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How to balance a motor?

colbystephens

colbystephens

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So I'm building a 6.2L diesel motor for my '83 K5 - it is an educational project, a passion, and the beginning of my offroad machine. So I've almost come to the point of reassembly after having the motor entirely dismantled and the heads rebuilt, worn parts replaced, etc. I hear people speaking about internal balancing of their motor, and I'd like to see a concise definition of what this entails. I get the general idea of the balanced motor, but how does one accomplish this? For instance, I'm having to replace my pistons, and was thinking to use the rods which I have, but I saw somewhere that one can get pistons and rods together balanced from the manufacturer. How do I balance my new pistons on my old rods - or is this a stupid thing to do? Anyway, as much info as you can think of in terms of motor balancing would be greatly appreciated. THANKS!

Colby Stephens
www.web.pdx.edu/~colbys
 
In laymans terms , its making every moving part the same weight .

I.E the lightest piston and rods weight is ground off the heavier ones .

Of course theres more , and of course a machine shop is doing it for you :wink1:
 
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so a balanced motor is simply based on the piston and rod weight - no other components or factors? that seems far more simple than i'd imagined. thanks for the info. any other input out there? thanks!

colby stephens.
www.web.pdx.edu/~colbys
 
Is it really needed??

I wonder if its worth it on a 6.2 diesel,thats only going to spin at a maximum of 3600 rpm's??,before the govenor kicks in.....it would be nice to have it perfectly balanced,but I doubt if its really nessasary or worth all the trouble and expense..I could see the point if it were a small block screaming at 7500 rpm or something... :confused: :crazy:
 
colbystephens said:
so a balanced motor is simply based on the piston and rod weight - no other components or factors? that seems far more simple than i'd imagined. thanks for the info. any other input out there? thanks!

colby stephens.
www.web.pdx.edu/~colbys
Click to expand...

if you don't even know what balancing really is, you CANNOT do it. I discussed it before and the more I learn of the technical aspects of engines, I was crazy. Balancing rods and pistons is just a part of it, the biggest part is the crankshaft and you simply cannot do that without the proper tools to spin it and find the weight that needs to be removed(far too much $$$ for any simple hobbyist).

Like noted, balancing is removing weight from a heavy spot to create a smooth rotation. Same as wheel balancing basically. Tape a quarter to a frisbee, try and spin it on your finger with your finger in the middle, it will spin right off your finger. In an engines rotating assembly those forces are controlled by the bearings, but if they don't have to fight the crankshaft much, they last longer and the engine will rev better(quicker and higher).
 
thanks for all of the pieces of info. it's certainly stuff to consider. is the general concensus that it probably is not worth it to have all of the balancing done for a motor that will spin less than 3600 rpm?

colby stephens.
www.web.pdx.edu/~colbys
 
the place that balances my stuff

they want your crank, rods, pistons, harmonic balancer and your flexplate or flywheel which ever 1 you will be using, so that they get everything that is spinning balanced...they have yet to let me down....where as most places just want crank, piston and rods...then you put on ur balancer and its not perfect then say your flex plate is out a little just the oppisite of the balancer, well you've wasted money because it is back out of balance,...definitly use a shop that wants the balancer and your flywheel to balance with or your really just wasting money, and its cheap insurace pro block in memphis where i get mine done charges $125 to balance from balancer to flex plate and they guarantee its +/- 1 gram!
 
that's a pretty sweet deal. i'm going to ask about this at the machine shop tomorrow - if they don't do it, I've got a buddy who may direct me better. I've researched the cost of balancing since the start of this post, and the general figures i come up with are between 125 and 225. Seems like a decent price to pay for a little assurance on reducing the amount of wear on the motor, plus a possible small increase in power. i guess i'm kind of going back and forth a bit. what would you all do? invest, or no and please explain. Thanks!!

Colby Stephens
www.web.pdx.edu/~colbys
 
I don't think its a waste. Take the RPM and smoothness out of your head, its better on bearings, that alone might be worth the cost.
 
The engine builder that just moved his operation into my shop, I've watched him do it a few times. It's pretty detailed. He balances the rods weighing them from both ends. He also polishes the rods at the same time. Pistons get balanced and the crank gets balanced from one end to the other. If he's working on a High RPM motor some of the valvetrain components get some careful going over as well. I think this is to remove vibration from the valve train but I'm a little sketchy on all the details. I just watch, definitely interesting.
 
Back when I was building dirt track motors for fun, we just did the assembled rod and pistons. First, grind the forging marks off the beams of the rods, then a light polish on the rods. Next attach pistons. Oh yeah, first step, get the rods re-sized. Sorry, been a while since I did this, have to activate dormant memory files. You can do the rods yourself, save a little dough. You need a good postal scale, a die grinder, elec or air, and patience. After you attach the pistons, weigh all the assemblies. Set the lightest one aside, and polish/grind the others to match its weight. When you are done, be sure to clean everything thoroughly to get rid of metal particles from grinding. If ya need to, get a junk rod and practice on it till you are comfortable with the tools. Have fun!!
 
diesel4me said:
I wonder if its worth it on a 6.2 diesel,thats only going to spin at a maximum of 3600 rpm's??,before the govenor kicks in.....it would be nice to have it perfectly balanced,but I doubt if its really nessasary or worth all the trouble and expense..I could see the point if it were a small block screaming at 7500 rpm or something... :confused: :crazy:
Click to expand...

Definately worth doing to any engine, especially if the pistons have been replaced.

In general:
Weigh all pistons and find the lightest one; machine the others to match that weight.

Weigh all con rod small ends & find lightest one; grind others to match weight.
Weigh con rod big ends & find lightest one; grind other to match.
Note that if you're going to polish the beams that you want to do that first. Some shops also weigh the whole rod as a check that they're all now the same weight.

The crank is a bit more involved. Once the weight of the rod & piston combo is known a chunk of metal is made to weigh the same by stacking large fender washers on some attached all-thread. Those are then put on each rod throw. The crank balancer uses a strobe light to point out where rotationally the inbalance is. they can either drill/machine/grind the heavy side or add Mallory Metal to the light side. Mallory Metal is spendy stuff, usually only see it added to spendy cranks.

Anyone who does not want the flywheel and balancer is not really doing the job. Even with an internally balanced engine they should be checking that those parts are correctly balanced. The only reason for not checking is if the engine is known to have been previously balanced and you're only compensating for the new parts.
 
Dont get me wrong, I'm a beliver in balanced rotating assemblys but I really dont think the slow spinning diesel will benifit that much.Great info in this thread, do your research and make your call. Cary NOT A DIESEL TECH!! Sadler
 
Other Factors

A lot of other factors should be considered if "ballancing" an engine,,

As well as making all of the reciprocating parts the same,,Cylinder volumes should be checked,,Heads CC'd,,Deck height checked,,Ring end gaps checked etc.
Any thing to make all the elements in each cylinder as "equal" as possible.
 
if new parts are added

ntsqd said:
Anyone who does not want the flywheel and balancer is not really doing the job. Even with an internally balanced engine they should be checking that those parts are correctly balanced. The only reason for not checking is if the engine is known to have been previously balanced and you're only compensating for the new parts.
Click to expand...

even if engine has been balanced b4 the balancer and flywheel should be done again when you add new parts(pistons or any rods) because if flywheel and balancer were balanced for one combination doesnt mean they will still be ballanced for the new rotating mass, and i always carry mine even tho they were balanced with my last combo, i like them to recheck to make sure its still in balance with my new combo!!
 
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