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crankshaft scratches - how big can they be?


rustdogbrown

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I recently got my crank, block, pistons, rods, etc. back from the machine shop and started to rebuild. I have plastigauged the main bearings and big-end bearings. Everything looked good. Main bearings installed and on to the big end bearings, right?

Well, I noticed some scratches on the #4 big-end journal. The scratches run mostly in the circumferential direction and are 4 or 5 in total. I can feel them with the tip of my finger nail but not with my finger(skin on metal).

I do not know how the scratch happened or when, but I have done my best to be extra careful. Now, I am typically really picky about this sort of stuff so I decided to ask the advice of the forum.

Is this a regrind defect? How deep is too deep for a scratch? Has anyone been faced with a similar dilema?

Rusty

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Try to polish the journal with 400 grit or finer whet or dry sand paper and WD40. Don’t worry about removing too much materiel, it would take you half an hour to remove one thousandths of an inch. You can do the final polishing with crocus cloth (the red stuff). Be sure you take extra care to clean the grit out of the oil passage. Also make sure there is no grit embedded in your bearing shells.

No amount of skill or education will ever replace dumb luck
1971 2002 (much modified rocket),  1987 635CSI (beauty),  

2000 323i,  1996 Silverado Pickup (very useful)

Too many cars.

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dharvey,

You would be surprised how small a groove or ridge you can feel with your fingernail. Rustdogbrown has nothing to loose by polishing the journal.

Besides that, a tiny groove around the journal will make no difference.

Remember, the main bearing shells have huge grooves in them.

No amount of skill or education will ever replace dumb luck
1971 2002 (much modified rocket),  1987 635CSI (beauty),  

2000 323i,  1996 Silverado Pickup (very useful)

Too many cars.

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oog.

You say you got the crank back from the machine shop- did they grind the

crank?

If the scratches were caused by the previous set of bearings and are

recessed into the surface of the crank you might

be OK. If something has RAISED metal by scoring the crank after grinding,

that's going to chew up the big end bearing.

The big ends do NOT have grooves in them, and most of the wear happens on

the 'inside' of the throw where it's not easy to detect with a casual inspection...

Try the polishing- I actually have polished a used crank with 220- 400- 600

crocus cloth and had it smooth out a LOT more than I expected-

and it took less than a tenth off the measurable diameter. Just be careful

to do it evenly... and don't gouge the chamfers on the ends of the journal.

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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TobyB

"Big ends do not have grooves in them". That is true.

I'm just saying that main bearing shells do have grooves in them, to point out that a circumferential groove does not make a bearing unfunctional.

post-13223-13667598532793_thumb.jpg

No amount of skill or education will ever replace dumb luck
1971 2002 (much modified rocket),  1987 635CSI (beauty),  

2000 323i,  1996 Silverado Pickup (very useful)

Too many cars.

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Have your machine shop polish it.....Scratches are not good.... The perfect engine has the crank riding on a film of oil(Duh) A scratch says there will be metal to metal contact. If you have seen a crank journal machine you will understand why hand sanding is a no-no...... on the other hand if its a cheap rebuild(no offense) you will score the bearing and probably have no problems.....A concern is that these cranks were nitrided and a scratch could pose a problem.....My guess is that to polish the scratches they would have to do all of them....$$$ for new bearings.....

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I pulled the crank. Took it to the machine shop. They said that it is not a concern as long as the surface is not raised around the scratches. They polish the area just to be on the safe side. No charge, and he gave a cap full of sticky assembly oil for free. He saw that I was using some assembly grease from the generic auto part store and thought his stuff would be better. Anyway, the scratches are still there. At least I know that any raised features have been removed. Thanks for the input.

Rusty

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Your fine as long as its not a race engine..... Did they chamfer the oil passages on the crank....??? And you know to turn it over without spark and plugs out. Confirm the oil pump is getting oil up to the rocker shaft.......Have fun.... Remember to confirm TDC with the valves.... It will save time....Have fun:)

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main bearing shells do have grooves in them

yup, agreed- and your edit makes good sense, too.

If the machine shop's given it a polish and pronounced it good,

bolt 'er in and go. BMW bottom ends are hell- for- stout, and have

loads of bearing surface area. Look at a smallblock Chevvy sometime-

it's puny!

Good luck, and keep everything scrupulously clean...

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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