Engine balancing practices
 

Hi guys,

I keep getting billed for jobs which I am fairly certain arent being done.

Ive just taken an engine to a machine shop with crank, flywheel, bolts, clutch pressure plate, rods, bolts, pistons rings and pins.

the shop has come back telling me its all already within 5g and billed me only to check it. My understanding is that all the parts would at LEAST need to be numbered so I know which parts go with which cylinders but its not even had that done.

Is what he is telling me even possible?

Thanks guys,
Dann

My Supertech pistons were within 1 gram out of the box, my Carrillos were within 0.2 grams.

So... maybe?

--Ian

Several of the builders I talked to said that there was no need to balance the crank on the miata motor, as they are generally balanced well right from the factory. I didn't pay attention and had mine balanced anyway... both times they were spot on. You still do need to bolt up the flywheel and pressure plate (and mark them) and balance with the crankshaft.

If good quality rods, then the big ends are likely well matched. You can then adjust total weight by mix/matching pistons, pins and rods (if all are new). If further adjustment is needed, then you can remove some metal from the piston pin boss with a die grinder. You should be able to get them +/- 1 gram easily enough.

But you are right, the pistons should be marked, as they are measured and honed to a specific cyld. And there should be an alignment mark on the flywheel + pressure plate to let you know how they were bolted up when balanced.

Forged rods do not weigh the same as stock and neither do forged pistons. Rotating assembly can't be balanced without being measured and numbered as you say. Mass needs to be removed or added to the counterweights to be true.

Thats the piece of common sense I was looking for.

Thankyou,
Dann

This. If your put rods that are 10% lighter, and pistons that are 10% lighter, and do not reduce the mass on the counterweights, there will be more vibration/deflection on the crank than stock. Even if all the individual pieces are balanced relative to each other, the assembly would be out of balance and more vibration/wear would be the result.

From what I recall the forged rods are a fair amount heavier than the stock rods. And there was pretty healthy variation in weight between different brands of forged rods. My Brian Crower rods were heavier than the eagle or scat rods, iirc. To add weight to the counterweights, they are drilled in the appropriate location and tungsten is epoxied or brazed into the holes.
https://www.google.com/amp/www.hotro...rminology/amp/

You guys correct me if I am wrong here but the way I understood it is that the crank on a flat plane 4cyl only needs to be balanced to itself, ie already done from the factory. The only balancing that needs to be done is the rods and pistons to each other. This is bc the mass of the pistons and rods counter each other from being 180 degrees apart. On V8 or V6 you have to balance bc the weights are not directly opposing in the same plane.

I may be wrong but this was my understanding and I can assure you my miata that has lighter rods(by about 15grams) but still stock pistons runs buttery smooth compared to stock. I did balance each to 0.1 grams which was way overkill but I had the tools and time so why not lol

I would say that you are right if you look at the Kia BP crankshaft that has no counterweights, just one crank throw offsetting the other. But then the Miata has significant counterweighting for some reason. Looks like a fine time for Google to offer his advice to us all. Or see if you can get @BogusSVO to notice this threat.

OK, so you do suggest the use of Bob Weights (calculated in some fashion) be used during balancing when changing the weights of rods and pistons from stock. Must admit that it makes sense.

If that is what Pat has done, that may be why he gets away with high revs and stock harmonic balancer (damper). That and not circuit racing.

I'm pretty sure Tony Montana's engine was balanced by the shop that built it. I wonder if he has any pictures of the counterweights before he added the oil pan.

Edit : I would think that the two ways that you can balance an engine would be- 1. let the adjacent moving assembly counterbalance it <or> B. have each one balanced individually by its own counterweights. I think that's what we are getting with the Miata crankshaft versus the KIA crankshaft

The counter weights are placed on each trow to "counter the weight" of the reciprocating assembly bolted to it. There are variations of design that have full counter weighting, no counter weighting, partial counter weighting. All can be balanced, but the loading on the main bearings and vibration will be higher the less from ideal the counterweights are. No counterweights, as suggested the opposing throw that's 180* out will "cancel" the force, but it puts the crank in bending which is a second order vibration. So at higher revs, the crank is bending a lot and putting a lot of load on the main bearings, and the deflection is hard on rod bearings too.

Yes my crank was balanced correctly, and I do run an ATI damper.

Oh, sorry for my mis-understanding, and thanks for the correction.

No problem, I used to run a stock damper to 8700 years back. I switched to an ATI around 2013 or so.

This is not a crossplane V8 crank, the opposing pistons balance themselves. For the same reason, you do not need bobweights when balancing a BP crank (or any other flatplane 4cyl crank).

If what you are saying is true, then just slapping rods or pistons into a motor would shake it to death with massive imbalance. That is not what happens.

The only witness marks you would look for on a balanced rotating assembly would be alignment marks for crank>flywheel and flywheel>pressure plate. Dampers should be zero-balanced independently. The pistons should be matched to holes independent of the balance.

My opinion of balancing with flywheel/clutch is that it's too pedantic for 99% of customers who request it. If you ever are required to change the clutch or flywheel, you lose that balance. You also have a spinning disc internal to both which will switch clamped positions every single time you engage/disengage the clutch, and that becomes part of the system when the clutch is engaged, so I see little use in including the clutch/flywheel in the system when the disc is just going to dick it all up in the end. Flywheels and PPs are balanced independently, that's plenty for 99% of builds (the 1% are not discussing their motor plans in a public forum).

To recap:
-Damper is zero-balanced independently, not by your machine shop
-Crank is balanced independent of piston/rod weights, with or without flywheel/clutch
-Pistons are matched to each other to <1g
-Rods are weighed big end to small end including wrist pin weight and adjusted independently (big ends matching, small ends matching)
-Pistons are matched to cylinders based on p2w clearance, not balance

Thanks andrew.

When Marine Crankshafts worked over my crank, I was told the same thing when I specifically asked about . Absolutely no need to bob weight balance an inline four motor. Granted, when some folks' motors go subsonic in RPM, Newtonian physics goes haywire and all bets are off.

Couple posts mention "forged rods." Ah, I know I have my moments, but this day in age, all rods are assumed forged.

One simple link- Done. Havacat.

That is wrong. There are many difference processes manufacturing a connecting rod: cast, forged, machined from billet, powdered metal.