I bought a balanced engine assy from eagle to put my 357 into the car. Whatdya know, I have a piston to valve clearance problem again. I am using aluminum 64 cc heads with 2.05 valves. I should have known better. I whipped out the dremel and radiused the reliefs on one of the pistons just for kicks. I got everything I needed out of it, But I don't want to do that 7 more times and check every single one.
So, I am looking to find a set of pistons with big eyebrows that are close to 552g so I can forego the rebalance. Does anyone know if the speed pro H631 piston will accept a larger valve?
BTW, the ones I am working with are H345NCP

They do have a rather large valve relief and we've yet to have issues with 2.02's, so a slightly larger intake valve shouldn't be an issue with them. Of course, you still have to check. Another good indicator is the combined valve relief volume between the 2 pistons is the same, with the 631's having half as many reliefs.

If you replace the pistons it wont be a balanced rotating assy without having it re-balanced.

If you replace the pistons it wont be a balanced rotating assy without having it re-balanced.
If the pistons are within a couple grams, it will still be balanced.

Yeah, within a couple of grams it will be but tollerances add up. When I get an assy balanced, the pistons are balanced with its corresponding rod attached. ya get a couple grams off on the rod and a couple grams off on the piston and it is no longer a balanced assembly.

Not that it will be way off but it wont be like having it balanced as a rotating assy.

Couple grams here, couple grams there adds up...especially when you take into account the speed at which everything is moving.

WTH So, if I change pistons, I am out of balance. How many grams am I removing by grinding? Can't be more than 5 or 8. All I am doing is opening up the intake radius just slightly.

I have definately learned from this fiasco!


Does anyone have any experience with modifying pistons on there own? I may start a new thread for this. I don't have the flycutting tool. But I want it to be very accurate. I am thinking about templates and such. Any help is appreciated.

tmorand,
I assume the pistons are on the rod and you are using caution to not get any grindings anywhere near the pin. If you decide to grind the reliefs yourself, do the best you can to make all eight uniform as possible.

Balancing is a critical art, first a shop will weigh all the pistons and remove material from each one to make them all weigh the same as the lightest one. then the rods are weighed on both ends on a special scale that supports one end enabling you to weigh the ends independant from each other. They will make all the rods weigh the same on both ends.
Then they will put the pistons on the rods and do the balancing of them once again.

If you do it yourself get a good scale and make sure they are all weighing the same when you are done grinding. Heck if thats the case get the scale befor you do any more grinding and see for yourself how close they weigh to one another.

I don't mean to get ya all worried about this but I wont build an engine without balancing the rotating assy and if I found myself in your shoes I wouls be hard pressed not to bring em back and at least have the shop that did the balancing check em after you do the reliefs.

Thicker header gasket

That would be thicker HEAD gasket. except he allready did some grinding on one of the pistons......

A properly balanced engine will allow any rod and any piston to be paired together from that set, IE all the pistons weigh the same, all the pin ends weigh the same, etc.
Flycutting the pistons will typically remove 2 to 5 grams per piston and will not affect the balance negatively. It will change the balance factor slightly, but in a positive way. (very slight overbalance) Think about what happens once oil is stuck to the piston and some carbon adheres to the piston lid. In a dynamic situation, the pistons aren't all going to be the same weight anyways.

Agreed rust,
It was not my intent to induce worry or anything. and you are right if the pistons were all flycut the same any change in the piston weight would be nothing to worry about

I would still get a scale and double check. But I have been known to be anal at times.

Mark

No problemo:D

What works in the real world is best experienced first hand. The first 440 Dodge crank I balanced was out about 2 ounces from one end to the other when I first spun it up. :eek: For stock rebuilds if the pistons are within 25 grams of the stock ones they pass, and the engine never exhibits any vibration as a result. When I balance I avoid removing the bobweights whenever possible (and flywheel) since you can never EXACTLY reinstall them in the same place, and the balance will be out a gram or two when you spin the crank up again.