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

 

[bamatoolmaker]

From experience I would reccomend a matching set of pistons,and sending the rods and crank to an auto machine shop to be resized for the correct bearing clearances.The result would be a dependable project

 

[Jimbrown]

Well I am going to step in and dissagree with most of the answers. There comes a time when you got to teach people to use what they got and make it work. The world ain't perfect. Any one can take all the corerct parts and make something work the challange is to make things work that others say won't work. I had a MG B that had two different types of pistons in it an it ran just fine. I autocrossed it around three states and took a lot of first places. Two of the pistons had three rings and two of them had five rings. I think it is common practice to bore one or two cylinders with out boring them all and running engines. I have heard but never done it of simply pulling one piston and running egines with one piston missing. I think that was common during ww1 and ww11. Next I bet that thing has a 200 lb fly wheel on it and 11oz of imbalance anin't goona matter hill of beens.

 

[ovrszd]

Well I am going to step in and dissagree with most of the answers. There comes a time when you got to teach people to use what they got and make it work. The world ain't perfect. Any one can take all the corerct parts and make something work the challange is to make things work that others say won't work. I had a MG B that had two different types of pistons in it an it ran just fine. I autocrossed it around three states and took a lot of first places. Two of the pistons had three rings and two of them had five rings. I think it is common practice to bore one or two cylinders with out boring them all and running engines. I have heard but never done it of simply pulling one piston and running egines with one piston missing. I think that was common during ww1 and ww11. Next I bet that thing has a 200 lb fly wheel on it and 11oz of imbalance anin't goona matter hill of beens.

So you agree with the post above when they said if your answer to the question of 2 plus 2 eguals 3, you get a pass???

 

[whitetiger]

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.

Pistons are manufactured to within millagrams of each other and you are asking about 9 ozs difference. You are correct, it will shake it's self apart in a very short time.

 

[patrick_g]

THANK YOU LARRY

I think that is the definitive explanation from someone with credentials speaking in his field of experience. Sort of cuts to the heart of the reality of the matter and doesn't leave a lot of wiggle room for I N T E L L I G E N T dissent.

Pat

 

[Ford960]

Other than the diameter of the new pistons why would anyone think that they are the correct ones for this engine. All this talk about modifying them to work is a waste of time and money and using them as is will likely result in destroying the engine. It will certainly vibrate and self destruct and when it does this poor girl will be the subject of unjust scrutiny.
Get the correct pistons and put the engine together correctly the first time.

I have first hand experience of two wrong pistons in a 4 cylinder engine. The engine was purchased used like that and shook noticably when we started it the first time. The correct pistons fixed the problem and made this engine a very smooth machine.

Measure the distance from the crown to the wrist pin and compare to the original pistons. If it's different at all then the new pistons will not be usable in this engine especially if the new piston dimension is greater.

BTW, I read all of the posts and if I missed anything that was already stated reflecting my post then I stand corrected.

As far as donations are concerned, I'm not well off but I'd throw a Jackson toward getting the right pistons.
Wish that I could get donations to rebuild the engine on my tired 960.

Good Luck

 

[SPYDERLK]

Thanks Pat. I cant be quite that immodest. I do have some uncertainty, but, as well, some feel for this owing to a physics background and racing and rebuilding of 4 cylinder motorcycles. At 10000rpm things can be very critical. In a slow reving 4cyl tho I know you can get away with more. I am concerned that the verry heavy pistons may overstress the rods, particularly the bolted joint, and would rely of the original manufacturer for a statement of design margin if available. Without this I would not put in 4 because I would consider that twice as much rod risk as 2 would cause. Two, set up adjacent to one another on #s 1/2 or 3/4 would cancel one another reciprocally, and the rocking couple due to their forces not being exerted at the same point on the crank would be minimized to the extent possible. The engine would have a net rocking couple because the lighter pair on the other end of the crank would only partially counter it. The neat thing about a 4cyl set up with 4 equal reciprocating masses is that each cylinder pair sets up its own rocking couple. These duel elegantly and cancel each other out within the constraints of the rigid engine block. There would be a net rc present in the rebuild with the heavy and light pairs [but less than there is in a 2 cyl engine] . It may not even be noticeable, but even if not, I would not want to crowd the top rpm due to the higher forces on the rods and bearings serving the heavy pistons.
Larry

 

[patrick_g]

Larry, Let me put you on the spot. If you were supervising the project, remembering it is a student wth little previoius experience, do you think you would just "stick it together" and hope? Wouldn't you be inclined to look into the parts applicability a bit and try to put it together as right as was practical? Note that more than one of us has volunteered to help a little with $ if that is required.

Some say it is GOOD to teach her how to bubble gum and bailing wire it as a learing experience. I say it is best to learn to do it right FIRST and then with experience later learn what sort of improvising could be done for an emergency fix.

What is your take?

Pat

 

[cp1969]

Some say it is GOOD to teach her how to bubble gum and bailing wire it as a learing experience. I say it is best to learn to do it right FIRST and then with experience later learn what sort of improvising could be done for an emergency fix.

Pat

Absolutely right. Once having learned how to "do it right", one then has the basis upon which to know when bubble gum and baling wire are appropriate and might work.

 

[HGM]

Larry, Let me put you on the spot. If you were supervising the project, remembering it is a student wth little previoius experience, do you think you would just "stick it together" and hope? Wouldn't you be inclined to look into the parts applicability a bit and try to put it together as right as was practical? Note that more than one of us has volunteered to help a little with $ if that is required.

Some say it is GOOD to teach her how to bubble gum and bailing wire it as a learing experience. I say it is best to learn to do it right FIRST and then with experience later learn what sort of improvising could be done for an emergency fix.

What is your take?

Pat

My point exactly..... Explaining all the physics of the issue has nothing to do with it.... If it were mine, and I felt like an experiment or needed it to run for a particular project where time was essential, I'd try it... Heck, if nothing else, I'd learn one way or another... But to tell a kid to shade tree it from the begining is just building shoddy work ethic from the start... When the pot goes around, I'll kick in for proper teaching too..