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Old 01-15-2007, 07:53:19 PM   #1
1981z28owner
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sbc crank end play

what causes a crank to have too much end play? the crank i got has .014.

thanks
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Old 01-15-2007, 08:44:50 PM   #2
rustbucket79
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Worn crank thrust surface or worn/incorrect bearings. (#5 specifically) Is this a new assembly or an older running engine?
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Old 01-15-2007, 08:52:10 PM   #3
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The crank is "captured" on both sides of the rear main bearing. That's the thrust bearing.

In front of the rear main bearing it's a machined surface on the side/face of the rear-most counterweight. Behind the rear main bearing it's a special "lip" built into the crank, just in front of where the rear main seal souches and seals off the crank.

How to fix... sorry, you need someone smarter than me to tell you. I recall max spec is about .008 or so.
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Old 01-15-2007, 09:17:38 PM   #4
1981z28owner
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it is brand new clevite H bearings. the crank was turned .020 on the mains. the power manual says .005-.007
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Old 01-15-2007, 09:22:33 PM   #5
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OK, Rust, you're up. I'm wondering how turning mains could prossibly get into the "walls" on either side of the rear main journal. I THINK that's not likely to happen, but you're into areas of machining that I guess I don't understand as well as I might think I did.

I'm thinking the crank is just worn out OR the rear main bearings are just out of spec in the width dimension for some reason.

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1990 454SS pickup. Original "tow truck motor" 454 upgraded to something a little more "aggressive." Seems pretty stout but the fuel bill is nuts.

78 Malibu. Inherited with only 35K on the odo! Mild 383 with Weiand 142 blower on top. Mid 12s appears to be the best it's gonna do on street rubber. Runs so good I may never drive it again.

"Last remaining QJet tuner on planet Earth!"

Last edited by Damon : 01-15-2007 at 09:24:42 PM.
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Old 01-15-2007, 09:27:46 PM   #6
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we have never had this problem before. i am going to measure my old bearings tomorrow and see if there is a difference.
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Old 01-16-2007, 12:31:36 AM   #7
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I had an old GMC Sprint that had a 350 in it, it must of had about .025 to .035 end to end play. When I pulled the motor apart I realized the thrust surface on the crank was shot, it had a deep groove in it. How it got so worn I don't know, maybe some type of tranny problem or something, so I have seen it before.
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Old 01-16-2007, 12:38:08 AM   #8
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Quote:
Originally Posted by 73RS
I had an old GMC Sprint that had a 350 in it, it must of had about .025 to .035 end to end play. When I pulled the motor apart I realized the thrust surface on the crank was shot, it had a deep groove in it. How it got so worn I don't know, maybe some type of tranny problem or something, so I have seen it before.
I have seen it before as well.. if it is a freshly machined crank, then it needs to go back where it came from. If not... time to talk to your machinist and decide if it is worth repairing....
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Old 01-16-2007, 03:53:07 AM   #9
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My theory here is assuming it was a used crank that needed grinding (on the mains) that it had previous damage in the thrust area caused by the clutch or torque converter. When you grind a crank you dress the thrust surfaces with the sides of the stone, and it will add .001 to .002" end play but that isn't generally an issue. Assuming the thrust surface wasn't already oversized the only other possibility is the previous damage that the operator ground until it cleaned up without measuring the final size. (we use a normal thrust bearing and whatever feeler gauge fits in is roughly your final installed thrust clearance. You can buy main bearing sets that have a .010" or .020" overlength thrust flange, and the crank is ground accordingly, again using the overlength bearing and a feeler to determin how much has to be ground away. 1981 has 2 options here, he can get the overlength bearing set and dress off the front halves of the thrust bearings until his thrust clearance is within spec, or he can take the crank back to the machinist and have him clean up the thrust on the crank to achieve the correct clearance. You can weld up the thrust and regrind it back to standard but we use that as a last resort to save a valuable crank.
Before the engine goes back into service, correct the problem that damaged the thrust surface in the first place, or the same damage will occur again.
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Old 01-16-2007, 08:31:44 AM   #10
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where do u get the overs sized thrust bearing? this is a crank we bought off bay several years ago. i will try to get ahold of them. aka, i wont. if i can get a .010 bigger thrust theni should be ok.

thanks
mike
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Old 01-16-2007, 01:04:44 PM   #11
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Clevite and ACL make oversize thrusts, or more accurately a main bearing set with the overlength thrust, but I'm not sure they have the H bearings in that setup.
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Old 01-16-2007, 01:53:03 PM   #12
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what would be a good series if h will not work? it is a 1182 forged crank.
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Old 01-16-2007, 02:29:46 PM   #13
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Also, be sure that your machinist dresses the thrust correctly. If he trues up the thrust when he grinds the mains and his stone is rough on the side and leaves a rough thrust surface, that will self destruct in no time.
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Old 01-16-2007, 03:15:21 PM   #14
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The stock "P" series bearing is perfectly fine for a factory forged steel crank, that's what we used for years prior to FM's race series.
Clevite 77 MS-1744P-10 .020 is the part number you're looking for with a .010" overlength thrust, MS-1626-P-10 .020 is for a .020 overlength thrust, these are both trimetal design bearings.
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Old 01-17-2007, 08:40:26 AM   #15
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i can add to "how" it gets that way. as an example in production building,, they don't care as much as to how bent the crank is before grinding or as to how close the end to end is set for true parallel mains. and they want fast cleanup's by eliminating "straightening time",, so the crank is indicated in on the worn sections or closest centerline for cleanup,, which are off center to the true axis of the mains, to get that fast clean up. everyone wants a .010/.010 crank and it's not always possible with alot of wear when centering off the snout/seal area.

example,,,, when grinding a crank with .015 run out bend in it,, where the mains will clean up by .020 the thrust flange/snout/damper will spin like a warped brake rotor or flywheel and take more to clean it up. maybe it was just gound till it cleaned up and not checked as mentioned......

when you get the timing set on it,, it will usually tattle tale - tell you if the crank mains were bent and not straightened before grinding or the end to end not being truely perpendicular to grinding stone,, or parrellel to the true main centerline by,,,,,,, the timing chain getting loose then tight as you rotate the crank/cam assy with the timing set on.

hope it helps. once you see the bad,, you really appreciate the good machining that can be done.

ditto with rust on the oversize flange length bearing deal. some 400-600 grit sandpaper on the rear bearing flanges on a flat plate,, will keep the crank in the same location in the block and provide the running thrust clearance you need.
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