[Keithc8]

Balancing an enigne is not exact sience but there are formulas that work with most engine under 7000 rpms just fine. You get into race engines over that and you may want to over balance or under balance the engine. That is a topic for another time.
The 302 Ford came originaly as a 28 oz balance engine. It had a weight on the damper and on the flywheel. The weight is bigger looking on the damper because it is closer to the center. The weight on the flwheel is not as big as the flywheel because it is farther from the center of the crank.
When you are balancing the farther out from the crank you get the less weight you need to balance. Most Fords are external balanced with a weight on the damper and flywheel. this is problem because most balance shops will drill on the damper and or the flywheel when balancing the crankshaft and all. If you have a problem down the road and need another flywheel or damper your engine will be out of balance. We use a factory 28 oz damper and flywheel to balance these engines to then check the damper and flywheel that we will use to make sure they are to spec. This is more time consuming but the right way to do it. you want to drill on the crank and not the other parts. If our customer have a problem with the damper or flywheel or want to change them we can then get them a new 28 oz damper or flywheel and they will be ok. We can even check the new stuff.
This all causes a problem and later Ford went to a lighter crank in the 302 with a 50 oz damper and flywheel, 1980 I think to help with the mass of the crank and to help power and economy. The external balance is very bad for race engines and anthing over about 6500 rpms. The longer the stroke the worse the case is on this. Well when Ford came out with this 50 oz stuff it caused even more problems because people would install a 50 oz set up on a 28 oz engine and vise versa. Got to love this Ford stuff.
The 352, 390 and 427 Ford are zero balanced but the 428 is external on the back only and the 428SCJ is external on the front and back. this is has also screwed a few FE guys over the years.
To make matter worse a lot of the pressure plates are out of blanced and need to be balanced with the flywheel and then marked so that they bolt back up the same as balanced. If the flywheel pressure plate bolt pattern is even shifted just a little it will affect the balance. If you surface the flywheel it will more than likely knock the engine out ot balance because most of the time a little more comes of one side than the other, I have seen that several times. And everybody thought this stuff was easy.
Then you get into making cranks that were orignally external to zero or internal balanced which takes heavy metal in the crank counter weights. You can also drill the rod pin side if it has not been drilled.
I have balanced engines now for about 30 years and seen about everything. On the first race steel cranks we were doing we have put as many as 16 pieces of mallory in a crankshaft and ended up with a 800.00 to 900.00 balance job. We now have much better cranks that are made for zero balancing. I remember working with Scat on the first Ford steel cranks back in the late 80s, counter weight placement and size to try and get them to zero balance much easier.
Well this could go on for a while and I would have even more mis-spelled words. Hope some of this helps. Street engine external ok up to about 6500 rpms, race engine just go internal/ zero balanced. The external weights start to flex the crankshaft at high rpms and can even break it but normally takes the #2 and #4 main bearings out first. Road Race and Circle track enigne are harder on them than Drag racing. We need to write a book on this stuff.

Good luck, Keith Craft