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

[JonT]

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

[afterburn 549]

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 .

[ChrisBennet]

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.

[jesus]

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

[JonT]

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?

[afterburn 549]

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

[JonT]

Originally Posted by ChrisBennet

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.

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

[afterburn 549]

The flywheel will compensate better . ( mass )

[Henry Schmidt]

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.

[JonT]

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

[afterburn 549]

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

One of many on youtube

[Pat RUFBTR]

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

[JonT]

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

[93nav]

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

Thanks

Originally Posted by Henry Schmidt

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.

[afterburn 549]

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 .

[Henry Schmidt]

Originally Posted by afterburn 549

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.

[afterburn 549]

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.

[Henry Schmidt]

Originally Posted by afterburn 549

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.

[JFairman]

Originally Posted by Henry Schmidt

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.

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.

[JonT]

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?