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Balancing rotating bits
luke2152 - 18/5/19 at 08:51 AM

Building a toyota 3sgte with forged pistons and rods an not sure how seriously I should take balancing. RPM limit will not be increased. It's a side project so I can take my sweet time about it.

I can think of 3 areas to affect balancing:

1. Balancing crank statically (on a grinding stone balancer) - won't tell me if the front and back are unevenly balanced.
2. Balancing crank dynamically - to counter the above point.
3. Getting pistons and rods to the same weights on digital scales.
4. Dynamic balance of the crank with weights simulating pistons and rods. Seen a few videos about this and it looks to be mostly on vee engines where the adjacent cylinders don't balance each other out like they do on an IL4. That said if the rods and pistons are significantly heavier than stock units, I guess the crank counterweights don't do their job on the individual cylinders.
5. Flywheel, clutch cover - can probably be done static or dynamically as the crank is being balanced.
6. I'm probably over thinking this!

I'm guessing when you pay 150-250 for a full balance you get 2, 3, 5. Done at home you get 1, 3, 5 (or you pay for 2 and do 3, 5 yourself). And guessing 4 is solely for big ticket race engines and would probably involve adding quite a bit of weight and I should stop worrying about it.

I'm tempted to just check static crank balance at home (and toyotas are said to be quite good from factory) and get dynamically done if it isn't right. Partly because local machinist can do my engine work but I'd have to go further afield for balance. Would be a waste to not do it while its apart if its a big deal though.


SPYDER - 18/5/19 at 12:35 PM

Hi Luke. I have just built a 3SGE. During the planning phase I discussed the matter of balancing, dynamic or otherwise, with a knowledgeable chum. He said" It's a Toyota. Don't waste your money".
He was right.
The crank came back with one tiny, tiny correction. Similarly, the flywheel had one tiny mark not much more than a dot punch would make. Waste of money.
It might be the case that using forged pistons could make a difference but apparently some people add a notional figure for the weight of oil clinging to the crank whereas others don't so what accuracy are we aiming for?

[Edited on 18/5/19 by SPYDER]