Opinions sought, mechanical question

[daTeacha]

I just wonder what you guys think about this one. One of my students brought in two pistons from a tractor she is restoring for FFA. The engine is a gasser, tops out about 1600 rpm. The pistons are 5 inches in diameter on a 4 cylinder engine. She has two new ones and two old ones. The new ones are about 9 ounces heavier than the old ones -- around 2.1 kg vs. about 3 kg. The plan is to place the new ones in cylinders 1 and 4 so they are travelling up and down together but opposite the old ones in cylinders 2 and 3. The things are pretty costly so they don't want to buy 4 new pistons. They tried it with the heavy and light running opposite each other and got a very shaky result.

Two schools of thought are these: 1) That's simply way to much difference in mass between the pistons for this to work, regardless of how slowly the engine turns. 2) Since it won't be running fast, it should be okay.

I suggested putting it together enough so it would spin on the starter, leave the plugs out, juice the 6V starter with 12V to spin it pretty fast, and crank it to see what happens. Any other ideas? I tend toward the idea that it's going to shake itself apart pretty quickly if they run it with the mismatched pistons.

 

[MrJimi]

How about lighten the heavy ones to match the light ones? And if you can get them and the rods perfect, you would have a balanced engine
Jim

 

[schmism]

or add weight to the lighter ones?

or do both (take half the weight from the big one... but it on the little one)

 

[MrJimi]

or add weight to the lighter ones?

or do both (take half the weight from the big one... but it on the little one)

How would add weight to a piston? Weight can be taken off with a lath
Jim

 

[Farmwithjunk]

I just wonder what you guys think about this one. One of my students brought in two pistons from a tractor she is restoring for FFA. The engine is a gasser, tops out about 1600 rpm. The pistons are 5 inches in diameter on a 4 cylinder engine. She has two new ones and two old ones. The new ones are about 9 ounces heavier than the old ones -- around 2.1 kg vs. about 3 kg. The plan is to place the new ones in cylinders 1 and 4 so they are travelling up and down together but opposite the old ones in cylinders 2 and 3. The things are pretty costly so they don't want to buy 4 new pistons. They tried it with the heavy and light running opposite each other and got a very shaky result.

Two schools of thought are these: 1) That's simply way to much difference in mass between the pistons for this to work, regardless of how slowly the engine turns. 2) Since it won't be running fast, it should be okay.

I suggested putting it together enough so it would spin on the starter, leave the plugs out, juice the 6V starter with 12V to spin it pretty fast, and crank it to see what happens. Any other ideas? I tend toward the idea that it's going to shake itself apart pretty quickly if they run it with the mismatched pistons.

There's going to be exceptions to every rule. You MIGHT get away with what you described. Or.... You might have a broken crankshaft, or bearings pounded out in no time, or a host of other troubles. It all depends on the type (brand?) of engine and how it's balanced.

I had a set of pistons balanced a few years back. A GOOD automotive speed shop/machine shop can do the trick. It's a little more complicated than just shaving weight. It has to be shaved in a place that won't effect the integrety of the piston. Often, pistons will have a raised "boss" just below the wrist pin location where weight is INTENDED to be removed as needed. That's where the "good speed shop" kicks in. Knowing WHERE to cut weight.

I'm also having a debate with myself on how you'd best pair those overweight pistons. 1 and 4/2 and 3, OR 1 and 3/ 2 and 4. Let us know what you all do and how it turns out.

 

[Pooh_Bear]

Let me see if I understand this.
You have a 4 cylinder engine.
Two pistons operate in tandem,
and the other two pistons operate in tandem.
Meaning two go up at the same time and down at the same time.

So why not put a heavy one and a light one together.
Wouldn't this keep everything balanced.

Note: I've never seen a 4 cylinder engine where the pistons
acted in tandem like a 2 cylinder engine. V-8 and V-6 engines yes.

Pooh Bear

 

[Farmwithjunk]

Let me see if I understand this.
You have a 4 cylinder engine.
Two pistons operate in tandem,
and the other two pistons operate in tandem.
Meaning two go up at the same time and down at the same time.

So why not put a heavy one and a light one together.
Wouldn't this keep everything balanced.

Note: I've never seen a 4 cylinder engine where the pistons
acted in tandem like a 2 cylinder engine. V-8 and V-6 engines yes.

Pooh Bear

I've worked on a couple old 4 cylinders where pistons ran in 2 pairs like described. One was on ignition stroke/cycle when "mate" was on exhaust stroke/cycle. On a low speed engine like that, vibration isn't as difficult to control as on bigger, higher RPM engines. Pairing cylinders like that supposedly gives more low end torque.

 

[JerryG]

I tend toward the idea that it's going to shake itself apart pretty quickly if they run it with the mismatched pistons.

I think that you are right. With that much weight difference, even at a slow RPM the shake is going to be awful. If you put heavy metal in the light pistons the cost will be almost as high as two more pistons. If you lighten the two heavy ones that much chances are that you couldn't remove enough weight in the right places to preserve the integrity of the piston. What I would do is have them look for a couple of used pistons. That would be the least expensive option and it would save the engine from self-destructing.

 

[EddieWalker]

I'm with Pooh Bear here. If you absolutely have to use the wrong pistons here, than his way would be the cheapest and easiest.

How much are the pistons? Why are they so differnt in weight if they fit that engine?

Do they fit the wrist pins? Do you have to buy the rings seperately, or will a kit come with a set and they fit both sized pistons?

Seems to me that when I rebuild an engine, I want it to last a very, very long time. I wonder how long this engine will last with the mismatched pistons.

Eddie

 

[daTeacha]

It's not my tractor. One of my students is restoring it for a 4H project. She doesn't own it, she's just doing the work for the project.

The pistons without wrist pins are about 12 ounces different. The new, heavier ones are shorter than the old ones -- about an inch less skirt. They take a smaller wrist pin, which means they will need a bushing around the pin when assembled.

If I remember right, the tractor is a '53 Oliver with something like an 11 inch stroke. I don't know if it's going to ever do anything more than just start up and drive onto a trailer or not.

Thanks for your input, I'll pass it along and keep you posted as things progress.