Balancing pistons and rods (and a mysterious missing part discovered)

[JFairman]

I had the rods resized and new small end bushings installed at an automotive machine shop first but thats the only area I removed metal from on the rods. I would have liked to bead blast or soda blast them first to get all the old brown staining off and make them pretty but you can't see them in the motor anyway.

The 3.3 Mahle pistons and cylinders I installed were new from a sealed box and I didn't want to grind or remove any metal from the pistons.

I weighed the pistons and wrist pins seperately and removed some metal from the inside outer edges of the wrist pins in a conical shape to balance the piston and pin assemblies down to 1/10 of a gram. I started with a carbide bur on a die grinder and finished up with a cone shaped grinding bit doing it slow and careful by hand.
I found this diagram on the forum some years ago.



In the bottom pic you can see where I removed some metal from the outer inside edges of the wrist pins to balance the piston and wrist pin assemblies so they all weighed the same.

[afterburn 549]

Thats the scale I use .
Remember to dust it off clean it up and it is VERY accurate .
It gives repeatable results.
The electronic ones do not seem as accurate. (to me )

[JonT]

thanks JF I don't have a die grinder is there something else I can use to take material off the pins? Dremel? Seems like that would take forever.

[JFairman]

I used an air pressure powered die grinder from Harbor Freight with a swivel adapter on the air hose connection so the air hose hanging off the back end doesn't limit movements of the die grinder. Some of them work really well and last a long time if you put a drop of air tool oil in them every 3 to 5 hours of use.

I've seen an electric die grinder at Harbor Freight that would work too. It looked well made but was pretty long so it wouldn't be as easy to maneuver around as an air powered one.
Electric Die Grinder with Long Shaft

I got 1/4" shaft carbide burs from toolsource.com Astro Pneumatic 2181 Double Cut Carbide Burr Set AST2181 AO2181
I went around and around at about a 45 degree angle with a carbide bur to start because it removes material quickly but it doesn't leave the area ground on smooth.

Then I finished up with conical grinding bits like this one in the die grinder.
Mounted Point Grinding Wheels - Shape A4W Size 1-1/4" x 1-1/4" | Shark | A4W
Stopping every few seconds to weigh the wrist pin and piston assemblies with rings installed until they all weighed the same on the triple beam.
The conical grinding bits wear down gradually so I went through 5 or 6 of them before I was finished.

[JonT]

thanks JF. any idea how much weight can reasonably be removed from a 141 gram piston pin?

[JFairman]

I don't know, probably not much.
You can buy after market lighter weight tool steel wrist pins that are thinner wall and very strong. Not sure where but EBS, or JE Pistons would probably know.

[JonT]

1-2 grams possibly? it looks like you took quite a bit off yours

[JFairman]

Looking at the range of bare piston weights I wrote on the masking tape on the sides of the pistons it looks like you can lighten the pins 3 to 4 grams without hurting them.

[JonT]

ok this gives me a good excuse to go buy a die grinder.... thanks

[JonT]

After failing to find a die grinder at Menards or Lowes I ended using my Dremel with a grinding barrel on a piston pin and was able to take 3 grams off the heaviest pin. That was a really slow process. I didn't want to take any more out of the pin so the rod was next and out came the angle grinder. This worked MUCH easier as I was able to take off 1 or 2 grams in a few minutes. I now have the totals for all rod/piston assemblies within a gram and will reweigh everything and fine tune. Next up will be assembly onto crank.

[JFairman]

Looks like you did a good job
When I did my rods in the same place I used an air powered 6" DA sander with the pad locked down so it spun real fast like a grinder. I had 400 grit sandpaper stuck to the pad when I did it.

[Green993]

When I did this to my pistons I used a dremel with a round grinder head and removed material from the insides of the skirts and re-weighed every couple of minutes until I got them mighty close. It took a while but I enjoyed it.

[afterburn 549]

It is not magic folks get the material off, leave NO sharp edges, always smooth edges, ESP on piston skirts , Just a little, you do not want a burr creating a Rembrandt on your cylinder walls.