Piston Balancing ?'s
 

I am finally getting around to the top end of the 2.7 track engine. The P&C set I picked up is purportedly a used set from Andial, which looks like about a 10:1 CR. These were not cheap, but cheaper than finding Nikasil cylinders and new JE pistons. The P&C are Mahle, in very good condition.

Here's the rub: I bought these from someone who had three sets in the shop. The weights (according to my triple beam balance) run from approximately 494g to about 514g (I'm too lazy to get my notepad from the basement at the moment). I would ordinarily just take some weigh off around the pin bosses, but these pistons are sort of flimsy, for lack of a better term. Weights are including the pistons and pins, no rings or clips.

My options are:
contact the seller, and see if I can match up the weight groups better,
try to get the things to weigh somewhat the same (strive for perfection),
try and get them right on in opposite pairs on the crankshaft, and get the entire set within six grams.

I seek guidance!

Pat

Swap some wrist pins around and try to get them closer that way first.

I just went downstairs and measured the pistons and pins separately. The pistons are good. Some of the pins weigh 18 grams more than the others! I'll get the chart.

I have a spare set of 2.4 pistons, whose pins are longer. I have to check them out. I've never seen pins so far out before, unless they all came from different sets.
Pat

http://forums.pelicanparts.com/uploa...1198810066.jpg

I'm not sure how close they have to be, but if you juggle those pins around, putting the heaviest pin with the lightest piston and vice versa, you wind up with the heaviest set being 507.0 grams and the lightest being 493.1 grams.

Average = 502.0g

Piston 1 + Pin 3 = 493.1g (avg - 8.9)
Piston 2 + Pin 2 = 499.3g (avg - 2.7)
Piston 3 + Pin 1 = 505.6g (avg + 3.6)
Piston 4 + Pin 4 = 503.5g (avg + 1.5)
Piston 5 + Pin 6 = 503.6g (avg + 1.6)
Piston 6 + Pin 5 = 507.0g (avg + 5.0)

Definitely an improvement, but is that close enough? Also, out of curiosity, is pin #3 noticeably smaller than the others or made of a different material or something, given that it's 18.5% lighter than the average of the other 5 pins? If you could replace it with a pin about 9 grams heavier and pin 5 with one about 6 grams lighter, you'd be a lot closer.

Assuming pin 3 is replaced by a 9g heavier pin and Pin 5 is replaced by a 6g lighter pin:

New average = 502.5g

Piston 1 + New Pin 3 = 502.1g (avg - 0.4)
Piston 2 + Pin 2 = 499.3g (avg - 2.8)
Piston 3 + Pin 1 = 505.6g (avg + 2.1)
Piston 4 + Pin 4 = 503.5g (avg + 1.0)
Piston 5 + Pin 6 = 503.6g (avg + 1.1)
Piston 6 + New Pin 5 = 501.0g (avg - 1.5)

Disclaimer: I am not an expert, and I just did this math after getting home from a 10.5 hour shift @ work, so I'm a little brain-fried; before using my numbers for anything important, double-checking them would probably be a good idea.

olstyn:
Wow. I appreciate the effort on the math. The problem is that Porsche wants a six (6) gram difference, maximum in the weights.Since this will be a track engine, ie, high revs, the imbalance will be noticeable at 7500 RPM. And, no, I see nothing wrong with any of the pins, I just think some are less meaty than the others. Dimensionally, they are the same.

I sent a note off to the person I bought them from to see if he has any from the other sets lying around. I also sent a note to Andial to see if I can get any from them.

This always starts a debate..bwth....once you move the pins around, then you can shoot for what ever weight you want.... most everyone wants light.
This worked for me, and has been running for along time
you can take some off the bottom of the skirt ( in my case I took a lot off)
bore out the pins some
grind some off the ends

I ended up with pistons lighter then any group Porsche list, have seen rpms into 7000 on a few occasions.
Again, please no debate here.......... worked for me

No debate from me, I prefer lighter. I'm going to wait and see if I can get some matching pins, and take it from there.
Pat

I know in Wayne's book he specifically states that piston balancing is done with the pins - meaning, the weights of the pistons are not altered it is the weight of the piston with the pin integral. All metal removal is done on the pins itself.

Probably explains why you have such a variance in pin weights.

Hope it helps

Hey Pat

Those seem like different wrist pins. I've never seen such a big weight difference in pins. I'm betting the pins are not all the same. There are subtle differences in the pin types that are not always obvious. Measure the length, diameter and wall thickness for each pin with a hand held caliper.

I have boxes full of wrist pins and can provide a set that matches to within +/- 1 gram if you need some.

Tom:
Yeah, I'm pretty sure these pins are from different sets. I'll get the measurements and let you know.
Thanks!
Pat

You have gram scale.........just do it

When I bought 6 new JE forged pistons and lightweight wristpins back in '96 they were all the same weight right to the tenth of a gram.
I wouldn't settle for anything less, so I would send yours back to whoever you got them from and have them send a matched set of wrist pins... not just the weight but the alloy and wall thickness too.

Then get a closer matched set of pistons from him if possible too. It should be easy to remove/polish some metal in certain areas around the wrist pin boss to get them exact.

Isn't it enough to just divide them up into matched pairs for the opposing cylinders?
-Chris

Chris: Actually, in my mind, the imbalance in that method would introduce additional stresses radially in the crankshaft, at the main bearings where the heaviest pair is. Imagine a worst case scenario, where you had really heavy pistons/pins on say 3-6 and lighter ones on 1-4. You would tend to beat up the 1-2 and 3 main bearings due to the imbalances. On a street car, I might consider this, but not for a track engine. I have always felt that the closer you can get to complete dynamic balance, the better you can pull revs effortlessly. I think I just need matched pins.
Pat

I don't see how having heavier pistons on one end of the engine would induce wear on the other end. The engine is basically just 3 separate 2 cylinder engines. Each piston in the set moves in sync with it's mate. This is why the engine is so well balanced in design.

-Andy

If I recall, the factory allows an 8 gram spread in piston weight.

Tom:
These pins appear to be 22.02mm x 57.93mm (0.867" x 2.281").
Pat

I am ..anal (I guess) but I shoot for 0 grams for every thing...and hope for the best. If "o" grams is the target then you will not be off by much in the dynamic....thats my way way of dealing with the unkown factor

Yup, zero is optimum. In my case, looking at these pins, they are not something I can just take a rotary file to. So, we'll see what the pin gods can send my way, and wait it out in the meantime.

Pat

22X58 mm is a pretty common size for a Porsche wrist pin.

The differences in weight are caused by differences in the inner diameter of the pins.