Thread: Scratched Crankshaft

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Hey guys well I have a 400 crank im running in my 400 that im currently building. its a stock cast crank. but on further inspection, the crank has some scratches in the journals. they are not serious scratches by any means. now what im wondering is, will it be fine to run in my 400? I just got it polished at the machine shop. Does that put a slightly rounded edge to the scratches when polished? Whats your opinions on this guys?
Thanks

Originally Posted by facemelter

Hey guys well I have a 400 crank im running in my 400 that im currently building. its a stock cast crank. but on further inspection, the crank has some scratches in the journals. they are not serious scratches by any means. now what im wondering is, will it be fine to run in my 400? I just got it polished at the machine shop. Does that put a slightly rounded edge to the scratches when polished? Whats your opinions on this guys?
Thanks

It depends on what you mean by a scratch - if you can catch a finger nail while dragging it across the scratch, it needs to be ground to the next undesize. If it is a very minor surface scratch, and is cosmetic mostly, it MAY be fine and run forever. I have seen nasties run for many miles with good oil pressure, but..... it's your final decision (or with a good automotive machinists' input after seeing it in person)

well they can not be that bad if they polished it .the belt takes the hi.s off the crank the low spot is left . all this will do is hold oil

Originally Posted by pat mccarthy

well they can not be that bad if they polished it .the belt takes the hi.s off the crank the low spot is left . all this will do is hold oil

Yeah when i brang it there I originally just had the assembly balanced, but then they noticed some scratches. So I asked what they thought and they just said we'll polish it up. The high spots I suspose is what would rip the bearings up.

Originally Posted by Irelands child

It depends on what you mean by a scratch - if you can catch a finger nail while dragging it across the scratch, it needs to be ground to the next undesize. If it is a very minor surface scratch, and is cosmetic mostly, it MAY be fine and run forever. I have seen nasties run for many miles with good oil pressure, but..... it's your final decision (or with a good automotive machinists' input after seeing it in person)

And if the scratches ARE too bad, which im thinking they probably are not, like you said, I could get it cut undersized. Didnt honda make they're cranks to have smaller journals for more horsepower? As a result of less surface contact?? lol anyways, ill keep that in mind. Oh, and lets say if i do need to cut it undersized, will that make the crank unbalanced? as i just paid 175 to get it balanced!
Thanks for your opinion

Face, Pat's got ya covered. We just call it "oil clearance. Seriously, you should be fine if they don't appear to be the Grand Canyon.

Originally Posted by facemelter

Yeah when i brang it there I originally just had the assembly balanced, but then they noticed some scratches. So I asked what they thought and they just said we'll polish it up. The high spots I suspose is what would rip the bearings up.

yes it would .i have polish cranks that see hi loads and grab the bearings a bit and polish them and run them i try to not get them turn down in drag engines. to get some use out of the cranks and have run some that have been scratch a bit long as size of the crank is not way off i do not grind them i like them abit loose any ways for racing and they make plus bearings any ways

Originally Posted by DennyW

Run some 360º oil groove bearings, and you will never know it was scratched at all.

had one 454 crank oil grooved 360 mains work just fine and is still out there some were ?if you read this call home grinding the crank will not throw any thing off

i have had crank at my shop done with tape were they came from they look wavy . i can do a better job with my hand held polisher with 400 to worn 400 belt with belt dresser then with belt rouge sometimes or the 3m scotch brite belt.

Originally Posted by DennyW

I agree, I was just showing him about the different methods of polishing. I like to see around a 30-32 micro finish or better on them. Then, I am happy.

that.s cool i just was saying that i was not happy with there new tape machine .i hope my new crank will not look like the ones i have seen from the other crank company

hey denny thanks for that info, I read it all, and it makes things much easier

Originally Posted by DennyW

I agree, I was just showing him about the different methods of polishing. I like to see around a 30-32 micro finish or better on them. Then, I am happy.

If you buy that the oil film thickness is only .00005 from the article then a 32 finish would be the worst you would want. According to some bearing studies we did a (long) while back in school I think the minimum film thickness is more like .0001 to .0003. In this case the 32 finish would work great as your experience has indicated.

In high speed bronze hydrostatic bearing applications we usually spec out a ground 16 finish. These are bearings up to 8 inch diameter with a 3 mil per side clearance and a minimum film thickness of 1 mil.

Kitz

My days in manufacturing, a long time ago, we spec'd a 64 finish on high speed turbine and generator shafts. These ranged from 6 inches up to 24 inch diameter shafts. The turbines were operating at 3000 to 6000 rpm while the gear driven generators operate at 1500, 1800, 3000 or 3600 rpm, depending on the frequency of the country of destination. The bearings in these applications were not round but eliptical (wide at the horizontal joint, allowing for an oil wedge and the climbing factor of the shaft (hydrodynamic wedge) - the shaft would rise and rotate about 10-15 degrees from the vertical centerline of the bearing shell. The oil film was based on a rule of thumb bearing to clearance of .0015/in of diameter, the shaft climbing at rotational speeds and the flow in GPM at 25psig.

Then, to complicate this even more, throw in a rotated tilting pad bearing and try to make some sense out of that feed and drain system - I tried including talking to the designer.

How does that translate to automotive technology? You still have clearances to worry about, pressure and flow and a hydrodynamic wedge - look at the direction of the feed holes in the crankshaft to determine where the wedge is formed. This provides the semi metered flow to the babbited bearings, and provides the cushion for wear resistance. This flow can be disrupted by heavy scratches or it can introduce early wear to the bearings. If the measured fit, by micrometers is correct, and there are no significant scratches, i.e. feel by dragging your fingernail across the journal, there should be no problem. But you also need a good finish in the 64 range as well. Pur it together and run it!!!!

Originally Posted by Irelands child

and there are no significant scratches, i.e. feel by dragging your fingernail across the journal, there should be no problem. But you also need a good finish in the 64 range as well. Pur it together and run it!!!!

I wouldnt consider them "significant" scratches, but as Denny stated, even if I have scratches, I have it polished, and the scratches will just hold oil.
I'm halfway done with the rotating assembly at the moment. Starting to look pretty nice!

Thanks for this info

Originally Posted by DennyW

If you mean the 10lbs for every thousand rpm's, no, I don't buy that. Slick is slick. Good oil pressure, and a super fine finish, only give you good results. Knock the friction out of turning, and it has to move easier.

You put it like it is. Thats how it should be all the time