All I need for my bottom end is to balance it!! I got a digi scale ( www.americanweigh.com has some great prices on digi scales. Mines 3000g in 1g increments), and I weighed each rod and piston seperately. The rods are pretty well balanced to each other, only 3 grams from H-L, but the pistons are about 8g from H-L. So, me being as **** as I am , Im gonna shave off a few grams to see if I can get the rods and pistons to 1g from heaviest to lightest. But after that, I still gotta balance it to the flywheel and balancer.

BTW: this is for a 350 block, 4BM, 1pc RMS with 383 crank

I used a GMPP flywheel, says its for ext. balance, but the flywheel itself is neutral, and a GMPP 400 balancer (with the big counterweight). Ive been reading in all these engine building books about bob weight, and it says to find the rod little end, big end and all that. but with this scale, if I weigh the little end then the big end, the numbers dont add up to the total weight of the rod (this is with the piston/rod combo). And even if I get the bob weight perfect to the crank, do I STILL need to get it balanced to the FW and balancer? Im leavin for college in a few months, and moneys tight (same sob story, I know alot of people are in that kinda situation), but I really wanted to get the whole shortblock together for the final time. Any input?? Could I just get the bob weight correct, and get another balancer, for neutral balance?

One more thing, does anyone have a pic a "rod hanger", something to hang the rod from to find bob weight? Or just get a block of wood and lay the damn thing on their and weigh it??

 

Take it to a machine shop that has an engine balancer. There's more to balancing the rotating assembly than just weighing the pistons and rods.

I'm about to get my rotating assembly balanced as soon as some more parts arrive.

New Eagle H-beam rods. There's no reason they don't all weigh the same at both ends.
New JE forged race pistons with the pins and locks (floating pins). Same thing, they should all weigh the same.
Weight of rod bearings
Weight of rings
Eagle only produces internally balanced BBC cranks now. 454 and up were always externally balanced so now I need a new vibration dampener and flexplate.

Once all the weights are known, the crank needs to be balanced in relation to all the weight. A neutral or internally balanced crank just means it doesn't use the vibration dampener and flexplate/flywheel as counter weights. The balance weights on the crank still need to be counter balanced in relation to the rotating mass on the end of the throws (rod, piston, rings, bearings). This means either holes are drilled to remove metal or drilled and have a heavy mallory metal weight added to balance it out. Bob weight is the weights they install onto the crank to simulate the weight of the rotating mass.

 

well, whats the maximum deviation of weight between the heaviest and lightest rod/piston? Or even the whole assembly. I figured I can get that as best I can, then get it balanced at a shop. I think the rods are only 3 grams from H-L, but pistons are about 8g. all pins and locks weight the exact same, so Im not worried about those.

 

If I were you, I wouldn't do anything to the rods or pistons just yet. *edit* How exaclty are you weighing the small end and big end? The shops have fixtures to do that. And are you wieghin it wik the piston still on? GOOD luck! Pistons most likely have to come off to get the big end and small end. /*edit* These weights are used to calculate the Rotating and Reciprocating weights. And as far as getting the pistons to weigh the same, where are you going to take the weight off? There's good places and bad places. The won't save anything by doing it yourself. Equalizing the pistons etc is the easy part, building the bobweights and spinning the crank and then balancing the crank is what is time consuming... which is what you're really paying for.

To answer your question. Zero grams deviation. Then balance the crank to under a gram or two.

Some more things to consider. Will you be replacing the rod bolts? maybe some ARPS? Could change the weights. What about resizing the rods? Minimal, just still could change the weights.

 

Im weighing the big end and little ends by suspending one end, with the rod at a 90 degree angle to the scale, with the other end on the scale itself. And yes, right now Im doing it with the piston on the rod. The deviations I was talking about was when I measured everything seperately.

As far as where to take the weight off of, I really dont know of the best plae to start. Where would you guys suggest? Right below the pin? or inside the skirts maybe? I dont know where to start. But even if the machine shop does do that, thats one less thing Id have to worry about. If I do it, I know ittl be right. And if its wrong, or screws up due to it, then Ill know who to blame

The rod bolts are actually ARP cap screws, eagle SIR I beams. And what do you mean by resizing the rods? shaving off some material to equalize the weight?

 

Resizing (at least that's what we call it) is basically making sure the big end is round on used rods. If you're using new rods, then you don't need them resized.

Since the piston floats of the rod, getting an accurate rotating weight like that is damn near impossible. Weigh it like that, take it off the scale, then do it 2 more times and none of the numbers will be the same.

Like I said earlier, the shop is going to have to build the bobweights to spin the crank, and they're going to at least check that you got the weights correct, and they're prolly not going to give you any discount if the rods and pistons are already matched.

The excess weight on the pistons is usually taken off a few places, extra castings flash, below the pin... but not too much... this is where experience comes into play. If you do it and screw up, really bad things could happen, like throwing a rod, ruining the block etc. It's not worth the risk, just pay the shop to do it right the first time.

 

And following that advice would lead to none of us learning to do it on our own.

Here's the inevitable stipulation: Should you do it on your own depends on how far you have advanced or how willing you are to lose an engine.

 

You could build a house on your own but if you're not a carpenter, there's no telling how well it will be built. A machine shop has trained machinists who know the proper way to do the machine work. Yes the average person could figure it out with trial and error and a few mistakes but the end cost will be much higher. Everyone has to start somewhere but that's also why people have schooling and training.

We've had apprentices start at our heavy truck shop. They're not even first year and they expect to be given a customer's $40,000 diesel engine to rebuild. Without training and knowledge, they're never going to given that job because if they screw up, the business ends up paying the cost. A qualified mechanic may still screw something up but at least he has enough knowledge that it's unlikely to happen. A qualified mechanic should also be able to complete a job quicker than an apprentices which makes the company more money justifying their knowledge and skills.

If the machine shop screws up your balancing job then it's they're fault and should correct the problem for free. If you screw it up, the cost is out of your pocket.

 

My main point is that there's really nothing to gain by doing anything on balancing it yourself. If knowhow is what you want, ask the shop you goto to explain/show how it's done, not that you're questioning them, but rather just want to learn.

And if you screw up before it even gets to the machine shop, it can cost more.

Classic example. The shop I worked at was known for getting out broken bolts/studs and repairing threads when everyone else in town can't. Ballpark figure about 10$ per hole. Well, had a guy bring in a turbo charger wik 6 broken bolts... now, his buddy thought a 60$ estimate was outrageous and that he could do it. Fine. A week later it came back, wik a broken drill bit or two and a broken EZ out. 100$ later the guy had his turbo back.

FWIW... I worked as the counterman at a automotive machine shop for 3 years, Vilas Motor Works in Bryan, Texas. I don't claim to know everything, but after working there, my knowledge of motors had gone up x100.