Thoughts on a different approach to rod resetting

wha-tah-hey Mon Oct 31, 2016 5:04 pm

Having found, after resetting an .049 rod using a Davis tool, that the skirt had expanded (don't ask me how - no clue) rather than the crown, I thunk on it some.
It's a common practice to expand the piston top on old McCoys to get more life out of them and one method is to insert the piston into the (removed) cylinder to prevent over-expansion (still needs care in hammering) with the cylinder set on a substantial "anvil" (say, a bench vise flat).
If skirt flaring during rod reset is an unrecognized cause of the common tight piston (or if, in fact, the crown is commonly expanded) it occurred to me that one might:
a) lightly clamp the reset punch in a vise (filed or ground flats might help to keep vertical).
b) set the piston in place with the rod extending down between the jaws.
c) set the cylinder over the piston, adjusting "reset-punch-in-jaws" depth so that the cylinder rests lightly on top of the jaws when the piston top is at "TDC"; now clamp vise jaws securely.
Tapping the piston top with a short piece of 3/8" brass rod (for .049s) as the cylinder/piston assy is rotated resets the piston rod, the cylinder preventing any part from expanding significantly.
I believe the cylinder is substantial enough to not be harmed unless a too-heavy hand is used.
Sounds good if I say it fast - if I had one needing reset I'd try it.
Comments?

wha-tah-hey Tue Nov 01, 2016 9:06 pm

OK, I'll comment then.

Earlier today I exchanged the thin wall single port cylinder on my GB with a BW twin port.
It occurred to me that many manufacturing processes require swaging to form parts, I swage cast bullets to proper size, I have an enlarged piston.... hmmm......... wonder if I can swage the piston skirt back down by running it through this handy die that just ended up as an unused spare????

Short answer - yes. I tapped it through twice with 2 notable results:

1) it didn't resize the piston skirt - too much spring-back; possibly work-hardened so I'm gonna try annealing the skirt tomorrow - results to be announced.

2) it DID NOT damage the cylinder. Even a thin wall shrugged off far more stress than a rod reset could ever put on it.

My point - resetting a rod in the cylinder as I suggested will NOT damage the cylinder and I see no reason why it shouldn't reset just fine with no piston expansion.
Excepting, of course, a careless duma** who shouldn't be allowed around a hammer. Very Happy

Surfer_kris Wed Nov 02, 2016 1:50 am

To me it sounds like you are overcomplicating things...

Perhaps there is a problem with the small anvil you used? Either in the machining of it, or in the way it was used. It cannot act as an anvil without a strong, heavy, support, e.g. a large metal slab to rest on.

All you need is a flat table top (metal one, don't know the proper name in english) to place the piston on, then work with gentle taps while turning the tool in-between each tap. Don't try to remove the slop completely you only need to reduce it a little. The need for resetting will also go down with time as the two surfaces are mated well.

KariFS Wed Nov 02, 2016 2:48 am

I used the flat part on a large(ish) vise as an anvil when I re-set the joint on my Tee Dee .15. The jaws of the vise are about 6" 5" wide, and the flat spot is meant to be used as an anvil. Seemed to work well, although it did take a lot more "taps" than I thought it would. I used a small hammer, maybe 2 or 3 oz and took my time as it was the first re-set I had ever done Smile

Oldenginerod Wed Nov 02, 2016 2:59 am

If the skirt is the only part that is expanded, but it can still be passed through the cylinder without causing any undue damage/stress, then I suggest that you should just fit it & do a break-in run. Annealing will damage the case hardening of the piston. If you try to re-harden the piston by heating & quenching then the whole of the piston will harden, including the ball socket, and potentially the rod which may become brittle. You stand to ruin a cylinder and more.
I found a similar problem once when re-setting a brand new rod. I did it, so I thought, carefully and gently. Turns out I discovered a fine hair-line crack in the skirt. Was it there before? I don't know, but it's junk now.
As Surfer_kris said, don't over-complicate it. Once a piston is damaged it's toast. Get another one, or run the engine to re-break it in.

wha-tah-hey Wed Nov 02, 2016 9:49 am

The idea came from a sense, after reading forum posts by others, that piston expansion is a somewhat common problem when attempting to reset a rod. I may have an exaggerated notion of how common it is though.
In any case the procedure I've outlined is not complicated and actually took more time to explain than to set up.
For those who have a suitable vise this *will* prevent expansion with no damage to the cylinder.
A "fine tuning" note - the piston should be positioned just below, not on, "TDC".

I've examined my piston and there's no crack, but a good call, Rod - the possibility hadn't occurred to me.
The skirt is much too large to run in (it had to be driven through the cylinder by heavy taps) so you're right that it's toast - unless my annealing procedure works.
If not, no harm done, just a little funtime spent in the shop . Very Happy

Surfer_kris Wed Nov 02, 2016 10:07 am

Is it really larger at the skirt or simply oval shaped there?

Most proplems with mushroomed piston comes from the tiny anvil I think, that has been one of the conclusions in the past discussions. It has to have a relief at the circumference as in the lathe one cannot make a full 90° angle at the bottom. For a similar reason there also needs to be a relief at the very center, as the lathe will struggle to make it dead flat there.

Insufficient support of the small anvil might be another common issue, one has to be on a solid (large mass) support. Ideally a full-size anvil and then there is no need for the tiny one anvil anyway...

crankbndr Wed Nov 02, 2016 10:25 am

I don't have a anvil so I get down on the concrete floor with the OEM Cox tool, found it better to use a heavier hammer with controlled hits than a light hammer tapping away.
Never damaged a piston yet, after a few you get the feel of how hard to hit it, a very lite hammer doesn't work well IMO.

wha-tah-hey Wed Nov 02, 2016 10:55 am

It's definitely larger, Kris, by a bit over .002", how I've no idea as it fit well before resetting.
I kept the punch vertical, no excessive force - a real mystery.
The Davis tool I have appears to have an endmill plunge cut in the base, with a flat center.
Paul Gilbeault says he's stopped using these due to piston expansion and uses his vise as you recommend.
No doubt the problem is a matter of technique so if yours, Paul's and Crank's methods (essentially the same) work with no problems then that's simple enough.

Marleysky Wed Nov 02, 2016 1:05 pm

Thoughts on a different approach to rod resetting Kk1710

Nothing Fancy. I've only done a couple of cylinders so I have not had any expansion problems.

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