streetpirate said:
what if you are using a heavy 258 crank, and aftermarket pistons? lets say, the off the shelf speed-pro's. would the difference in mass require the rotating assembly to be balenced? it would make sence to me to have to have some meat taken off the counterbalences.
i have never built an engine
Again, it's going to depend upon what you're building the engine
for. OEM/Production small block Chevvies, for instance, are typically not balanced as well as racing engine - time, after all, is money. I'm sure they balance them to within a couple of grammes or so, but I've been involved in racing engine builds where a tenth of a gramme meant getting another part.
Granted, most production engines are going to redline somewhere in the 5000-6000 range, and some racing plants can hit 12000 easy. More, if it's a Wankel rotary engine (I used to know a guy who specialised in Mazda/Wankel engines, and he could balance them to spin up around 22000RPM all day long.)
Bear in mind the maxim - "time is money." If you're going to build for high output or ultimate reliability, it could become worth it to have the entire reciprocating assembly dynamically balanced - but otherwise, probably not. Sure, you'll want to take time to balance your rods and pistons to each other (it helps to reduce harmonics,) but to what level? Within a gramme or so is usually just fine for a "production" or a "typical" engine - and if you're not going to see high crankshaft speeds, a gramme will serve neatly.
Sure, have the crankshaft itself spun - an unbalanced crank is a very good way to destroy an engine! However, if it was made properly to begin with, you're not offset grinding, and you didn't manage to bend the thing, "balancing" the crankshaft is probably going to be more just checking to make sure it's still balanced than anything else - unless you notice something obvious - like a hole in the side of the counterweight (where a plug of Mallory metal probably used to be... Lightening holes are usually drilled in the periphery of the counterweight - but "heavy plugs" are usually put in the side so they don't get flung out during operation...)
Unless you're planning on living at high RPM (anything over, say, 4,000RPM or so for "cruise" operation,) balancing the entire reciprocating mass
as an entire unit probably becomes more an exercise in showing the depth of your pockets than anything else. Since we're not building 1500cc Honda "wind it up and move" engines here, save your money for tyres and lift. All the "common literature" doesn't show any need for special balancing when building a stroker - I'd have to check some of my old textbooks, but I don't see a lot of reason to do so offhand, either...
5-90