Opinions sought, mechanical question

[BillyP]

Sounds kind of stupid to me. After further thought, not kind of, it is.

 

[JimParker]

The new ones are about 9 ounces heavier than the old ones -- around 2.1 kg vs. about 3 kg.

The tractor is an Oliver 99 and it does indeed have a stroke of about 11 inches...

Let's see... Nearly 2 pounds (.9 kg = 1.98 lbs) difference in weight, and moving 11 inches per stroke... Even at 1,600 rpm, that's a lot of unbalanced force being developed on each up/down stroke...

Not only would I not do it,.. I wouldn't want to be standing anywhere near it when it cranks up to operating RPM!

My main concern, however, would be for the kid who instead of learning how to do something the right way, is being taught to take short cuts and perform shoddy maintenance. If this thing blows itself apart, what do you suppose are the odds this kid will ever feel confident turning a wrench again, knowing she destroyed an engine the last time she touched one?

PM me, and I'll find a way to pay for the 2 extra pistons so she can do it right... That's how strongly I feel about teaching kids right!

 

[SPYDERLK]

Let's see... Nearly 2 pounds (.9 kg = 1.98 lbs) difference in weight, and moving 11 inches per stroke... Even at 1,600 rpm, that's a lot of unbalanced force being developed on each up/down stroke...

Not only would I not do it,.. I wouldn't want to be standing anywhere near it when it cranks up to operating RPM!

My main concern, however, would be for the kid who instead of learning how to do something the right way, is being taught to take short cuts and perform shoddy maintenance. If this thing blows itself apart, what do you suppose are the odds this kid will ever feel confident turning a wrench again, knowing she destroyed an engine the last time she touched one?

PM me, and I'll find a way to pay for the 2 extra pistons so she can do it right... That's how strongly I feel about teaching kids right!

This weight comparison holds only for the original plan for putting the new pistons on 1 and 4. This plan was as close to wrong as you can get. There is an opportunity here to teach students about balancing. The heavy pistons can be dealt with by pairing them so that they counterbalance each other. For a low rpm tractor engine the heavy weights should be ok - - altho the percentage increase in weight is huge and will stress the rods and bearings beyond the manufacturers nominal ratings. The manufacturer didnt use boat anchor pistons for a reason. But they did make things more than strong enuf for the pistons they did use. Perhaps strong enuf to use boat anchors with rings.
Larry

 

[BobRip]

I agree. If you don't teach them to do it right, you are not teaching them right. Even those of us who know what is right and wrong have to fight not to take short cuts. PM me and I will send you $20 if that helps.

 

[Partsman2]

The rotating assembly has to be somewhere close to balanced or it will self distruct in short time. Those are obviously steel or cast iron pistons to weigh that much. If they are beefy enough you might be able to take enough off of the heavy ones and add a little back on the light ones and get it close. The weights need to be at least within 5% of each other. I'm more familiar with aluminum pistons but I know they can add weight as well as take it off. That would be the MINIMUM you would HAVE to do. It's not optional.

It doesn't cost anything to take it to a machine shop and ask if they can balance them and how much. They will want to balance the whole rotating assembly. This is the crank, pistons, rods w/bearings, flywheel, and any balancer if it has one. It probably isn't as much as you think. Of course I don't know your definition of "pretty costly".

You don't want to go to a pure speed shop as they won't want to mess with it and may not have access to the specs. There are shops in rural areas that specialize in ag and heavy duty equipment. I have a shop and we do more ag and heavy duty than pure high performance. We don't have the balancing equipment so we farm (no pun intended ) that kind of work out to a shop in a nearby town.

I also agree with the others with the lesson learned part of it. Let's not set the kid up to fail! I guess that's easy to say when you're not the one paying for it but it is the right way.

 

[cp1969]

Using 1600rpm and 11 inch stroke yields an average piston speed of over 2900 ft/min which is pretty fast. A 350 Chevy has to be turning over 5,000 rpm to match that figure.

Still think the weight of those pistons might not matter?

Somehow, that 11" stroke length does not seem right. A 14 liter Cat engine has a stroke less than 7" and turns in all cases less than 2100 rpm. If the 11" stroke IS right, then balance is way more critical on this engine than you may realize.

 

[ovrszd]

As a 4-h project being done on a borrowed machine there are plenty of reasons to do this project properly including matching pistons. To teach an impressionable youngster how to botch a job is a disservice to her and to the next youngster that won't be lent a tractor as a result of the bailing wire job.

Cutting big corners to get a tractor running in time to harvest a crop is one thing. Botching a job as part of a learning experience is the worst time to do it wrong.

