radiateu2

If I just re-ring my engine do I need to have it rebalanced if all the pistons go back in the same bore they were removed from...of course I would replace all the rod, cam, and main bearings too....what do you guys think or do on a tight budget. I know I should have it balance but the shop...the only shop in town wants over 200 for balancing

Vader

If you're just cleaning eh pistons and reinstalling them, there's no need to rebalance. The factory "balance" got you through your first set of rings, so as long as you drive and maintain the car at least as well as you have, you should expect the same service.

Don't be too surprised if you see a little bore and piston skirt wear, however. You may want to at least hone the bores and could possibly end up with oversized holes requiring different pistons. If you've been running on synthetic for a lot of those miles, you might not see much wear at all. I had almost nothing on my '94 after 110K miles.

radiateu2

OK, so a little cylinder cleanup and new rod and main bearings and new rings shouldnt throw the balance off to much to notice it. Would this or should this be correct? But if I need new pistons I would need to rebalance..would this be correct or should I weigh my current pistons and try to buy pistons close in weight and take off enough to match the weight of the old pistons

ede

i've installed new pistons in engines without any balancing work being done. maybe not the smartest thing to do but they worked.

Vader

Most older stock cast pistons and rods are not even close to what anyone would consider balanced. If you're not planning on driving it any harder than any other stock engine, you should be able to install a stock set with similar results.

I would be tempted to try to match the pistons first, then match the big ends of the rods. If you have to remove more than 8-10 grams, you might want to rebalance the entire assembly for a stock, street driven engine. (I've seen as much as 9 grams difference in stock rod CAPS alone, on an engine that ran well for 100K miles.) The pistons should be matched to within 3-4g but aren't as crucial as the rods. Try to match the rod caps first, then the assembled big ends of the rods. If you discover that the crank ends of the rods are not within 3-4g after matching the rods caps, work the lower ends of the beams and rods to get them closer. Once they are within 3-4g, determine how much total mass you have removed from the caps and rods.

If you exceed 15g of total material removed from any one rod/cap assembly, you really should rebalance the assembly. Between 10-15g, you probably can get away without a rebalance. Below 8-10g, you definitely already have a better balance than many factory SBC rotating assemblies.

If you have a shaft balance stand, you can do the crank balancing yourself. If you have to pay someone $200 to do it, negotiate. Since you've already balanced the big ends by that point, the hardest part is done and all they need to do is balance the crank throws to match. They should do that for a better price.

Beefy89

If your rings are worn then you've probably lost some of the crosshatch pattern on the cylinder walls. Like Vader said you might want to have the cylinders honed.

Streetiron85

Is it possible to explain what a shaft balance stand is, and how I might balance a crank myself by using one?

Vader

The first step in balancing is to determine the mass of the rod caps, then match them as closely together as possible. The caps are then bolted to the mating rods, Determine the mass of the journal ends of all the rods by supporting the wrist pin end on a free-floating fixture (like a lawnmower blade balance stand) and resting the journal end on a scale or balance. Once the journal ends are weighed, add the mass of the bearing shells to be installed to arrive at the mass on each crank throw. Using clamps and tape weights (or my favorite, flexible vinyl magnetic tape) wrapped around each crank throw in place of the rods, set the crank on a balance stand. Adding mass to the opposing sides of each crank throw will indicate how much material should be removed from each journal to achieve a neutral balance on every throw. After a few adjustments, the amount of material required to balance will become very small, and achieving a perfect balance will seem almost impossible. That's when you're done.

Oh. The balance stand?

radiateu2

so where would one get the balancing stand and magnetic strip weights from? Couldn one use a stand at a fixed level to weigh the ends of the rods with one end on the stand and the other end on the scale?

Vader

That's the idea of the lawnmower blade unit. It supports one end and allows movement while the other end it on the scale.

Streetiron85

The concept of matching the weight of all the components of the reciporicating mass is simple to grasp. Pistons weigh the same, big ends, small ends and total weight of each rod match.
I get lost tho, when it comes to attaching a specific amount of weight to each rod journal on the crank, and then balancing the assembly with that weight attached.
Because if you attach the same amount of weight to each rod journal, wouldn't the weights on opposing throws just counterbalance one another? To the extent that it wouldn't matter whether you put 1000g on there or 0g. A crank that's in balance with any specific amount of weight on all 4 throws will also be in balance with no weight attached at all.

I'd really like to find out more about this because I have 4 motor projects that will eventually all need balancing and if there's a way I could do it myself, it could save me enough $$ to be able to afford a set of AFRs.... To put it in terms that everyone can identify with.

Thanks

Streetiron85

If the response to this is too complex or lengthy, would it be possible to show a link that gives some info?

RB83L69

Imagine a crankshaft with 2 main bearings, and one rod journal, and no counterweights. Just for the fun of it, let's put the rod journal right at one end, next to one of the mains.

Probably wouldn't run smooth.

OK, needs a counterweight. Let's add one. We're "cost-conscious" though, and we don't want to re-design the whole engine. So let's put it at the other end, near the other main bearing. Now, using the static lawn mower blade method, we trim this counterweight such that with the crank horizontal, there's no force tending to make the crank turn. In other words, it's perfectly balanced. Will it run smooth?

The answer of course is no. When we start spinning it, the end with the rod journal is going to try to fly off into space in one direction, and the CW is going to try to fly off in the opposite direction. If you look at the thing when the rod is let's say over on the left side, that end of the shaft will be trying to go to the left; and the CW will be on the right side, trying to push that end of the shaft to the right. In other words, the shaft is now going to try to wobble around its center of mass.

Not much of an improvement.

That's "static balance". It's the same reason why poorly built aftermarket wheels will vibe so bad when you put all the balance weights on the inside lip and use a bubble balance on them. Sure, they sit there and have equal weight in one plane; but not in the other.

Now imagine that we take the CW and split it in half, and put half of it on one side of the rod journal, and half of it on the other. Now we're getting somewhere. The moment of inertia resulting from the CW is applied to the same point on the shaft as that of the rod journal, so now it runs smooth. That's called "dynamic balancing"; at all times, under all conditions of rotation, all the moments of inertia tending to make the shaft move sideways, are exactly opposite and equal, so they cancel out, and there is no vibe.

Now imagine that we keep adding rod journals to our crank. We're going to have to do the same thing with the counterweight for every one we add, or else it's going to wobble again. Obviously if we end up with a situation where there needs to be 2 weights in between 2 journals, we can combine the 2 weights and make one that's the right size and location to produce the same effect as the 2 separate weights.

Make sense?

Streetiron85

Last night I searched Google on this subject.
I found out that a $50,000 machine is what you need to get started at crankshaft balancing.
It partially makes sense.... For it to fully make sense, I'd have to have about 200 more questions answered.
I'll just accept that crankshaft balancing is going to be something that's beyond my understanding for a while.
I have other things to focus on.

But thanks anyway.