well i was putting a lt1 motor together today and i was using rubber lines to cover the rod bolts as i inserted the piston into the block.the rubber lines were a little loose and one of the rubber hoses fell off as i was tapping the piston in. the exposed rod bolt put 4 marks on the crank were it had hit it. i cant beleive how easily the crank is damaged by such a light amount of force. well anyway i dont realy want to buy a whole new crank or have it turned down because the person im builing for said they didnt care about the marks just polish them out. so i hit them with 400 then 1200 then 2500 to polish it off and they apear to be fine. my question is.. is this going to ruin the crank and bearing without a doubt or should it be ok??

thanks

 

You have more guts then me, I'd have never gotten on the crank with a peice of 400grit. Dont get me wrong, I dont know that it will hurt it, I am just saying I wouldnt have done it.

I used the hose like you on my first ones, but where I had cut the hose with a pocket knife there were little peices of thread from inside the hose that got caught between the crank and the bearing, I had hell getting the thread out, after that I just had my buddy watch them and be super careful as they came up. Not one mark.

TTT for you.

 

As long as there are no raised portions from the divits which will quckly mar the bearings you will be fine.
Hell... I am using a cam in my car that has a groove all they way across a bearing journal due to having to use a chisel to get a siezed bearing off it.

 

It could be slightly out of balance now after using the sandpaper on it. Not much, but who knows how it could affect the motor?

 

I had the same thing happen to me. I did what you mention: polish the marks out with 400 and 600. I have over 20 000 miles on the motor by now.

 

lol, i doubt he removed even less then a 1 gram with the sand paper, so there is no way in hell anything will be out of balance.
Marcin

 

Thoughts:

You'd need to polish off well over .001 on the rod journal diameter to remove a gram. It would take you a LONG time to do that with 400 grit paper. Polishing might remove .0001 to maybe .0002 on the entire journal.

If you are polishing a journal, the best way is to spin the crank slowly in a lathe. Most folks don't have that option, so another way is to wrap a strip of the wet/dry snadpaper just the width of the journal completely around the journal, then wrap a piece of clothesline around the journal two or three times to cover the width of the sandpaper in order to load it evenly. You can then pull on the free ends of the clothesline and spin the paper. Use oil on the wet/dry paper. Don't do it dry!

Theoretically you should polish in the direction that the bearing rubs so that any little scratches or minute pieces of metal left on the surface face away from the direction of rubbing, not into it. Visualize the crank spinning and your sandpaper as the bearing and you'll figure out the direction.

If you spin the paper by pulling alternately on the ends of the clothesline, you'll be polishing in both directions, but you could finish up in the correct direction.

As far as grit choices, you want to end up with approximately the same finish as the rest of the journals. This is usually less than 8 microinches Ra (8 Ra) and often about 5 Ra. On a crankshaft with a nitrided (hard) surface, 400 grit run wet should give a 5-8Ra finish, and 600 grit will be closer to 2Ra. Personally I wouldn't go finer than 600 grit. Don't start there, because 600 won't remove the little "outies" from the nicks. If you can catch them on a fingernail, you might start with 320 run wet with oil for a few spins. When you get the high spots out, wash the journal with solvent or mineral spirits, then go to the finer grit(s) to just make the surface smoother. IOW, 400 smoothes the 10-16Ra finish from the 320 to around 8Ra, and the 600 if you choose, reduces that to 5Ra or so.

A fairly good way of comparing surface finishes (poor man's profilometer)uses a wooden "lead pencil". Use the standard 2B or Medium hardness lead you use for school tests. Don't sharpen it to a conical point, rather make it a flat chisel point using your sandpaper. If you angle the pencil at about 30 degrees and push the chisel point along the length of the journal across the scratches, you will feel some drag as the scratches rub off the graphite. Try this on the surface of the unpolished journals to calibrate your hand as to how it resists your scraping. Then transfer to the polished journal. With a little practice, you can feel a few microinches of difference. Try to get the same surface feel. I've seen guys who could do this and give accurate readings within a few microinches even without a known surface for a reference.

Anything finer than 600 is probably counterproductive. I certainly wouldn't advise 1200 or 2500. I also wouldn't advise using your finger to load the paper in one spot. Use the clothesline.

Good luck!

 

so if i understand it right it should be ok.. it is polished back down below so no burs are there. i just hope it holds together .... i guess there only 1 way to find out.

 

I have used a fingernail file and 600 grit and it turned out OK. If there are no high spots,I jokingly say it's room for oil storage,you just don't want a high spot that will dig into the bearing.

 

IMO, some good tips offered above. But to sum up, you want to remove the nick(s), not polish them. Use the correct grit honing stone to remove, and followup with polish if necessary or desired. Using polish prematurely, will polish surrounding area(s) as much (or more) as nick.