piston/ring land longevity

Last time I got 50,000 miles with stock Hepolite pistons on my 750 it was smoking like a pig. The vertical ring land play was getting sloppy so I bored it and went to longer rods.

Now it has another 50,000 miles on the rebuild, the ring lands are still snug and it doesn't smoke. Why?

Recently I realized that the longer rods reduce the intensity of the direction reversal "snap" at the top and bottom of the stroke. The rings don't have to crash as hard against the ring land with longer rods as they do with shorter rods because there is more "dwell time" near TDC and BDC. I had always thought that the combustion pressure is what wore out the ring lands but now I don't think so because I was blowing out the Hepolite ring lands in less than a year when I was over revving on the race track (back in the day). That's not happening on the track with the longer rods.

The thread below got me thinking about this. "I found 7-8 thou of play" and similar play was reported by another rider at:
https://www.accessnorton.com/NortonCommando/piston-question.24127/#post-354928

Might have something to do with the difference between the cast Hepolite pistons, particularly if they were the early ones with narrow oil rings, and the forged JE pistons in your long rod kit. At least I'm assuming that you used those pistons with the longer rods. Back in the '70s, I found the Hepolite Powermax pistons to get very worn in the ring grooves after a season or two of racing. The forged JE pistons seem to hold up better in racing use, although I don't think I ever put as many miles on a set of them as I did on the Powermax.

Ken

lcrken said:

Might have something to do with the difference between the cast Hepolite pistons, particularly if they were the early ones with narrow oil rings, and the forged JE pistons in your long rod kit. At least I'm assuming that you used those pistons with the longer rods. Back in the '70s, I found the Hepolite Powermax pistons to get very worn in the ring grooves after a season or two of racing. The forged JE pistons seem to hold up better in racing use, although I don't think I ever put as many miles on a set of them as I did on the Powermax.

Ken

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There is argument about what material holds up better - cast or forged. But I also think the JE forged pistons hold up better - but I don't know if its because of the material or the longer rods.

maybe the different in piston pivot position of the longer rod set up has the tendency to make the piston to rock less violently. Is there a known effect to piston wear based on how high the piston pivot is located on the piston??

o0norton0o said:

maybe the different in piston pivot position of the longer rod set up has the tendency to make the piston to rock less violently. Is there a known effect to piston wear based on how high the piston pivot is located on the piston??

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With the longer rod and higher "piston pivot position" - everything is smoother and less violent (stress is reduced). But I think part of the reason for longer ring land life is the material durability of the forged JE pistons that both Ken Canaga and I use. Piston ring land wear is the big problem with stock cast pistons and I don't think rocking is the cause of ring land wear. What you want is a piston made with better material (either forged or billet 2618 or 4032 aluminum) and if you can add a longer rod into the bargain so much the better.

It’s interesting because the cast Hepolites have a higher silicon content while the forged JEs have a lower silicon content and silicon hardens the surface. It could be though that greater hardness doesn’t help as much as we might presume and that the greater plasticity of the lower hardness forging somehow is a helpful factor.

My experience though is that forged lands bend while not breaking as cast lands do. And, bent lands, while far less catastrophic, still lose ring seal, can lead to seizure or other failure, and so are not much better than cracked ones

xbacksideslider said:

It’s interesting because the cast Hepolites have a higher silicon content while the forged JEs have a lower silicon content and silicon hardens the surface. It could be though that greater hardness doesn’t help as much as we might presume and that the greater plasticity of the lower hardness forging somehow is a helpful factor.

My experience though is that forged lands bend while not breaking as vast lands do and bent lands are not much better than cracked ones

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That's why I said earlier "There is argument about what material holds up better" because cast is supposed to be harder. But from what I've seen the Forged ring lands hold up better. They are also a more precise and tighter fit when new compared to sloppier cast ring lands. The rings are thicker and heavier on the stock cast pistons and that in itself could make a huge difference in the pounding effect on the lands. There is no "ring flutter" problem with the thinner rings we use on the JE forged pistons but there is ring flutter with the thicker heavier Hepolite rings for stock cast pistons which are a third thicker or twice as thick compared to the thinner forged JE piston rings (depending on the choice of forged ring thickness).

And, we are assuming that the old Hepolites actually did have a higher silicon content.

Yes, rod ratio and pin position are factors in ring land dimensional durability. Thin light rings might be a greater factor.

When I was messing around with Ducati singles I had direct experience with long and short rod engines of identical bore stroke and - of course - displacement.

I preferred the short rod engines because they had more snap in the mid range and, in Griffith Park, I needed that to hold off the RD 350s by coming off corners as hot as possible. That was the one edge I had on them and the later wide case singles with long rods just didn’t do it as well.

That said, the wide case long rod engines’ top ends lasted longer, especially when run a lot at high revs. If I had been road racing on a track, then my preference would have been for the long rod.

The wide case long rod engines were desmodromoc spinners and the long rods went with that theme

xbacksideslider said:

And, we are assuming that the old Hepolites actually did have a higher silicon content...

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Yes the Hepolite alum is questionable - especially when you consider Ken Canaga's experience that his version of forged JE pistons (using original short rods) have longer lasting ring lands than the cast Hepolites. But this could be because of the thinner JE rings.