Balancing pistons and wristpins

[JFairman]

I have a set of new Mahle 3.3 turbo pistons and cylinders and wristpins and I want to balance them as close to .1 gram as I can with my old triple beam scale.
The wristpins are all within .2 grams of each other and the pistons are all within 4 grams of each other with rings on them.

I have Waynes book and he says to grind a little metal out of the inner diameter of the outer edge of the wristpins with a pointed carbide bur on a die grinder. I know it's also possible to grind a little aluminum off the inner corner edges of the wristpin boss in the pistons but I don't feel good about doing that to these new pistons.

Anyway, I'm asking what the experienced engine builders on this forum would do. Where ever I remove some metal with a carbide bur I'll finish with a 320 grit sanding cone on a die grinder arbor so the finish will be nice and smooth instead of rough from the bur.

Thanks in advance for any advice and here's a pic I took of some of the stuff. There is alot of meat in these thick wristpins so I'm leaning toward doing balancing on the innner outer edges of those.

[HawgRyder]

Balancing is a fine art.
Even individual pistons can be incorrectly balanced.
We used to be so picky...we balanced pistons using the centerline of the pin as the secondary balance point.
In other words...we not only took material off some pistons...but if we needed to take ... say 5 grams...we would take 2 1/2 off each side of the centerline of the pin hole at the samt height and distance from the top of the piston.
Very time consuming.
Also...don't forget to include the wrist pin clips in your measurements.
Bob

[JFairman]

Thanks for the reply.
I think I'm probably best off removing a little material from the inside diameter of each end of these thick wristpins and polishing the area smooth as final balancing.

Then install the pin with the area I removed material from toward the bottom of the piston with the lighter pins in the heavier pistons and heavier pins in the lighter pistons along with 2 circlips on the scale until they all weigh the same or within .1 gram of each other.
Here's a few pics of them.

[Trophy]

Wow those wrist pins are thick walled... How much do they weigh?

[JFairman]

136.7 grams plus or minus .1 gram

I used after market lightweight tool steel wristpins with JE 10:1 pistons in a 3.5 liter BMW euro 635 motor I built years ago and they were thin walled compared to these.

[Trophy]

They are quite heavy... The JE's ones I used in my 4.0l build were 40g lighter per pin..

Slow and steady with the removal for balancing is the way to go. Are you also weighing the Rods? It is always nice to play mix and match with all the components (Rods, Bearings, Pistons, Pins, Clips and Rings) I did this in my car and got down to .4 between all of them, and perfectly balanced left engine bank to right. I didn't have to do any machining in my case.

[JFairman]

I will be balancing the rods but in the meantime they're in this car and it runs great but the exhaust valve guides are worn out.

[Trophy]

That's a nice looking car...

[Walt Fricke]

Isn't there a point of extremely diminishing returns on this gnat's ass balancing? Isn't different carbon buildup from cylinder to cylinder going to put one at least 1 gram out of balance from one other?

[HawgRyder]

Horizontally opposed engines are inherently in balance if parts are close to the same weights.
V8's and inline engines benefit more from balancing IMHO.
We once built a VW engine with 92mm barrels at the back and 70mm barrels at the front...just to see what would happen.
We were surprised when it ran very nicely...actually fairly smooth.
Micro-balancing is more for race engines (maybe longer life at high RPM..?) than a street engine...or even on that is used for DE once in a while.
I'm not saying don't bother...but perhpas there are other places where the cash can be used.
Bob

[DSPTurtle]

Jim, I really would wait until you have the rods out. They will have a much larger tolerance for mismatch and by playing the Tetris game of mixing and matching you can get pretty darn close without firing up the die grinder. Or else you will have to once again break out the burr for the rods to keep your balances correct.

[JFairman]

Originally Posted by HawgRyder

Horizontally opposed engines are inherently in balance if parts are close to the same weights.
V8's and inline engines benefit more from balancing IMHO.
We once built a VW engine with 92mm barrels at the back and 70mm barrels at the front...just to see what would happen.
We were surprised when it ran very nicely...actually fairly smooth.
Micro-balancing is more for race engines (maybe longer life at high RPM..?) than a street engine...or even on that is used for DE once in a while.
I'm not saying don't bother...but perhpas there are other places where the cash can be used.
Bob

That kind of reminds me of where people have taken old VW motors and made them into air compressors by running them on two opposing cylinders and using the other two cylinders to compress air into a tank.

[JFairman]

Originally Posted by DSPTurtle

Jim, I really would wait until you have the rods out. They will have a much larger tolerance for mismatch and by playing the Tetris game of mixing and matching you can get pretty darn close without firing up the die grinder. Or else you will have to once again break out the burr for the rods to keep your balances correct.

I balanced all the piston and wrist pin assemblies today. Went slow and got them all the same weight. I didn't remove any metal from the pistons, I ground some metal off each end of the wrist pins by hand with a die grinder. I've seen it done like this before and it worked fine.
The numbers I wrote on the piston skirts is the piston weight with rings and no wrist pin. Some were turned so the numbers are on the other side.

When I balance the rods I won't use a die grinder anywhere on them. I take a 6" DA sander thats used for body work and lock down the pad so the dual action is disabled and it spins real fast like a disc grinder.
Then depending on how much metal I want to remove from the rod caps and up at the top of the small end radius I'll start with 80 grit sticky back sand paper, then 120, 220, 320, and end with 2000 grit to get a smooth polished finish with no stress risers.
I've done it before on BMW rods and they came out great. Balanced end for end.
I know all this balancing isn't needed but I like doing it, it gives me satisfaction, and the motor will run real smooth.
I want to have the rods resized and new wrist pin bushings installed before I balance them. Offset wristpin bushings for a small increase in compression would be nice.

[DSPTurtle]

Nice work. A DA or belt sander is way more better (haha) then a grinder. I got lucky on the last few engines that just by mixing and matching I had .5g left to right and 1g front to back. I'm with you on the satisfaction of getting that stuff perfect though. I'm even more anal on timing the cams. So when does the big build begin?

[KTL]

Nice scale! Did you steal that from the local junior high science lab? Just kidding. As long as it's calibrated, they're quite accurate.

For comparison, JE uses the thick wall pins on their 3.4L (98mm) replacement pistons. Reason being is the cylinder pressure is what dictates the req'd pin strength?

The 930 23mm pin is a 0.18 in./4.57mm wall thickness (905-2500-18-93C, 130g, 93 series nickel carbon steel) whereas the same diameter pin in a 3.2L Carrera is a 0.15 in./3.81mm wall thickness (905-2500-15-52C, 112g, 52 series tool steel). So the 930 pins are a much stronger, and heavier, pin.

Here's the 23mm pin listings for reference

JE Wrist Pin Search - JE Pistons

Interesting you chose to shave the pins, despite them already being a tight group in terms of weight spread. By comparison, I was supplied a set of pins (JE 22mm pins for a 3.0L build) that were off by quite a bit. Even with mixing and matching I wasn't able to get them w/in a desired 0.5 gram combined spread



So i'd have to do some shaving in order to tighten up the weight differences. I agree I don't like fiddling with the pin boss area. Leave that to the experts! Here's some of the lightening locations used on JE pistons- see page 4

http://www.jepistons.com/PDFs/TechCorner/SCPDrawings/JE_Piston_terminology_and_features.pdf