sbc crankshaft -spun main -

I spun a main bearing in my car the other night, and was wondering is there a rule a thumb for telling if the crankshaft is junk ? or safe to turn
its std right now and your finger nail can grab pretty good on the one main

 

Will it clean up at .030 under? Is that cheaper than a replacement crank?

 

I'd start with another short block....... It would cost you to line hone the block and turn the crank...

 

If I Hot-Tank it and if the black shit don't come off the spun journal I shit can it !!! >>>>.

 

I wouldn't even spend the money to get it hot tanked.... Sorry to bum you out dude.. Just find another short block... Low milage if possible...

 

Ok. Then keep the crank.... That one can be welded up and turned to the stock size. Also make sure it get's checked for straightness....

 

i was in your position last summer, took the short block to local engine builder, he turned the crank, recon 2 rods, checked the line bore on the block and assembled the short block. all better. plan on spending around 1200.00 if you have them reassemble the short block.

 

I second that.

 

Can you post a picture of the crank and main housing bore? I'd be more worried about the block myself. If you have a good machine shop nearby, and you can do most of the work it might be worth it.

 

Depends on how bad you want to keep the crank.

I had a Callies in a race motor that spun three bearings and broke a rod, bending it.

That was a $1,200 crank, so it was worth the effort to fix it. It cost ~$500 to have it welded, heat treated, straightened, ground, and rebalanced.

I got two more racing seasons out of that crank (on top of the three it had seen before it's unfortunate incident) before it was retired, and it's still in fine shape. We retired that whole short block from racing duty (relegated to "backup mothball" status), but I'd run it in a street motor without a thought.

I can't imagine it costing less than $400-$500 to have any crank welded, straightened, and ground, so the stocker may not be worth it.

If you really want to save it, my advice would be to have your machinist grind it, starting with the worst journal, and see if it will clean to a standard undersize condition that you can buy bearings for. If it will, bully for you, have him finish the job and rejoice, if it won't, demote it to boat anchor and move on.

I will also second the "check the block" sentiment, as the aforementioned Callies spun it's three center mains because the webs had pulled plum out of the core of the block. The whole main web had pulled away from the cylinder bores and outer skin of the block from the pan rail all the way up to the cam journals. That crank was the only thing we were able to save from that motor (besides the heads obviously).

When the block broke, it dumped all the oil pressure from the main oil passages and caused the bearings to weld to their respective crank journals. The broken main webs and caps actually maintained the proper crush on the bearings, and the bearings maintained their alignment with the oil passage, there just wasn't any oil to speak of coming out of that passage anymore.

Sometimes you just lose the crush (or never develope it in the first place) on a pair of bearings, and the bearings spin, cut off the oil passage, and smoke the journal, but a lot of times, the crush goes away because something else broke.

Have the block crack checked, regardless of what you do with the crank.

 

Not dumb at all... Forged steet crank shafts can be welded.. Some crank grinders do that to either increase the stroke length or decrease the stroke length. I don't think that can be done with nodular or cast iron cranks.

 

Your finger is pointing to a MAIN !

 

Indeed.

How rough is that journal? Too rough to drag a fingernail across? I can't tell from the pic. If there isn't bearing welded to the journal you might get away with a 0.010" grind and oversize bearings.

If it were mine, I'd take it in and have just that journal ground to see if it'll clean at 0.010" under. If it will, do the rest, and put it back together with new bearings (everywhere, that metal got pumped through the whole motor).

If it won't clean at 0.010" under, demote it to boat anchor and move on.

 

That Don't look bad. Go get it ground. That's a steel 350 crank, they don't grow on trees.

 

#### This place makes me LAUGH !!! Rods Mains What the hell !!! Get the shaft reground and find someone to help you with the rest of the engine !!! >>>>.

 

I really get a kick out of the guys that think a crank can only be turned .010 and still be good. I turn welded steel and cast cranks ALL the time.. Nothing wrong with a .020-.030 crank. Hell they make bearings to .060 under and if worse, turn to small journal or Honda and start over...

Heat treat on most cranks is gone at .010.. So whats the big deal??

I have an old BBC 4.250 Callies Stealth that has been welded several times over the years (been in an Altered, a Pro Street car, a boat) and is at present at 2.100 (SBC) rod. Going into a Mudd truck..

Also a 4.375 crank that has been welded on two rod journals and no.4 main. Welded,ground, straightened and liquid magged for $250.00'

The old timers used to say the .020+ bearings don't transfer heat properly to the rod or main bore.. Thats a load of horse droppings. Thats been dis-prooved by all the bearing people..

 

Make sure the cranks not cracked and is true and straight,if so then turn it down as much as it takes to clean it up,Keep in mind you will turn the nitridded hardened layer off the rods if you have them turned too .Make sure your shop turning it leaves a large fillet and you buy bearings to clear the fillet and you will be just fine.You can turn em low as long as the fillets done right.over 50% of the factory chevy 350 steel cranks i've sorted through didn't pass the mag test.

 

There is absolutley nothing wrong with a turned crank. In fact it is often better than a stock one. Most rebuilders orcrank shops are fanatical about sizing. GM does givea rats tail as long as it is within tolerence. Most crank grider guys are fanatical. Yo can turn to 030 safely. . Some guys dont want to go below 020 but not because it s unseafe just because it gives them no repair room if the mess up. What is real important is that the Kournal s perfectly round and not tapered and that there is sufficient clearance.
BTW I once helped a young friend with aSBC deal in his 67 Camero. he had arash of Main probems till discovered he was using solid motor mounts. While he was a lunch one day I slipped in a rhs rubber mount. End of problem. While blocks are strong sometimes race cars twist morethan we think and it takes little block distorion to through the mains out expecially f they are on the tight side of specs. The left mount being steel gave him the protection andlaunch and reaction heneeded while the rhs rubber had just enough give to stop distorting the block. It did nothing to hurt his perfomance either since it is always being compressed on a run. You can argue till the cows come home was it right or not but it did end his problem right there and it never came back ever. He was geting all sorts of advice at the time and was unsure what to do or who to listen to. Since he was in my shop I made the decison for hmand even installed it while he ate his lunch next door. He was a bit mad at the time but I think by now he has cooled off. He never had to fix it again. WHen he was assembling his otors I looked at it a couple of times. The mainswerefree and the crank spun well with no binding. i figured the only possiblilty was the block was twisting on launch. it was a fairly high HP engine for its day.
Don