Author Topic: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons  (Read 328 times)

dkmc

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6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« on: October 26, 2020, 03:55:22 PM »

I've seen the warnings "do not exceed 650 RPM with a cast iron piston", or "if you're intending to run 850 rpm, better have an aluminum piston fitted"....etc.
Could someone please explain this requirement in detail?

veggie

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #1 on: October 27, 2020, 01:28:28 AM »
dkmc

The cast iron pistons are much heavier that the aluminum version.
As such, the cast piston creates greater reversing inertial loads each time it's stoke comes to a stop and reverses direction, thus limited to 650 rpm.
The reversing loads created by the lighter aluminum pistons are less, and can tolerate 850 rpm (or even 1000 rpm on some engines)

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dkmc

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #2 on: October 27, 2020, 01:35:42 AM »

So then can an Alu. piston be retrofitted into an engine that had a CI one?  Re-balancing required I suppose...

Hugh Conway

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #3 on: October 27, 2020, 03:27:44 AM »
I think the higher speed engines have non-spoke flywheels.
A spoked flywheel (especially a listeroid flywheel) may become shrapnel in an overspeed  situation.
Personally, wouldn't attempt replacement with Al piston, rebalance, then higher rpm
Maybe some more informed members will add their .02
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38ac

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #4 on: October 27, 2020, 04:55:18 PM »
It is as Veggie said, the forces involved quickly go up with RPM. Even with the lighter piston the 8/1 engines had a change to the same H-D bronze backed babbit shell that was used on the heavy flywheel engines.  A cast iron piston engine is not going to fly apart at 850 RPM but the service life of both the big and small end bearings will be quite short
I have never seen an over balanced CS or India clone, they are all under balanced thus a change to an aluminum pistion will always smooth them out some,, in my experience.
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cujet

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #5 on: October 27, 2020, 10:14:20 PM »
My 20/2 was purchased as a 1000 RPM engine and has cast iron pistons. I don't run it at 1000 RPM though. Instead, I run it at 800 RPM. It's held up just fine throughout the years and multiple extended power outages after the hurricanes. My 6/1 also has a cast iron piston, I run it at 730 RPM.

My 20/2 also has the larger flywheels with spokes. No issues what so ever, but again, I'm running it at 800 RPM. I did calculate the G force and it is just over 200G's. Which for a flywheel is exceedingly low. By way of comparison, a typical V8 engine's cast iron flywheel withstands about 4000 "G's" of inertia at the perimeter at a 6000 RPM redline.

It is my understanding that either piston will do just fine at any RPM we typically run. Iron pistons are used on other diesel engines running much higher RPM, often 3000 or so.

I did pull my connecting rod bearing apart some years ago and noticed no unusual wear.

It would be interesting to weigh each type of piston and compare. The cast iron pistons really are not all that heavy in the hand. Especially when compared to the weight of the piston pin and connecting rod.
« Last Edit: October 27, 2020, 10:17:49 PM by cujet »
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dkmc

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #6 on: October 27, 2020, 10:50:52 PM »

Thanks to every one for the insight. I think I'll stick to 650 RPM for now. But now I know the details.

Willw

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Re: 6/1-8/1 650 VS 850 RPM and Iron VS Alu. pistons
« Reply #7 on: November 06, 2020, 12:08:08 AM »
FWIW when I fixed my Lister VA I replaced the original aluminum piston with a cast iron one and it didn't seem too badly off-balance; it just skated rather gently across the floor.
I realize this is opposite to what you are proposing to do, and it is certainly not running at 850 RPM but hey, nothing ventured nothing gained. There is a lot of info on this forum about engine balancing.

I was unable to free the stuck rings on the original piston, which is why I replaced it.

I have not run the engine very often, but I do have a new aluminum piston for it, when I get a chance to change it.

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