How-to drill oil drain holes in 3.5L pistons

[89Vette]

Engine cleaners can dislodge sludge and deposits in the oil galleys and crankcase sending them through the engine to be redeposited elsewhere, like in the pistons drainback holes. As to balancing, I would be more concerned with the reciprocating part of the equation but not much due to the fact that you are drilling the same size and amount of holes in each piston and the fact that they are so small as to be of negligable effect on the rotating assembly. We're not talking 10,000 rpm race motors here...

First of all, I had a typo (or brain fart). The following sentence should have read...
I believe these holes are either fewer or smaller than ideal....

I don't know enough about this small weight change in engine balancing -- but if I were going to do it (with an engine rebuild), I'd do it before the rotating assembly were balanced -- just to be sure.

Sue,
There are giant "lobes" on a crankshaft. They are counterwieghts to the pistons. Without those counterweights, the forces generated at higher rpms would shake the motor apart. (Note: a giant weight on the end of the crankshaft -- called the harmonic balancer also performs some of these duties.)

What most people don't know is there are variances in piston weights. There can be small variance from one (like kind) piston to the next. Or, pistons weight can change due to the material used in casting/forging the pistons. For example, standard weight Chevy pistons were approx 600 grams. With my rebuilt 383 performance motor, they weight about 400 grams. Because different makes and weights of pistons can vary alot in weight (even 50% differences), the crankshaft must be individually counterbalanced to match those weights. This means those crankshaft counterweights need to be adjusted. (To do this, a machinist spins the crank along with weights (I believe equivalent to the pistons/rods being used. IOW, I'm not sure they use the actual piston.) Either way, just like a tire balancer, the machine tells you how much weight change is necessary to balance the counterweights to offset the weights of the rods and pistons.

Here is the rotating assembly in a typical engine....


Because I used very light pistons in my Corvette rebuild, the builder decided to drill out (remove) weight from my crankshaft. Here's my Corvette crankshaft after balancing. Notice the holes in the lobe. It didn't look that way before balancing.


Any compared to tiny extra holes in the pistons, you would assume it might not affect the outcome of balancing. OTOH, you'd probably be surprised at the force a piston generates when moving up/down at even 5-6k rpms. Also, know that a VX crankshaft balancing principle is the same -- though it only has 6 rods/pistons vs the eight in a V8 engine.

Note: For factory balancing, automakers demand a weight tolerance (from manufacturers) so they can weight every crankshaft the same -- and assume it will match the pistons going in a given engine. Factories don't match balance every single engine. There is some "tolerance" that won't be felt by the consumer -- as mentioned above. Because of the small amount of weight being removed from each piston, they effect on balancing may be negligible (as Scott suggests). But, it wasn't the major point of my first post in this thread.