Engine balancing.

[StressCraxx]

Some engine builders tack the ring gear to the flywheel after shrinking it on. This is really common if you have a motor with lots of timing advance that may kick back on the starter when cranking. If the motor is hard mounted to the chassis, tack welding helps prevent the ring gear coming off.

That explains it....but I'm still a little concerned about balance!

Kurt

Please forgive me for my earlier lack of clarity. Hard mounting the motor to the chassis takes the cushioning of the shock from the starter contacting the ring gear or kickback. Both Arnie Loyning and Jay Ivey do this for our Kent derived engines. The flywheel is balanced after the ring gear is tacked in place.

I thought this web page explains balancing pretty well.
http://www.neponsetvalleymachine.com/crankshaft_balancing.html

Other considerations
Before balancing, check the flywheel bolted to the crank for lateral and radial runout. If there is more than 0.003" axial runout at the face of the flywheel near the OD, check the face of the crank and the mating face of the flywheel for burrs. Stone the burrs, make sure all is clean and re-check. If there is axial runout or the crank/flywheel mating surfaces are bodged, you won't get the desired result.

I learned the above the hard way.

[rgh0]

.

I thought this web page explains balancing pretty well.
http://www.neponsetvalleymachine.com/crankshaft_balancing.html

.

The article is a little misleading as it is focused on V type engines not our straight 4 cylinder engines. In 4 cylinder engines, balancing is simpler as the weight of the pistons does not effect how the crank is balanced as it does in V engines.

In a 4 cylinder the engine the pistons balance themselves as there is 2 going up and two going down at any one time. So matching the piston weights is critical to ensure they balance themselves out. There are some higher harmonics that do not balance themselves out as the pistons going up versus down are not totally symmetrical if you look at the geometry. This is why some big 4 cylinder engines have a twice speed balance shaft ( eg the Porsche 944 engine) to counter these higher harmonics which become significant with the heavier pistons and rods in big 4 cylinder engines . Keeping the piston and rod assembly as light as possible keeps these higher harmonics as low as possible.

The crank can be balanced independently of the piston weights as the counterweights are only required to balance out the weight of the crank journals themselves and does not need to contribute to the piston weight balance.

Balancing the flywheel assembly and front pulley ensures they are individually balanced but then balancing them on the crank allows for any small unbalance caused by any small offset in their mounting on the crank versus the balancing machine which is why they then need to be mounted with the same orientation each time to maintain that balance.

cheers
Rohan

[nomad]

Very good information from you guy's and I've bookmarked it. Thanks!

As I mentioned though, I never became concerned about my flywheel balance till after the engine was assembled! I don't plan on anything other than a street engine but having to replace the tack weld's has me concerned about the imbalance near the periphery.

Back in my motorcycle days I had a parallel knife edge rig made up for crank shaft balancing and i suppose I could come up with something like that if there is no other way.

Kurt.

[StressCraxx]

.

I thought this web page explains balancing pretty well.
http://www.neponsetvalleymachine.com/crankshaft_balancing.html

.

The article is a little misleading as it is focused on V type engines not our straight 4 cylinder engines. In 4 cylinder engines, balancing is simpler as the weight of the pistons does not effect how the crank is balanced as it does in V engines.

In a 4 cylinder the engine the pistons balance themselves as there is 2 going up and two going down at any one time. So matching the piston weights is critical to ensure they balance themselves out. There are some higher harmonics that do not balance themselves out as the pistons going up versus down are not totally symmetrical if you look at the geometry. This is why some big 4 cylinder engines have a twice speed balance shaft ( eg the Porsche 944 engine) to counter these higher harmonics which become significant with the heavier pistons and rods in big 4 cylinder engines . Keeping the piston and rod assembly as light as possible keeps these higher harmonics as low as possible.

The crank can be balanced independently of the piston weights as the counterweights are only required to balance out the weight of the crank journals themselves and does not need to contribute to the piston weight balance.

Balancing the flywheel assembly and front pulley ensures they are individually balanced but then balancing them on the crank allows for any small unbalance caused by any small offset in their mounting on the crank versus the balancing machine which is why they then need to be mounted with the same orientation each time to maintain that balance.

cheers
Rohan

Hello Rohan,

You are right, the article on v8 balancing is a bit misleading. What I hoped the reader to understand was the process steps of component balancing the reciprocating bits, then the components added to rotating balancing, including harmonic pully, flywheel, presure plate. I must admit its the first time I saw discussion on balancing cams and sprockets. Its all rotating mass, so it seems reasonable.

Regards,
Dan