Opinions sought, mechanical question

[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..