy finish. I figure after the first 500 revolutions my 4 hours of work will be reduced to a pile of nice hard carbon

Pretty.

I'll reiterate what 2.7Racer said. Your attempt at balancing most likely has made your motor more imbalanced.

Imagine if you will, a playground with 3 see-saws and 6 children balancing them out. 2 45lb children on one 2 56b children and another and 2 64lb children on the 3rd see-saw.

45--45 (The "--" between the numbers represents the see-saw)
56--56
64--64

That is how the rods and pistons are matched up in you motor as it comes from the factory. It costs them nothing to sort all the rods and pistons by weight and them pick matched pairs when they built the motor.

Now imagine a well meaning teacher comes along and notices that the children on the left side of the see-saws all have different weights so he takes some books out of the heaviest and puts them in the backpack of the lighter children until they all weigh the same - at least on the left side.

55 \ 45 (The "\" or "/" represents the tilted see-saw.)
55 / 56
55 / 64

Of course now that we've "balanced" all the children on the left they no longer weigh the same as the children they are sharing the see-saw with on the right side. Now none of the see-saws are balanced anymore.

Does that make sense?

-Chris

[DSPTurtle]

That is an excellent description. Fortunately I did not do anything to change the weights of the pistons other then record their weights. One wrist pin was .5 grams heavier then the other so I put it with the lightest piston. I have read some great posts on here about getting within 2 grams being pretty darn good so to be there without actually touching metal makes me happy. Plus, my scale is probably only really accurate to +-1 or 2 grams anyway. Chinese made HF scales are really only good enough for the folks selling stuff on street corners
A few years from now (hopefully) I will be back in here and get to do the whole shebang!!!

[ChrisBennet]

So you are going to put them back the way your found them right?
-Chris

[DSPTurtle]

Yup... only a little shinier