View Full Version : how do you self-BALANCE an engine?

07-19-2006, 04:51 PM
how do you self-BALANCE an engine?
I didnt take the crank out. Just the connecting rods/pistons.
I'm gonna go weigh them today.

of course I'm retarded and mixed the order up of how the rods go in the engine. Thats the only reason why I have to re-balance it. Someone slap me!

07-25-2006, 10:33 PM
If your concerned about the order of the rods then you can relax about balancing. When the rotating assembly is balanced then each rod should weigh the same, and each piston the same. That in mind each piston/rod combo should weigh the same, so if that were to get swapped around no problem.

However what you should be concerned about is the bore size and bearing clearances. If you are not replacing the bearings/rings then you would want to make sure that you put the piston/rod with the bearing that was in that rod in the hole it came out of so that the bearing tolerences and piston ring end gap are ok. I would put new bearings and file-fit rings in and check all of your tolerences to make sure everything is correct.

What was the reason you tore the engine apart?

07-25-2006, 10:37 PM
ahh, you may have many more problems than just balance. assumming the engine was intrnally balanced to begin with, all you need is for every rod and piston assembly to be the same weight. You can't balance the crank yourself.

07-26-2006, 05:46 AM
assumming the engine was intrnally balanced to begin with. You can't balance the crank yourself.

I have heard that the factory doesnt balance the rotating assembly from the factory. This is the first time the crank and rods have been removed from the engine.
Maybe you cant balance the crank yourself, but you can do all the tedious grunt work saving you alot of $!

I had a head gasket blow, which hydrolocked the motor and bent 1 rod. The rings went to crap because my compression was only 80psi on every cylinder!

I got Total Seal rings on it now. The rod bearings looked like crap, but the crank bearings looked fine.

07-26-2006, 07:43 AM
im nit sure about you guys in the states but over here there are guys that will completely balance your engine at mid rpm (exsperts say there is no advantage to high rpm balancing with acurate equipment) for not a lot of cash! they are very god and not a million miles off F1 levels!!!!

thanks Chris.

07-26-2006, 09:32 AM
If your replacing the rod bearings and rings then the order each piston and rod assembly goes back is not important. You need to match the weight of each assembly. Weigh them all and match the others to the lightest one. I think factory torarance is 1 gram.

07-28-2006, 02:06 PM
get a digital gram balance, without disasembly use carb cleaner to DRY UP the oil in all Piston rod assembly, use a dropper(from a kids medicine) and pour O N E drop of WD40 in each piston pin hole. remove casting flashing from the pistons, AND THEN begin to weight the PR combo, the piston end and the big bearing end, use very light dremel long swipes under pistons, or in the sharp ends of the botom bearing where the casting flash is located.... DO NOT! grind along the rod, try not to nick the middle of the rod... untill all weight the same.

I balanced my flywheel (m6) after a kiss from a lathe and a lightening in the edge, a friend has a wheel shop and he let me use a balancing machine for a while (it was 1.5 oz off) then brought my pedestal drill, again use the balancing machine trying to rotate the flywheel many times relative to BM axle, and fixture adapters until I got a solid reading of the unbalance, then drill what you consider CLEARLY LESS than the unbalanced weight... rince and repeat....
after fumbling a lot with the machine and giving the machine diferent combos of size, dia and weight I finally got 0 unbalance from all combinations....

I dont know if true but my engine is CLEARLY less harsh, at iddle is much more softer (even the aftermarket engine mounts) and the steering wheel vibrates a lot less at high revs (really weird feeling) since im hitting the rev limiter more often and uneventfull!!!!!


07-28-2006, 02:13 PM
Your stock reciprocating mass should all be about the same, as it is I assume a facotry build. As stated, you need to figure out which holes the pistons came out of as that is much more important. You've proably got enought wear it isn't as critical, but you don't want to get the piston to wall clearance to loose or too tight.

Also, you didn't mix up the connecting rod ends, or the main caps did you?

For future reference, when disassembling a motor, use a punch, and put a punch mark on each rod/piston combo. Mark . for cylinder 1 mark .. for two, etc... Or, invest in a set of inexpensive stamps.

07-28-2006, 02:16 PM
I forgot, about puting the pistons in any hole, if you stuff a rag very tightly inside the bore, then swipe a 400# sandpaper wetted in wd40 many times in the direction of the crosshaching (spelling?) to break the glassing inside the cyl, and make it microscopically harsher (ridges) it will be like (REMEMBER I SAID L*I*K*E) new cyl surface so the rings can rub and seat (not ideally but better than nothing) more eficient.
then you have to put A LOT of effort in washing the bores to take our the alumina residue out of the cyl, then take out the rag you stuffed in the cyl (new rag for each cyl),
after rebuild inside the engine will have a lot of dirt and lint, the very first start use the cheapest 30 oil, the biggest oil filter, use it for about 30 min- one hour then throw that oil and put your regular oil and new filter.....

07-28-2006, 02:18 PM
If You Thigthened One Rod Cap In A Wrong Rod.... Well......
Youre Screwed!!!!!

07-29-2006, 03:14 AM
If You Thigthened One Rod Cap In A Wrong Rod.... Well......
Youre Screwed!!!!!
yes, i got rod caps mixed up with which rods they go with

If your replacing the rod bearings and rings then the order each piston and rod assembly goes back is not important. You need to match the weight of each assembly. Weigh them all and match the others to the lightest one. I think factory torarance is 1 gram.

now that is smart... maybe make them all lighter, but matched to the lightest one
I dont really plan on running rpms over 6100 rpms, now I dont have to get it balanced!

08-01-2006, 09:00 PM
Give up then, the rods are cracked cap rods meaning they need to match up with the original rod.

You basically need to rebuild the shortblock with new parts. Otherwise its a bomb waiting to go off.

08-02-2006, 09:58 PM
Time for a re-set here!
If the problem is which parts go where, there is no reason to think about balance. The factory does a fine job of matching the weights of each rod, piston, etc. and unless a full race build-up with many new parts is carried out, it is worse than pointless to attempt the various proposed partial balance procedures.
As far as which rod cap goes where, having cracked rods is an advantage, as the unique pattern of each will cause an obvious mismatch unless the cap is mated with the correct rod.
The next issue is which assembly goes in which bore. Rod bores and crank journals are also controlled to very small tolerances and since you're replacing the bearings, you'll be using plastigauge or bore gauges, etc. to check the clearance, so it doesn't matter which rod goes where. Now we're down to which piston belongs in which bore. Modern engines no longer use selective fit pistons, so all will be exactly the same diameter, and since you'll be doing some kind of hone/deglaze process to allow the new rings to seat, it doesn't matter which piston goes in which bore either, as they will all be essentially seating on new surfaces.
There, all your problems solved!