hi guys its been a few years and alot of beers since i assmbled a small block chevy. i say assmbled because i think machine shop build motors. anyway the last motor i assmbled was 383 stroker kit i bought the kit from paw they "balanced" the rotating assmblely. i remember in the 80's the big word was blue printed and balanced. im assmbleing a budget 350 right know for a friend. is going to make any differnace if i weight the rods and pistons and shave them to same weight because i dont think i have the tools to balance the crank. Thanks
Weight matching will be a good thing and shouldn't affect the balance of a SBC at all. It sometimes does, but it depends on when and how it was last balanced. SBC's overall are designed to have neutral balance (with a few exceptions, 400, '86+). This means you can balance the individual parts, then when assembled, the total imbalance should be the sum of their tolerances. Weight matching the rods and pistons is just running on the assumption that the crank is truly neutral balance. I put mine together this past summer. I should have weight matched the pistons before assembly to the rods, but didn't catch it before sending to the machine shop. I did buy a set of weight matched rods, +/- 1 gram. Assuming anything beyond 1 gram would be extra weight on the pistons, I was able to do it with them assembled on the rods. Then I added the rod bearings to the scale before a final tweak to each assembly. The shop weighed all this to check my work, then made up the bob weights for my crank. The balancer indicated I had added enough to the reciprocating parts that I would need to add heavy metal to the crank. After they marked the location and net gain required, I ordered the metal and then drilled and installed the weight myself. They spun it a few more times, drilling to remove the excess in the right place and ended up getting it down under 1 gram of total imbalance. It was fun to learn that stuff and only cost about $200 for the metal and their labor. These tolerances should be great on a hot street motor. By comparison, NASCAR builders will strive for .1 gram of total imbalance.
Just remember that if you replace the clutch, the pressure plate should be reinstalled in the same clocking to maintain that balance.
It will not make much of a difference from the standpoint of durability etc. There will be no performance gain at all. If you want to do it just for shits and giggles or to say you did, fine, otherwise it is not worth the effort/time. I base this reply on the fact you are just rebuilding/freshening a stock 350. If you bought some oddball pistons and or rods then maybe you might want to check them but a budget motor that is not going to be wound tight and raced on a regular basis, then I'd say no need. The stock factory weights/balance of an sbc is more than good enough for any mild budget build.
The pressure plate is not part of the assembly when a motor is balanced, so it's clocking shouldn't matter. Balancing is a lot more important when you start jacking around with nonstock parts. For the average street engine, if you stay with stock rods and choose pistons close to stock weight, you can probably sneak by without a balance. But something like a 383 has a smaller diameter piston (lighter) than what the 400 crank was balanced for at the factory. If you're using anything other than GM crank & rods matched up like a factory combo, balancing is mandatory. Scotty definitely went the long way to a $200 balance job! Around here that's the going rate for full service balancing. But sometimes you just want to be involved, and I respect that. good luck
I think that the gentleman who mentioned clocking the pressure plate is refering to when you have the clutch assy. balanced with the rotating assy. In that case you will usuallly find a punch mark or stamp on the outer portion of the flywheel with a matching punch mark or stamp on the outer diameter of the pressure plate. It is important to have these marks "clocked" together at assembly because that is how the parts were assembled when the balancing was done. Frank
Interesting. 20+ OHV V8s balanced and no shop has ever asked me to bring in the pressure plate. Maybe babbited blocks cause the balance job to go to extremes? Or is the pressure plate a larger percentage of the rotating mass on a model B engine?
Could be, the engine I have right now has a lightened flywheel and a V8 pressure plate, which may be why. (This is also the first engine I've ever purchased, I figured it was commonplace, it sure is a thing of beauty all sparkly and new looking)
A professional, competent, knowledgeable machine shop will ask for everything that is going to be bolted onto the crank assembly. Reciprocating weight and rotating weight. Pulleys, dampener, disk, pressure plate and so on. Also rings, bearings and rod bolts and nuts, timing chain sprocket. At least that's the way precision race car engines are balanced. If the pressure plate and/or clutch disk is replaced with new...no more balance. Of course, all this is for dynamic balancing. If you are using balance or beam scales (for static balance) then it's sorta like measuring journal size with a ruler. A better way to spend time than watching paint dry.
There is more to balancing connecting rods than making them weigh the same. Because of the motion, the crank ends and pistons ends have to be balanced separately, as well as total weight. Here is a how-to of someone doing there own. http://www.angelfire.com/ca4/CorvAIRCRAFT/RodBalance.html
Shifty, I just don't trust anyone unless I'm right in the middle of it! I need to know where you're getting your work done. The rate around here is more like $250 plus metal. It was the $2 per gram on 'Mallory' brand metal that blew me away. I needed 200 grams to get the net gains needed to offset my heavy ass rods and pistons. Good thing I like the internet. I found the same alloy for pennies, literally! My shop did ask for the pressure plate, but it really shouldn't matter. You can always take the flywheel in to have balnce confirmed and then checked and adjusted with the pressure plate.
When we measured the weight of a rod we measured the weight at the ends, not overall. Overall was just to match them, each end was to balance them. This was while doing a dynamic balance though, and I know not many guys have a balance machine in their home garage. I've never balanced an engine outside of my college hi-po engine building class though, so my experience is limited. Thought I'd add my .02
I worked in a high performance shop and to me it is worth the balancing if the engine is going to be run hard and at higher rpm's. Something alot of people don't think about is that the off balance of a rod and piston, might not seem like much, but the higher rpm's the engine see's the small amout of unblanced weight will become huge. Plus your bearing's will last longer. I can tell the difference between factory engine's, if one happen's to have better balance than another by the smoothness as it idles with less vibrations.
Balancing is far more important than most guys think about. The link that is posted here by Relic Stew is very good at showing the way to correctly balance a set of rods.. Someone with more knowledge than I,should post a total Tech on the complete engine Balance procedure....There is a formula for making up the "BOB WEIGHTS" that are affixed to the crank in order to balance it.Both Static and dynamic... Balancing is manditory on a "Dependantly " balanced engine when changing out the stock flywheel and front harmonic.as they are used bring the crank into balance.Can someone do a Tech on this??
I am bumping for more info please. I am running a '57 283 / t-5 in my truck project. My question is - I had a new clutch/pressure plate made up a couple of years ago and I am just now ready to put it together. Looking at the factory manual for '57 leads me to believe that the pressure plate needs to be clocked when replaced due to factory balancing, I assume. The problem is that mine is new. Do I need to remove my flywheel and have it all balanced together? This is just a street car. I did notice an extra rivet in one of the holes on the pressure plate on the outer edge and was figuring it to be attributed to weight. Any thoughts?
I always went by this rule and was told this by a machinist that balances race engines for a living . Anytime you change something in "Rotating Assembly" you should always "BALANCE" it to make sure everything is OK . On a street car you can get away with a lot more than something you are racing . Bottom line is when you use parts from different engines or replace any parts of the rotating assembly , get the engine rebalanced . It's worth every dollar spent ! Just my opinion ! Retro Jim
You are confirming what my gut feelings are telling me. Let me ask this, can I just get the flywheel/ clutch assembly balanced together? The flywheel is already balanced to the engine so as long as I put it together the same way ,it should be alright, correct? At this point I am really just putting it together so I can get my clutch linkage figured out and make sure leverage is sufficient.
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