Engine balancing question.

gunslinger

Member
My quick bearings and gasket job is quickly getting expensive.
I took a perfect running engine apart and found two slightly bent intake valves, the number three main bearing is trashed, a cam with a couple lobes smaller than the others, a pitted and worn front cover....etc.
According to the machine shop, the crank isn't going to polish out like I had hoped, and the cost of cutting it is almost what he'll sell me a crank and bearings for.
If I replace the crank, does the entire rotating assembly need to be rebalanced?
He assures me that it's a turbo crank, but I won't know for sure until I see it.

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turbodave231

Moderator
As stated above, Its always a good idea to have the rotating assembly balanced, however.................When these cars were under the GM warranty and the engine had a problem, the dealerships would replace a damaged crank with a new crankshaft and not balance anything. So If you swap out a stock GM crankshaft for another stock GM crankshaft, you should be no better or worse than what the factory did when engines were originally built. The parts were built with a fairly wide tolerance for balance and were not sorted or matched during assembly.
 

hiboom

Hiboom
I used to have my sbc balanced when I turned them to 7500 rpm my buick rarely sees over 5500 I have threw together several different motors for my buicks (i have had 3 ) over the years using a variety of parts from na cranks hypertech pistons stock pistons just about anything to get one going when money was tight.never ever had a vibration or one fly apart because it wasn't balanced ,but I also believe a good balanced motor will outlast a non balanced rotating assembly. If it was something I wanted to do once and not have to mess with again and I was putting a fair amount of money into it I'd probably do it .My cars rarely see the track so they were only high 11 cars If it's something that you were going to push daily or are shooting for 10's and looking for longevity ? mite idle a little smoother and last a little longer.Some people swear by it some don't couldn't hurt for a couple hundred more LOL
 

Pronto

Believe nothing you hear and half of what you see.
Eric, is the crank he wants to sell you stock sized or has is been cut already? If so do you know how much it's been cut? It may be a "turbo" crank but if alot has been cut off it it may be too weak. Did he say how much would have to come off the original crank?
 

longball

I fought the lawn...
[quote="gunslinger, I took a perfect running engine apart and found two slightly bent intake valves, the number three main bearing is trashed, a cam with a couple lobes smaller than the others, a pitted and worn front cover....etc.[/quote]

ya sure that was a perfect running engine?
 

Paul Lohr

Member
I wonder if a balancing shop could match balance the old and new crank (using a dynamic balancing process)? If so, would that be substantially cheaper than balancing the entire assembly?

hiboom is probably making sense. I'm just not sure that dynamic balancing adds even a small amount of benefit when the rotating assembly does not exceed 5500 RPM. Unless component weights are changed. Many times the component weights are changed (lighter pistons, aftermarket rods, forged crank). Then it makes sense to dynamically balance the complete assembly.
 

gunslinger

Member
[quote="gunslinger, I took a perfect running engine apart and found two slightly bent intake valves, the number three main bearing is trashed, a cam with a couple lobes smaller than the others, a pitted and worn front cover....etc.

ya sure that was a perfect running engine?[/quote]

I was shocked at the condition of the engine. A few weeks prior, I ran a 12.17@ 110. I also drove the car 40 miles a day, every day, before I pulled the engine. It ran great, and made no noises at all.
The only reason it came out, is because the car was going into storage, and I was getting a little oil out of the breathers so I suspected that it had a head gasket leaking into the valley. (which it did)
You know how it goes. I thought "While it's out, I may as well put bearings in it." And the whole project snow-balled.
I just wanted to do a quick rebuild that would last me a few years until I could save up enough money to have a nice stroker built. :banghead:
 

gunslinger

Member
Just cut the crank .010 and be done with it we have had 10 sec cars with .030 cranks in them that were fine
I'd love to do that. There's one local machine shop that's hit or miss, and another one that's $200 to turn the crank. I've got a crank kit coming for $240 with bearings. I'll see if it's a turbo crank when it gets here, and how much it's cut. Good shops are getting harder to find around here.
 

gunslinger

Member
So it was either a NA crank, or a turbo crank that had been welded and cut. I told the guy at the machine shop that i didn't want it, and he told me he didn't blame me.
I brought my old crank back, cleaned the aluminum glaze off of the #3 main journal, and measured everything. It's all well within spec, so I sent it out to the hit-or-miss shop to have them polish it. If they can't, I'll send it somewhere else to have it cut.
This is turning into a giant PITA.
 

turbodave231

Moderator
If it were my engine, I would measure the crank carefully and figure out what undersize is needed to clean it up 100%. Buy the new rod and main bearings in that undersize and install them in the block and rods. Oil and torque the bolts to spec. Use a precision bore gauge set to the undersize dimension to figure out exactly what journal size is needed to hit .0018-.0021" on the mains and .0016-.0019" clearance on the rods. ( you can replace the clearance specs above with anything you like. These are my preferred clearances for a stock crank, stock block street engine)

A good crank grinding tech should be able to hit the number you supply. Locally I can get a crank turned for $90. So $200 is highway robbery unless he is supplying bearings too.

If you just drop off the crank and tell them .010 US, you get whatever clearances you get............there is no way to decrease the clearance, You can polish the crank to get a little more clearance if necessary, but what a pain in the butt!

To get this done, you need an accurate 2-3" micrometer and a bore gauge (both need to read in the tenths (.0001). Both can be bought from Summit, Shars, or Enco fairly cheap.
 

turbodave231

Moderator
Even better if you keep the crank STD/STD. Even if it is little undersize, you can still get .001 US bearings for mains and rods. I think Mike at FT sells .001 or can get them for the bearings you already bought. This allows you to mix and match STD with .001 bearings to get the exact clearances you want.

The mike and bore gauge are still needed to get the clearances right. Small investment for a big return IMO.

Ask questions and you will find the help you need. Good Luck with the rebuild.
 

Mike Licht

I was here first
What Dave said ^^^^^ and send the bearings back and we will send you and under size set when you know what size you need
 

Mike Licht

I was here first
For anyone who does not know TurboDave is a master machinist and teaches engine machine work, he knows what he is talking about
 
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