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

So I made it out to the garage finally after all the Christmas festivities and my goal today was to get my pistons and rods balanced up so I can determine where they will go on the crank during assembly. As it turns out one of the pistons (with pin and circlips) was about 10 grams lighter than the next lightest piston. When I decide to take a closer look at it to see if some material has been ground off somewhere (all the pins are within .3 grams of each other), I notice that the middle compression ring is missing!!! WTH!! So it only had the oil ring and the upper compression ring nothing in the middle groove. Have no clue how this happened. My mechanic tore the engine down before giving me back the parts.

Would my car exhibit any odd running behavior from this? smoking? low compression in that cylinder? Anyway the engine didn't smoke and seemed to have good power when it was dynoed at an estimated 305 hp at the crank.

I reweighed everything and came up with the following: Rod, cap, bearings, piston, pin and circlips)

1-1232.2 grams (lightest)
2-1233.5
3-1239.2 (heaviest)
4-1237.0
5-1233.5
6-1237.0

Does this seem acceptable to install? which order and where would I place these on the crank? Do I need to have some material removed from the piston pins to get the balancing closer? Thanks



http://forums.pelicanparts.com/uploa...1451177258.jpg

I always shoot for zero and leave the rest to fate .
Supposedly 1 gram is like 30 lbs of imbalance or some rumor like that at 6000 RPM.
I always put pistons back in the holes I found them.
Find the lightest rod, make all match.
Yes you can do end for end, but afaik it is not worth it while unless you are heading for the RPM stratosphere.
Find the lightest piston and make all match .
There are numerous places to carve weight off . A little off the skirts, a little off the pins . A little off the bottom of the rod end.
I would not make it a religious experience.
I have never had a engine blow because of where the weight was shaved .

I pair them up so they are opposite from each other. The heaviest pair go nearest the flywheel. The lightest pair go near the pulley.

you can alSo grind them a bit but the boxer is a design that takes care of some imbalance

So would the best thing be to take the piston pins to a shop and ask them to remove X amount from each pin to reduce all the total weights to the smallest total? Or is their a practical way I can do this myself?

You will do it with more love .
Its just not that scary.

Is there an explanation as to why this is done? just curious. It makes sense to oppose the heaviest to balance but not sure on why the proximity to the pulley or flywheel. Thanks

The flywheel will compensate better . ( mass )

Balance the pistons as a set and the rods as a set.
Checking end to end balance on the rods is very important. I've seen factory installed (matching rods) 4+ grams out from end to end. I've also seen rods that weigh the same 9 grams out from end to end.
There are many ways to accomplish this task but sending them to a qualified machinist is the best option.
Our target is 0.1 grams but sometimes that's a very lofty goal.

Can someone point me to some threads or info on how to diy this material removal for the rods and pistons?

No magic required
Some sort of grinder .
https://www.youtube.com/watch?v=QLpiF0E0EJU

One of many on youtube

Personally, I did not hesitate to buy a precise balance of laboratory in 0.01g, of the blow I balanced everything in this value. :)

So am I seeing this right Im supposed to drill divets into the backside of my pistons to remove weight?

Excuse my ignorance, but can you explain what you mean by 'end to end'? And how to measure it and accomplish it.

Thanks

Please look at the posted Video .
There are lots more there too.
I have HAD to take weight off the most inappropriate places in various kinds of engines to make it all work . ( I do more then just P car stuff )
That said a 911 engine is EZ .

Keep in mind that John is a general machinist and some of his techniques are not Porsche specific.
Case in point, the ridge on the rod cap on a Porsche 911 rod should not be ground on to lighten the rod. Weight should be removed from the area around the connecting joint.

True, But I think the idea can be construed out of the video .....
I think us old farts job is to encourage the young generation to "get er done".
Every trip starts with that first step.

I agree to a point. There are things that are truly Porsche specific that, when ignored, render some pretty valuable parts useless.

My amatuer home rod balancing setup... i balanced them down to 1/10 gram with the ceiling fan off.
It runs good and smooth.. i think it's OK.
http://forums.pelicanparts.com/uploa...1451353720.jpg
http://forums.pelicanparts.com/uploa...1451353699.jpg

Thanks JF. So is that the only place you took material off (where the cap and rod bolt together)? What about your pistons did you have to remove material on them? if so where/how?

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.

http://forums.pelicanparts.com/uploa...1451403794.jpg

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.

http://forums.pelicanparts.com/uploa...1451403751.jpg
http://forums.pelicanparts.com/uploa...1451403777.jpg
http://forums.pelicanparts.com/uploa...1451403809.jpg

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 )

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.

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.

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

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.

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

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.

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

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.http://forums.pelicanparts.com/uploa...1451784038.jpg
http://forums.pelicanparts.com/uploa...1451784063.jpg

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.

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.

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.