Allowable piston weight variance.

[machine shop tom]

I'm working on a BBC, 4.500" bore, 4.250" stroke. The customer bought an Eagle balanced assembly. I'm checking the weights and balance at his request (and my curiosity). The rods were out 2-3 grams on the small end, 4-5 on the big end. Those are now with .5 grams on either end.

The pistons are JE. The box weight and what the original balance job has for the pistons is 583 grams (without pin). The pistons actually measure between 580.8 and 585.4 grams. To me, that seems like quite a variance for an expensive (at least to me) performance piston. I don't see any real good way to remove weight from these pistons, so I'll probably leave them alone. I l know the weight variance isn't that big in the total scheme of things, but it bugs me that there is that much difference. Heck, I've see stock replacement pistons for many engines come in sets closer than these.

Does anyone here share these thoguhts? If I had ordered this kit, I'd be calling JE just to see if they find this acceptable..........

tom

[Procision-Auto]

A machine shop owner I do business with tells me that a few grams of difference is nothing to worry about. He says some pistons come out of the engine
with varying levels of carbon build up, and oil deposits that he doesn't get too crazy with matching everything to the last gram.

That is his opinion however; I don't know if a few grams will throw a typical street/strip motor out of wack, but perhaps a higher rev'ing performance motor
would care to have closer tolerances.

[Dave Koehler]

There is "usually" some variation in pin weight. Weigh them with pins and move the pins around to get a close match. As far as piston weight removal. No place under the pin to remove weight?

As far as "what is close enough" or "is nothing to worry about", this may or may not be true and will be debated til the end of gasoline. I look at it like this. We are being paid to be picky so none of that "close enough" flies too well for me.

[Wolfplace]

There is "usually" some variation in pin weight. Weigh them with pins and move the pins around to get a close match. As far as piston weight removal. No place under the pin to remove weight?

As far as "what is close enough" or "is nothing to worry about", this may or may not be true and will be debated til the end of gasoline. I look at it like this. We are being paid to be picky so none of that "close enough" flies too well for me.

Very very well said

Does it make a difference in the real world?
Probably not
Does it make a difference to me if I am doing the work,,
Yes it does

Tom
Let us know how the crank turns out,,,
I checked one a year or so ago that was out a ton on one end,, something like 50 or 60 grams at 3.5"
The good news is it was heavy,,,,,
To be fair the guy bought it off of Ebay but it was, according to him anyway... "a kit done by Eagle from an authorized Eagle dealer"

[Mark O'Neal]

Most piston companies have a 5 gram tolerance on pistons. We all try to keep it closer, at least I do, but in the end pistons are sold as "semi-finished".

[gem]

As has been stated here before balance is not exact. Variation in oil allowance, debates about over and under balance percentages, and real world examples of what will work abound. My perspective is that my customer is paying for the best that I can produce with my given equipment. The only time I would accept the degree of variance that you have measured is if I believed that reducing it would compromise the integrity of the parts to such a degree as to make them weak.

[gem]

Most piston companies have a 5 gram tolerance on pistons. We all try to keep it closer, at least I do, but in the end pistons are sold as "semi-finished".

In my experience you do much better. The best balanced set of pistons I have ever encountered were Probe. My customer (another shop) phoned back after we returned the job and said "Hey, you never balanced the pistons" I had to tell him I didn't have to. Less than 1 gram variation across the entire set. Only seen it once mind you.

[machine shop tom]

Thanks for the input, guys.

These pistons are pretty well finished as light as can be. I'm afraid to do any cutting on them, there's no real good place to take off 2-3-4 grams. I don't want to hack on them and end up causing a bigger problem than being a little less consistent in weight than I like.

The pins vary only .3 grams low-to-high, not enough to make up the difference. I did put the heavier pins with the lighter pistons.......

My bobweight figure is 14 grams lighter than the documentation provided with the kit. I can qualify my measurements, but the bobweight sheet they provided doesn't give the small and big rod end values, or oil value. My total rod weight is 1 gram lighter than provided on the customer's bobweight, but I did remove some metal from the rods' big and small ends. I'm not real impressed with the documentation (it's from Eagle......).

