Piston Lightening from skirt- your view?

[304065]

In an effort to achieve perfect internal balance I weighed all my JE pistons with an electronic balance to a tenth of a gram.

The pins all weighed within a tenth, and they are hardened, so removing much material there was unlikely to produce the desired results. (They all weighed 109.1g)

The pistons themselves were fine, but once the rings were added there were some small weight differences. The standard practice is to mill material off under the crown, but I don't have a Bridgeport handy. . .

. . . so I removed a tiny amount, less than a gram of material, from the edge of the skirt and the pin boss. There's a very small lip on the end of the boss that didn't appear to serve any purpose other than providing a pad for balancing, so I polished it off. . . and polished the entire area to a mirror finish to eliminate stress risers.



So, pundits, have I done catastrophic harm? Remember that this is a street engine with an absolute safe limit of 7000 and a powerband that ends in the mid sixes . . .

With this change, all the pistons are balanced under 1g, and the heaviest will go at the flywheel end.

[sww914]

You know more than I do, but you did ask for my opinion, I'm quite sure that you're in good shape.

[cgarr]

It looks like you just took off the sharp edges? thats good!

[304065]

I took off the ridge (you can see it on the uncut piston on the right) and removed a tiny amount of material from the bottom of the pin boss (the rectangular section).

It's hard to see because the mirror finish reflects the flash, but there are no scratches in that section. I was concerned that scratches from the abrasive I used to remove the material would present stress risers where a crack could get started and then propogate into the pin boss-- so I spent the better part of an hour polishing the areas down-- then another hour cleaning the pistons to remove any trace of the polishing abrasive.

I'm not sure I would do it again, probably just leave them alone and weight-match to the extent possible. The factory limit is something like NINE grams?

[Porsche_monkey]

Not an expert (Henry is, not me), but a) I do not think it will matter in the least and b) I personally would not have bothered.

[Walt Fricke]

I've taken way more than a gram of aluminum off of the underside of the crown with a die grinder. When I got worried about thickness, I got some more off the inside vertical edge of the pin boss, especially at its top (viewed as it would be grinding on it). Seems to me that the farther from the pin axis boss material is in the crankshaft direction, the less it is apt to contribute, especially where it is farther than the shortest such radius. But I may be overlooking some buttressing effect.

Stress risers and all, these held up at 8,000 rpm racing. Mahle 90mm nominal 10.3/1 racing forged pistons in a 2.7. Had to match a singleton for the usual reason. Done this twice, in fact.

I see that J&E has some interesting structure there, with things thicker toward the inner edge. Perhaps that little kick down you ground off is designed to strengthen in a similar way? Hard to imagine it doing much, as it looks so small.

Just like the tiny amount you took off. Isn't differential carbon deposition likely to put them all off by a gram or so after you run it a while?

But that probably isn't the point. Aesthetics and perfection count for something. I can see this engine being a long time in production before it actually powers a car. But I can't see anything failing due to the one gram less aluminum from that location.

Walt Fricke

[304065]

Walt, thanks for weighing in (pun most definitely intended!)

I can't imagine why JE put that lip on the pin boss, perhaps it is actually a pad to be removed for weight-matching?

Certainly carbon deposits will form, like all things in life entropy will set in. I cannot control that, all I can control is the starting mass.

LOL, this engine will be a VERY long time in production, but given the pace of the restoration of the CAR, it will be first to the "marriage line!" The car has taken far longer than anticipated due to some discoveries immediately prior to media blasting, e.g. somebody clipped the nose section with a '69 and then went to elaborate lengths to make it look SWB, plating over the battery boxes and brazing the ARB support panel from the original car back in, rather than just drilling out the holes in the stamping!

The reason for my concern (and the poll) is the oft-repeated worry about the strength of the pin boss in JE pistons. . . polishing may be overkill, but my mind kept returning to why we file down the nicks in airplane propellers. . .

[tom1394racing]

John

I'd say your extra work on the pistons was overkill for a low revving street engine.

The factory does allow an 8 or 9 gram spread. Most of the factory built engines I have disassembled have had around 2-4 gram spread.

[tctnd]

The ridge you removed appears to be an artifact left after machining the skirt cutaways. As such it would have no function (except to add useless weight).
regards.
Phil

[turboteener]

Your fine. You could still remove more material if need be.
If you want light check out these pistons.

[cgarr]

That looks like the piston on my Coleman Powermate Air Compressor I just replaced!

[turboteener]

As a matter of fact, it was from my shop air compressor. Turns 15K rpm. You ought to hear this sucker fire up.