Crankshaft Polishing - how effective is it...

I have a couple of crankshafts laying around that I have been thinking about taking to the machine shop for a polishing and I have been wondering just how effective the polishing process is when it comes to leveling the journal surface. I have always relied on the "fingernail test" to get a rough idea about how deep a groove is in the journal as the first test to determine whether a grind is needed but I have never heard anyone talk about the machined surface after a polish other than in terms of the depth of scratches in the metal...micro-polishing, etc.
Any machinists out there who can set the record straight?

 

When I worked in a machine shop, the owner always made me polish the cranks. I have to say, it really did an excellent job at getting rid of any blemish. Also, just because you can't REALLY feel it with your finger nail, doesn't mean that your motor isn't choking on it! Treat them like your cyl walls and you'll never run into trouble.
x
Brandy

 

I know this is NOT what you said...but that is what my mind read and I nearly died laughing...at first I thought you were joking and then I reread it...

Sorry, I just had to share...
Mike

PS do you still work at the machine shop? Do HAMBers get a break?

 

In my opinion a lot of the spit and polish stuff is generated by the magazines and bench racers who have never built a engine. Crankpolishing in theory reduces the microscopic indentions and grooves so the bearing and oil have less resistance. Now some folks would say that the oils job is to take away heat ,and if thats true less surface area would not allow as much heat transfer.
There are so many other items that come into play we could discuss it for years . Basically it needs to be round, free of defects,and have the proper clearances.
I say polish the sucker if it feels good.

 

Now here is something i do know about,generally polishing a crank will have very little performance edge at all unless you are racing and really need that 0.2 bhp.What it does do is stop oil splash which in some cases engines are designed to rely on.Basically the designers of some engines actually rely on the amount of splash the crank produces when it's running through oil thus giving extra lubrication and sometimes this lubrication can be critical in times of stress i.e towing and hard acceleration up hill.If you want to take the blemishes out of a crank then 'shot peen'it,this will ensure that the surface of the crank will be stress free whilst keeping the oil drag,Suffice to say for any (even performmance) road engines i would not polish the crank but if you want to run it up the strip for a 1/4 mile polish away.................Marq

 

I must say there has been some INTERESTING things posted on this subject, a lot of which don't hold water or at best seem to the ramblings of uninformed people. I'll just leave it at that because a clear explanation of the factors involved would take more space than I have to comment.
I think, at this point, it would be best to say, check with your machinist and follow his recommendation.

Frank

 

Couldn't agree more. The only late model engines I know of that use a splash system are on lawnmowers. You really don't want your crankshaft running through the oil. It will churn it into foam and reduce the efficiency of the oil pump. Even if you did, the smoothness of a journal shouldn't affect the splash. The journals are covered by rods and rod caps.


As for polishing....also agree. I have ground hundreds of cranks, forged and cast, and to take a crank you have laying around and have it polished is throwing good money after bad. In other words, if it weren't polished after grinding, the process was not completed. If it is a used crank, it should be thoroughly and professionally checked against many different specs prior to use.

 

Speaking of cranks...

Is there a cheap source for some spray on crack checker? I dont remember the technical name... which makes it rather hard to source!

Sorry for hi-jacking...

Back to your previously scheduled crank polishing...

 

Magnaflux Spotcheck is the product you are looking for. OK for cylinder heads, especially aluminum. Wouldn't use on a crank.

 

I’m a long-time member of the Swedish working group of that is a part of the ISO committee dealing with international standard of surface texture (ISO TC/213 VG16 http://www.iso.ch/iso/en/stdsdevelo...tailPage.TechnicalCommitteeDetail?COMMID=4647 ), and I’ve also been involved in writing a few books about the subject of surface measuring so I guess I’m a specialist in this case...

Fingernails are not enough for examine the surface of a crankshaft!
To examine the surface correct you need to have a measuring equipment with an option to measure both waveness (W) and roughness (R) parameters. "Ra" is sometimes used as the only parameter for machine shops but the information given from "Ra" is not enough as it only gives average information about the surface.
A crankshaft bearing surface needs information from the following hybrid parameters: "Rk" that tells about the "peaks" of the surface, "Rvk" that tells about how the surface will keep an oil film and "Rmr" that tells how good the surface will hold up for wear. For some information lokk at http://www.taylor-hobson.com/faqsurfacedetail.asp?faqid=71

The Primary profile (P), just to filter if the waveness profile, (W) is an important part of the surface characteristic.

I wont go into what process that can deliver a certain surface, and for certain, I wont go into giving tolerances for the surface (that could get me into trouble). But check with your machine shop if they know these things: If they don’t, go to another shop. Car manufacturers sure know this stuff!

 

Crankshaft polishing and con rod polishing is a funky deal.
There is not point in polishing the crank unless you have it balanced.
Polishing only makes it weaker.
Same with connecting rods.
Normally when polished, rods are shot peened after to give them back sheer strength.
If you polish and don't balance it you're really decreasing the stress resistance.
Polish and balance it properly and you'll have a much smoother, more economical, and more reliable engine.
If you do it,
Go to a racing shop that knows what they are doing and pay the bucks.

