Re: [AMC-list] AMC factory engine balance

[tom jennings <tomj@xxxxxxx>]

On Fri, Feb 19, 2010 at 14:02, Frank Swygert <farna@xxxxxxx> wrote:


> I don't know what the "close" standards were though. I know the standard
> for balancing an engine after the fact (a shop doing it) is +/- 1 gram for a
> piston/ring/pin assembly, and for each end of the rod. That's 2 grams leeway
> from lightest to heaviest. [....]Modern casting/machining is very good! I'd
> say "modern" being from around 1960 up. You have to almost go to pre-war
> stuff to find really rough castings that vary more than 4-5 grams in weight.
>  <http://list.amc-list.com/listinfo.cgi/amc-list-amc-list.com>
>


... or go into AMCs stocks to find 50 gram differences...! Let me tell you a
little story about 195.6 rods...

I static balanced all my rods, and all pistons+pins. The pistons, .040 over
unknown brand new/unused, were all within 1.5g (my notes are out in the
lab...) Since the pistons are just flat-bottomed cylinders I just took metal
off the bottom (shortened) them to just about 0 gram difference. Polished
the tops too.

Rods... OK I have three sets, kept each set together, but used the set from
the NORS rebuilder. Those rods varied from 640.g to 653g, centered around
642. I made 'em all 640.5 within 0.5g. That one I had to take almost 13g
off, so I went to the other disassembled set to get one closer to 640. Plunk
them on the scale one by one, quick check... hmm that's odd.. 680, 690, 680,
690, ...!

Check part numbers. Exactly the same. A couple of one and three digit
numbers elsewhere on the part, but there are differences there even within a
set.

Called Doug Galvin. He said, in 63 or 65 AMC cast new rods with the same
part number and DIFFERENT MASSES. Yup, no way to tell them apart except on a
scale! No factory instructions/tech notes on them. So the lore is, as long
as they're the same as a set you are OK.

Sheesh!!

Luckily you can do that with the six, but mix'n'match, now that would be
Unpleasant. Or maybe not, who knows, maybe Grandma wouldn't notice a bit of
vibration at 3500 rpm!

I am sooooo glad I did that balancing now!



But even within a "set" (no such thing of course, but they're the same
"cohort", probably all six rods came from the factory together) there was a
13 gram spread in the rod weight.
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