There is naught in the info provided to indicate that there is any intentional overbalance. I did calculate my #s at 51%, but it ended up way over their bobweight.

I guess I'll leave the pistons at plus or minus 2 grams and rebalance the crank at my BW figures.

Thanks again,

tom

PS, I just checked a flexplate balanced at another shop that is out 10.6 grams. I checked my machine and mandrel, it's withing .1 gram from the test weight. This guy just had his engine balanced there.

Oh boy........

[Wolfplace]

This is how I take a little weight out
You can usually get a couple of grams out here, more on some


ABS makes a cool tool for this with a nice radius
http://www.abs-products.com/balancing-s ... ters.shtml

[Dave Koehler]

Thanks for the input, guys.



My bobweight figure is 14 grams lighter than the documentation provided with the kit. I can qualify my measurements, but the bobweight sheet they provided doesn't give the small and big rod end values, or oil value. My total rod weight is 1 gram lighter than provided on the customer's bobweight, but I did remove some metal from the rods' big and small ends. I'm not real impressed with the documentation (it's from Eagle......).

You are dealing with an "averaged" balance job. They might go through 50 rods 5 years ago and take the average weight. Same with the other parts. Doesn't mean that this year you got one that is "in the averaged ballpark" anymore but that's why you get the big bucks. Personally I feel these types of kits being sold are good for our balancing business....once the client figures out what that good deal really costs.

[Mark O'Neal]

Most piston companies have a 5 gram tolerance on pistons. We all try to keep it closer, at least I do, but in the end pistons are sold as "semi-finished".

In my experience you do much better. The best balanced set of pistons I have ever encountered were Probe. My customer (another shop) phoned back after we returned the job and said "Hey, you never balanced the pistons" I had to tell him I didn't have to. Less than 1 gram variation across the entire set. Only seen it once mind you.

My son packs them.

The employees I can speak to.........I can beat HIS butt......

(Actually he could level me any time he wanted to, but he lets me pretend.)

[SchmidtMotorWorks]

If you don't want to cut on the parts but want a perfect balance job, you can set the each bob weight for each pair of parts.

[CNC BLOCKS]

If you don't want to cut on the parts but want a perfect balance job, you can set the each bob weight for each pair of parts.

We have had to do that a few times rather then hack up a set of rods or pistons. If the pistons are with a couple of grams and rhe rods are with in a couple of grams we will balance to the heviest one which may put you into over balance which not hurt nothing.

I spent some time with the great Smokey Yunick years ago and asked his opinion on balancing and his comment was balancing is an imperfect science my friend

I was at a shop last year in another state where a guy was doing a balance job for a friend and he was explaining how he got the rods and pistons with in a tenth of a gram and when he was done a took of the bob weights off I asked him to put them back on and spin it up again, So he did and one end was out just over 3 grams and the other end was out 4 grams, He looked like the little boy that shit his pants LOL And I said you were worried about the rods and pistons being off a tenth of a gram.

[k-star]

Tom, by any chance did you check all the pistons for size yet??? That seams like alot of difference for a modern piston. Last time I had a set of pistons off more then 2 or 3 grams one of them was machined from the factory .015" undersize on the od.

I would have to fix it if it was me. Maybe get the heavy one close as you can and cheat the small end of the rods some.


Keith

[Tony Mamo @ AFR]

Ive built my share of quality engines over the years....thinking back I dont think I had TRW slugs that were off by that much.

Is it the end of the world....certainly not as others have chimed in but regardless I wouldn't be happy about it.

Time permitting send them back to JE and let them tune them up, or get out the cutter like Mike featured a few posts up and have at it. Either way a call to J/E would happen regardless. Im curious as to their position on this.

I just dont like knowing I'm assembling something with a tolerance error that shouldn't of been there....regardless of how minimal the real world impact may be.

Most of the stuff I get is within a gram or so....but I could see even two being "reasonable".

Five seems a bit much for a quality piece.....(maybe for a set of Ebay pistons....LOL)

JMHO of course

-Tony