494 crank question

ok so I bought a stroker kit off of a board member.was only run on dyno time.put under the bench for a year,now going to use it.all the pistons and rods look real good,clean like dyno time.crank appears to look good.the rod bearings from dyno motor looked like dirt ran through them.thought I would have the balance checked,and it was out some.more holes drilled in the throws.machinist went to polish and check bearing clearances and sees a problem.the crank is worn,scored only on the opposite side of the throws.thinks he can clean it up at .020.ws .010.is this a problem with 494 cranks or because the balance was off?it's a stock crank.

 

Umm, if this isn't suspicious...
How does one not notice that before bolting on the bobweights? Even if you are only looking at the topside you can see the other throws.
Scoring deep enough to wear .010 off a crank (on one side) can be seen from across the shop, esp. when there's dirty bearing damage.
Usually you're not looking to do the minimal amount of work. Things are looked over closely both for the purpose of CYA as well as to upsell services as needed.
Funny that they obligated your $ for balancing beforehand, isn't it?

It wouldn't throw it off much.
Not an entirely accurate example, but for thought...
A 1.500" long x 2.000" wide x .015" thick piece would be about 5.8 grams, but the crank wouldn't wear evenly. The weight doesn't sit in a flat plane, so the imbalance would read even less to the balancing machine (it's wrapped around the crank).

Using a 2400 gram bob, an imbalance of 1% would be 24 grams. You can balance them much to less than a gram, but it doesn't repeat well. That crank might have only been off 2 grams by wear.
If a correction was made the holes would be close to opposite on the c-weight, meaning you would see the scoring when looking at the new holes. (of course it would be covered up by the bobweight)
Bear in mind they never balance the same from machine to machine anyways, so the corrections are always off a bit....

Something else to consider is that you normally grind a crank before balancing.
The rods are ground first, because it moves the crank around a bit. Rods are located off the chucks being offset. The crank is returned to the machine for the mains after the chucks are re-zeroed.
Theoretically, the balance would be off a bit from that, but it doesn't show up on a machine as anything different than the margin of error normally found anyways.

***If the crank cleans up and is free of cracks, you can use it but it would need more balancing. IF....it showed up off balance with the underside side of the journal worn, it would show even more out from the rest of the journal being trued up at the same stroke, as there is more material to remove on the outer side and it's further away from the rotational centerline.***

Sorry for the huge novel, but I'm seriously questioning some things about this shop.
Why would they create a worse situation by doing the procedures in the reverse order?
It may very well be within a 1% error, and it might not show up any balance related wear on the bearings.
I think I would want them find you a different crank or be doing some free work.

 

Dan,

What Rod bearing do you have?

If it's an H series Clevite or similar "hard" bearing, and the crank has not been Nitrided, then what he is seeing is exactly what happens to a stock crank, when you put a motor together dirty with hard bearings.

JW

 

jim,i have the rod bearings that were left in the rods.they are clevite stock small journal chevy.crank looks good for the rod journals.maybe the main bearings were the hp bearing,i'll never no.you bring up a good point.thanks.

 

They were a "P" bearing Dan? The back of the bearing would say CB 745P10 if they were .010 under.

If so, then that type of wear would be unusual.

JW

 

the rods bearings are cb 745p std.just stock type bearings.but i only bought the crank rods and pistons,so i'm in the dark on the main bearings.i'm going to try to find out about what was used in the build.

 

Ok, then issue is related to the last build.. obviously it did not go well, or the parts would still be in that motor.

I would not worry too much about this, clean up the crank, get it balanced up right, and move forward.

To answer your original question... no there is no "characteristic wear" issues with 494 rod journals. Too many possibilities with "collateral damage" from incompetent work to say definitively if the balance was an issue or not, but I would say that is not likely.

Most crank wear issues are the result of oiling issues, debris contamination, detonation, or mismatched bearings/surface hardness issues.

JW

 

thanks for the info jim.:kodak:

 

Dan, I've had a 494 that's lived a good life for many many years in the gold car. I would highly suggest you convert to internal balance if possible to help take some additional strain off that crankshaft. After 488 passes, and a little luck, it's going back together for a new project. Try to keep the reciprocating weight as light as possible as well.