 

Thanks RPW, being an engineer I understand the data and measurements presented on that website. I guess that is at the root of my question, assuming that the crankshaft journal surfaces are not out of tolerence and in need of a full grind, will polishing remove some of the peaks as shown in the following chart? I guess what I am looking for is a means to get reference level "C" on the Rmr(c) chart down to the bearing level.
I am not sure if there are any shops in this area that get into this detail but I will certainly ask, thanks for the info...

 

When I saw the subject line, I thought... well it depends on what crank and where on the crank.

I have a 4" Merc Flathead crank that Mike Bishop (av8) did some work on for me.
Ford did some very rough casting way back when; and cleaning up the casting flash will improve it's resistance to stress risers. (or breakage)

Now before anyone jumps on that statement; the crank IS polished, ALL OVER, not just on the journals. The leading edge of the counter weights were knife edged and the final product is about 8 pounds lighter. (Merc cranks were 68 pounds in stock form)

Now that more technical discusion has been added; (very good I might add) I should add that the journals were entrusted to an experienced auto machine shop. They did the journal grinding and if they did, polishing of the journals as well as radiusing the cheeks of the weights. The journal grinding/polishing was done after the counter weights had been cleaned/shaped/polished.

 

Thanks! I’m happy if I can help!
When measuring Rmr, make the first reference line a little bit down the highest peaks of the surface, normally that one is set 5-10% down from the highest peak (reference levels differs between manufacturers and manufacturing methods). The reason for the 5-10% is that the highest peaks wears down directly in the very moment the engine first is started.
The next reference level, the bearing line Rmr(c), is set a certain amount of microns (1/1000 mm, I’m all metric) down from the first level. Lets say 4-5 microns, that is quite common... (Notice: human hair diameter is about 40-50 microns; the grease of a fingerprint is about 2 microns).
Then the percentage of remaining material on that level is measured, for instance 60% of material on that level. That shows if the bearing will last!

An important thing to notice is that shown curves (taken from measuring equipment) are not in scale, the length is compressed about 200 times to make the peaks and valleys visible. This surface might be a polished surface!

About repolishing the bearings of a crankshaft: I’m not sure if that is a good choice. The roundness is also an important matter and that comes mainly from the previous step in the manufacturing process. A long time used crankshaft has lost some of its roundness ( during its life in the car and another polishing wont get that back. As a matter of fact, at my company its strictly forbidden to even touch a polished surface of a crankshaft. We use this kind of equipment when measuring roundness http://www.adcole.de/L_english/index.htm .


Well, its Saturday evening and I’m here talking about my work...
Send me an e-mail at Monday and I’ll give detailed information!

 

Is that why a crank that comes out of an engine that has had its oil changed often has a very polished look to it? Except for the lines from whatever got caught going through?

I noticed that mine was nearly a mirror when I tore it down... Guessing 60 something thousand miles on it.

 

Yes, thats right, it´s that easy: Surfaces rubbing each other do also polish each other. On the other half, if the bearing area is worn out it might look highly polished (to high percentage Rmr), but if no oil cavaties at all remains, the parts will not be lubricated. Then damages to the surface soon will appear and the components will break down... The "worn polished" surface might look perfect to the eye, very similar to the new polished surface, but probably it is not good at all for function! And even if an engine is well maintained, moving parts will wear from use. Sad but true.

One odd thing is that even professionals do make mistakes about this, making "better" surfaces than they should! Some manufacturers seems to think that a mirrorlike surface is perfect. Well, in an technical application it is not!
Several years ago I was involved as a specialist in an investigating a plane crash. The cause of the plane crash was that a mechanical part had too fine surface, suddenly the parts acted like gage blocks and an important part in the steering system did struck, causing it to crasch. And I´ve been involved in similar problems in many different technical fields. How about objects as different as F1 shock absorbers and sewing machines!

Some basic information about the material ratio curve (Abbot Firestone curve) can be found at http://www.siu.edu/~cafs/surface/file10.html

 

Heh, someone thinks I am mis-informed.I am ugly ,slow witted ,smell bad , and a very big pain in the arse. I take being mis-informed as a huge compliment!!

 

Your mentor was right of course. You should always polish after grinding. That is standard procedure to remove the burrs you created during grinding. The original post was talking about polishing cranks he had sitting around the shop. Different deal altogether.

 

I work for GM europe ,we have a reliability shop on sight .Now what i meant by splash was not where a crank relies on splash to lubricate the crank but where splash is used to help lubricate the cylinder bores and it is still used widely and no use you shooting me down i went in on saturday and asked one of the designers.Its not hearsay but fact!.Infact i'm told it is used more these days than in previous years due to tight tollerances agains longer periods between services and as oil viscocity is thinner these days throwing up a good amount of oil into the bores has very little effect on power loss as it may well have had with a 50 weight.Manufacturers design these things in to keep waranty claims to a minimum and thats why they don't like you messing with them.............Marq

 

Luton, huh? Used to live in Hitchin. Had an 8HP Morris Minor that would barely climb Offley Hill, so I would stop and cool me and the car off at a pub. Green Man if I remember properly.

