marc1973

so im rebuilding my 1973 l48 it has 114k miles on it but the previous owner rebuilt the motor, i cant tell when but to say the least there's almost no wear on any rod bearings and main bearings, im rebuilding it because i was just going todo the top end but after measuring i found my piston to deck clearance to be 0.050 and they were dished, i wanted more compression so i am putting flatops in and while i was their i get new rods aswell, my question is should balance the rotating assembly or should i be okay without doing it, i never plan on spinning it past 5500 rpms

im using the the stock crank and the rods are standard length and the pistons 4.000 bore. if i should get it balanced what is a good price to pay for it?, ive talked to one shop here in tampa and they wanted $350.

t.l.d.r- should i get the rotating assembly balanced and whats a good price to pay for it?

63mako

IF you reuse your crank and rods and IF you don't recondition them or IF the new rods weigh the same as the old and IF the new pistons weigh the same as the old and IF the balance on the last rebuild was right you won't need to rebalance. That is a lot of IF's. $250 is a good price for balancing.

marc1973

okay thanks, is the engine that sensitive to the change in weights because im getting mixed signals from people i talk to some say it doesnt matter and others act like if you change one thing the motors is going to internally destroy itself, id like to get it balanced just to be safe, im just still in the process of finding a shop that i can afford

63mako

It has to be rebalanced.

drwet

It has to be balanced. You won't regret doing it and there's a very good chance you will regret NOT doing it.

Indiancreek

Balance it$350 is a bit high. $200 to $250 should cover it around here. If you're getting new pistons from a good manufacturer they should be balanced. Using the old rods have then reconditioned then balance the crank to the engine. If you don't and it has a vibration at some rpm you won't be happy.
Hate to turn the same bolt twice needlessly.

gkull

I don't think that a motor with 114K miles could use non overbore pistons. I would have somebody measure the block bores. You probably need it bored to .020 or even .030 over. I found an old 4 bolt block once that was so out of round that it took a .060 boring

jnb5101

I had a 454 completely rebuilt by a professional drag racer a few years ago with a mild flat tappet hydraulic cam, an engine that won't see more than about 5000 RPMs. I asked him about balancing, and he said that it was "overrated" for that type of engine. To this day, it pulls strongly and without vibration past 5000. Just my experience.

GUSTO14

I find that a lot of folks seem to equate balancing a rotating assembly with racing only. The truth is, that is really only one part of the equation and I would even go as far as saying the least significant reason. Every engine requires a certain amount of horsepower to simply spin the rotating assembly. I like to think of it as the parasitic drag of the assembly. It varies based upon a number of things, such as the weight of the rotating assembly, the clearances on the bearings and rings, as well as things like the type and weight of oil used and any number of other things that go into assembling and spinning a rotating assembly. But, it also varies based on the balance of that assembly. All rotating assemblies generate harmonics. The greater the harmonics the more resistance there is to rotation and the more power it takes to spin the assembly. Of course once the harmonics reach a certain level, the rotating assembly will typically self destruct and that seems to be the level of balance most of us are concerned with. Hence, "I won't be spinning it beyond XXXX rpm, so it's probably not necessary to balance my engine...".

The point of all of this is that the level of balance is most directly related to the amount of power it requires to spin the rotating assembly at any given rpm. The better the balance, the less horsepower required to spin it. If your particular engine produces 350 horsepower on an engine dyno and also requires, say 50 horsepower to spin the assembly at 5,000 rpm, it is actually producing 400 horsepower, but only 350 is available for your use. If you can reduce that amount from 50 to say 40 that extra 10 horsepower is now available for your use.

In the past, "most" manufactures have really only been concerned with engine balance as it related to destruction within the warranty period. Today as fuel mileage has become a prime concern (CAFE), you're seeing more and more manufacturers using things like synthetic oil, roller bearing rotating assemblies and yes paying more attention to internal balance, to increase fuel mileage by reducing the parasitic or induced drag of the rotating assembly.

So the next time someone asks if balancing an engine is necessary, don't forget to factor in the horsepower (and yes economy) that might be lost if they decide it's not necessary.

Good luck... GUSTO

63mako

454 is externally balanced It is balanced with the damper and flywheel. 350 is internally balanced, Apples and oranges.

63mako

There is no way the 2nd rebuild @ 114,000 miles will get by with standard bore.

Indiancreek

If the balance isn't important then there wouldn't be any reason to balance any part of it. Dealing with degree of out of balance is in relation to the RPM. as the rpms increase the effects of out of balance increase. Take a window fan and put a little lead tape on one blade and run it on slow. You'll see the vibration created. Increase the fan speed to high and the vibration increases in proportion with the increase of the blade speed. The distance of the over weight from the center line of the crank has a similar affect.

TheSkunkWorks

The only times that I've omitted balancing has been on non-performance rebuilds that re-utilized the same crank, rods and pistons. Personally, I could never bring myself to skip doing it (or any other fundamental blueprinting step for that matter) for a high-perf/sportscar application. My $.02





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Mick71

All good advice, but I wonder how many SBC's have been "rebuilt" in Bubba's driveway and keep screaming along (for a while anyway )

marc1973

would you be surprised if i told you it has been checked and is in fact usable with standard bore, idk when the last rebuild was but it wasnt that long ago, literally nothing was hard getting off i.e. exhaust mainfold bolts, there is very little wear on everything internally. almost seems like the guy rebuilt it and then just didn't drive it that much

marc1973

as much as id like to get the engine balanced i seem to be having a hard time finding a shop that will do it for less than 300$, im at the point of just weighing everything and if its all the same testing it out

augiedoggy

It doesnt work that way. how are you going to weight the counterweights that are part of the crankshaft that offset the piston/rod?
you can weight your old pistons and rods and see it will be close or you could just chance it by not balancing.

marc1973

im going to weigh the old set up and the new set up and if its close as in almost the same weight within like 2 grams or so im going to chance it, seems to me that the previous didn't mess with the factory balance so if its off slightly near the center of the crank the theory is that is has less effect on the crank than it being off farther away

63mako

If the block has 114,000 miles it is NOT useable with standard bore. No way, no how. If the guy you are having rebuild it told you that get a new guy.

scottyp99

It's possible the car has 114,000 miles, but it's a NOM that has less miles. Have you checked to see if the VIN numbers match?

Scott