Whether or not the botched job will get by long enough for the tractor to be loaded onto a trailer is going to be tough to answer but I don't think it is the right thing to do in this situation.

I'm going to stick with HB on this one. Whether it can be rigged up to work is irrelevant. As well as whether this engine will run long hours or not. The important thing here is to teach this student the correct way to build an engine. This student is going to remember whatever you teach her for the rest of her life. It's that first impression thing. I remember in great detail the first tractor engine I overhauled as an FFA project!!!! When I tried to start it, it would not turn over. Locked up tight. The teacher never educated me in the procedure of keeping the rod caps with the appropriate rod. It embarrassed me that I had to take it back apart. How is this student going to feel if all these "great shadetree ideas" fail in the first few minutes of operation???

 

[HGM]

I'm going to stick with HB on this one. Whether it can be rigged up to work is irrelevant. As well as whether this engine will run long hours or not. The important thing here is to teach this student the correct way to build an engine. This student is going to remember whatever you teach her for the rest of her life. It's that first impression thing. I remember in great detail the first tractor engine I overhauled as an FFA project!!!! When I tried to start it, it would not turn over. Locked up tight. The teacher never educated me in the procedure of keeping the rod caps with the appropriate rod. It embarrassed me that I had to take it back apart. How is this student going to feel if all these "great shadetree ideas" fail in the first few minutes of operation???

I really cant believe this is a discussion..... I COMPLETELY agree with the last few posters about teaching the kid the right way to do it rather than half a..... Anyhow its not a difficult choice, the machine shop recomendations are right on and fairly cheap.. How much ate these pistons? Is that money really worth the lazy/cheap values that this kid will learn? If she puts it together with these 2 pistons and it shakes badly, but stays together, what kind of grade would she get anyhow? Its not right..... Thats like teaching her that when adding 2+2 , 3 is close enough.. I dont intend to hurt anyones feelings here, but since its a kid, my veiwpoint is entirely different than it would be if an adult was just trying to cut a corner.. The adult should know better and would definitly lear a lesson if it failed.. If this student fails because of your advice, not only will she look incompetent, but she will never trust her teachers advice again.. It could also set a habit for her to always cut corners.. Do the right thing as her teacher and recomend she do it right or not at all....

 

[patrick_g]

Looks like there is a majority in favor of doing it right for the kid's sake. GOOD!!!

Now about the engineering... Having two pair of similarly mismatched pistons does not constitute ballance. Just adding up the weight of pairs of pistons moving in the same direction at the same time and getting equality does not constitute dynamic ballance.

The right choices are not that complex:

1. use 4 new pistons or
2. have a competent speed shop ballance the pistons, preferably along with the entire rotating assembly but at least get a static ballance on the pistons.

If it comes to $ of the additional pistons, supply the address and name of the school to whom contributions should be sent and I probably will not be the only contributor. My expectation is that there will be more contributed than needed and as far as my contribution is concerned, keep any excess and apply it to something the kids need or could at least use, even if it is just a few cold sodas.

Pat

 

[SPYDERLK]

Quote: The rotating assembly has to be somewhere close to balanced or it will self distruct in short time. Those are obviously steel or cast iron pistons to weigh that much. If they are beefy enough you might be able to take enough off of the heavy ones and add a little back on the light ones and get it close.
Yes, and not needed, respectively. This is not a rotating assembly. It is an interaction between rotating and reciprocating components. Within a very close approximation the rotational part is comprised of the crank and the big ends of the rods. This part is not changing at all, so the manufacturers spec for rotational balance is exactly preserved. We have a change isolated to the reciprocating components. In a four cylinder engine crank throws are set up 180 from one another in order to evenly split power pulses over the 4 stroke cycle. The reciprocating weights move equal and opposite to one another. These are arranged with with the 2 end cylinders opposing the 2 centers. If they were opposed as 1up-2down-3up-4down, a rocking couple would result tending to wiggle the ends of the engine up an down alternately, and so would require a more sophisticated balance scheme to negate that. The former arrangement gets around this and you wont see too many designers pass up this free money. The case presented in this thread has a free money solution. Its an excellent opportunity to exercise this growing mind in a way that promotes ingenuity based on an understanding of pertinent parameters. ---- For instance:

If you had no other choice than to use boat anchors and you only needed one, would you just put in one?
If not, how would you best arrange the replacements?
Would you replace all four if 2 originals were still good?
What part of the engine is most threatened by reciprocating boat anchors?
How would you treat the engine knowing they were in it?
If it ran perfectly smoothly, just as the original, would you assume that you could treat it as the original?
What special treatment would you give it, if any?

If the student will dig in to questions like these you can rest assured that you are not spawning a parts changer.

Whatever you do, dont add weight to the originals.
Larry