 

shit it is a small world yeah offley hill is still there but the hitchin road is now a big bye pass you can reach well over 165 mph along if you try hard enough and me n the gsxr 750 have lol.Green man is still there and is still real pretty and the countryside is still there.I live on Cannon lane which is by the motor garage that sell chrysler pt's and the new funky sporty thing ,you know just past the old petrol stations that are no longer there and have been replaced by a big roundabout and the bye pass that goes through butterfield green.Well perhaps you didnt know all that just keeping you a little up to date......Marq

 

Thanks for the update. I enjoyed the mental image I got when reading your post. I loved it there. My wife of 37 years is from Hitchin, and I met her while stationed at RAF Chicksands in Shefford, Beds. Also lived 2 years in Essex and 10 years on the Suffolk/Norfolk border. Wish I was downing a pint of Brown and Bitter right now.

 

Glad you liked it,i have family in essex,some in loughton some in Brentford some in romford and some in basildon..........Marq oh p.s i go to the Hitchin market regular on a saturday it's still good fun especially the antique bit

 

Wow, there’s a pile of comments and ideas on this one, and I think some people are talking about different things. I read the original post to mean that he wanted to polish the rod or main bearing journals of the cranks, not the as-cast or as-forged surfaces. So my 2 cents is that: if the choice is to polish or not before running them, by all means, do it IF you know what you’re doing- it’s better than nothing. But if they were mine, I’d take them to a good machine shop, and have them check them out, make sure they were straight and round, and if they were, then think about polishing them.

Now, much more than 2 cents (and this is probably more technical than a lotta guys care about- sorry):

Regarding polisher’s comment "There is no point in polishing the crank unless you have it balanced. Polishing only makes it weaker. Same with connecting rods. Normally when polished, rods are shot peened after to give them back shear strength."

I think you’re talking about grinding/polishing off parting lines and stuff like that. It will remove material, which makes it a tiny bit weaker, but it also removes places where cracks are likely to develop, and there’s less inertia with a lighter crank, but probably only enough to make a difference if it was aggressively modified or knife-edged. And you are very right that it should be re-peened and then re-balanced afterwards. The shot-peening introduces residual compressive stress, which good stress, as it improves the FATIGUE strength of the material (which does help it resist any tensile stress- shear or otherwise. In the case of a conn rod, there is not much shear loading, but plenty of axial tensile loads, which is why shot peening helps them too).

Now, all this talk of using the crank to splash oil around inside the engine. I don’t believe it- otherwise, why would almost every high performance engine have a windage tray, which is supposed to make sure that the crank DOESN’T whip oil up out of the oil pan? Lots of windage (oil traveling with the crank) costs horsepower and hurts your oil consumption by driving oil back up through the rings or into the PCV vapors that are then burnt. If we're using modern OEM's as our examples, then in these days of fuel economy regulations, the less energy that is spent dragging oil around, the happier the OEM’s are. And longer service intervals are helped by having LESS windage, since less oil mist is flying around to be sucked up and burned by the PCV system. Furthermore, parting lines aren’t real consistent features- engineers wouldn’t count on them to provide lubrication. If anything, there’s a maximum allowable parting line spec on most crank forging/casting drawings. There’s some splash lube going on down there, but it’s not the main source unless you’re driving something that says “Toro” on the side of it.

Most cylinder wall lubrication (in engines that don’t have dedicated oil squirters in the block – like some Mercedes and diesels- or in the rods- like 60 degree Chrysler V-6's) comes from the oil that blasts out between the rod bearings and the rod journals. Oil is supplied there at something like 60-100 psi, and then the rotating motion of the journal squishes all that oil up to 1000+ psi, where it squirts out in the area between the side of the rod and either the rod beside it, or the crank counterweight. Journal surface finish has little effect on this.

Now, as for all the surface finish stuff. I can’t remember off the top of my head what specific surface callouts are on a crankshaft print (it’s been a couple years since I looked at one), but it’s more than just the Rk hybrid parameters. Those do apply to cranks, but are more important for cylinder bores and conn rod pin ends (if they run bushings/floating pins.) Those areas do not have pressure-fed oil delivered to them, and rely on the oil-carrying capability of the surface to avoid wear. In the case of the crank, there is that 1000 psi hydrodynamic cushion of oil in there between the metal and the bearing, and that operates in a very different way than a metal-on-metal contact situation where the surface needs a little more “tooth” to avoid wear (if you want even more surface finish nerdiness, check out that Taylor-Hobson site that RPW posted- those guys KNOW surface finish, and make the machines the OEM’s use in their engineering labs.)

A polish job is going to follow the shape of the crank journal as its ground. Like somebody said "if it weren't polished after grinding, the process was not completed.", and if there’s anything in that grind that’s not right, the polish isn’t going to fix it- it’s just going to make it look shinier- the bad contour will still be there. Like RPW said, you really need something like an Adcole to really KNOW what shape a journal is (some factory specs on journal shape are as tight as +/- 0.00008”!!!), but if you can see or feel an imperfection well enough to worry about a polish job, it’s probably worth taking it to a shop and having them check it all out, and grind or polish as they advise.

Which is what somebody else said, just in a hella lot less words.