Turning a crankshaft

[Jerry B]

I need to turn the mains on a hit & miss engine, I am going to try it my self, How do they get it so slick is there a secret to it Jerry B

[Cory Engines]

Jerry, first you need a crankshaft grinder. For the "slick" fininsh. Machinist call it AR. The finnish can only be gotten with a grinding stone from a crankshaft grinder. Even a small machine will be about 5000 lbs, this is to keep the work from
moving around when turning all that crank mass. There is a time to take it to a machine shop, this is one of the times.
It will be cheaper in the long run.
Good luck, Todd Cory

[Craig A]

A file finish will work just fine.
Don't press hard and don't turn the work too fast to avoid galling the surface.

[Eric M.]

Somebody please correct me if I am wrong, but would fine emery paper give a comparable finish?

[Andrew Mackey]

If the crank is galled, and there are small raised areas of metal. a fine jewelers file will take down the raised area. 1,000 grit emery should give you a smooth surface. If the whole journal is scarred and needs to be turned undersized, then a crank grinding shop is your best bet. The journal has to be true (round), and not tapered. It is not a job that can be readily be done at home.
Andrew

[OTTO-Sawyer]

Another thing that hasn't been mentioned yet is the need for a counterbalance weight to offset the weight of the contecting rod throw on the crankshaft. Without it, even running a lathe at very low speed it will be vibrating badly.

If you have a tool post grinder on your lathe you can mount the crankshaft between centers (or on an indicated chuck & tailstock center) and slowly turn it by hand while grinding, and get by without a counterbalance weight. Cover the ways to protect them from the grinding grit, and take light passes on it to avoid heat buildup. I would only grind one (or turn one) if it was out of round and/or tapered and needed to be true'd up. If it is straight and round, but has nicks in it from a rod bolt or something else hitting it all you need to do is 'knock down' the high spots with a file and then either take an oil stone or wrap the finest wet/dry sand paper/polishing cloth you can get around the file and use it to smooth the edges.

A raised edge around a nick or gouge will carve out a bearing just like the teeth on a reamer, but the bearing will never see the low spots (as long as there aren't too many of them). Small divits are no different than the crossholes drilled in a modern crankshaft with pressurised oiling systems. There's nothing in the hole to tear up the bearing as long as you remove the raised adge around it.

IF you FILE IT, slowly rotate the crank towards you while filing away from you, if that makes any sence the way I worded it.... If you don't, you risk the chance of filing a flat spot in the crank. Keep the file moving over the surface with very light pressure, You will feel it every time it catches on the high spot as it goes over it. When you don't feel it hit any more switch to the oil stone or wrap the file with emery cloth or the finest wet/dry sand paper you can find and polish it.

I cleaned up almost 2 feet of a 4 1/4 inch diameter crankshaft end on a 12HP Fairbanks Morse that way that looked like it had been used as an anvil for many years. I mic'd it in several locations and made a plug gage the exact diameter that I used to check a water pump drive pulley that I bored out to fit the engine, and then used the newly bored pulley to check my progress when filing the crankshaft. Slow, time consuming process cleaning up hundreds of hammer dings, but in the end the crankshaft was still at its original diameter and the pulley slipped on freely with less than .0015 clearance. If you've never tried this before, practice on an old piece of shaft before doing your crank. Paint up the shaft with machinist bluing, or even a magic marker so you can see where the file touches, and try to avoid removing the bluing while knocking down the high spots. I've been doing this for over 35 years, and with a good oil stone I can get a 3-7 finish on a profilometer where most bearings are ground to either 16 or 32 max. finish... Seal diameters are generally in the 10-20 range (which the engineers at the factory I used to work in didn't know why that was until I explained it to them that if you get a seal too smooth it will leak.... you have to have a certain amount of 'roughness' on a seal diameter just the same as you need to hone cylinders to certain roughness so the rings seat properly) (Too many engineers out there have their 'book smarts' but haven't got a clue what they're doing if you ask them a question about something)


And just a sidenote, warning.... If you do take it to a shop to have it ground, check them out first before leaving your crankshaft with them. I took a set of heads to a shop for a valve job 25 years ago (before I had a valve grinder) that used to have a good reputaion, but I didn't know that it had changed hands recently and the new owners didn't know what they were doing. They belt sanded my valve stems and made them look like rat tail files with all the heavy scratch marks in them. They told me that's how they do all the valves to clean the varnish off of them ! I had already polished them up on a wire wheel before I took the heads in to them.

[OTTO-Sawyer]

Originally Posted by Eric M.

Somebody please correct me if I am wrong, but would fine emery paper give a comparable finish?

Emery paper will give a good finish, but needs to be 'backed up' with something so it 'cuts' smooth & straight. Finger pressure is too uneven and would leave high & low ridges or grooves. Kind of like wet sanding a paintjob on a car, you need to back up the paper with a paint stirrer stick or something else to get a flat smooth finish.

I generally just loop the emery paper over the end of the file and then use it the same way I would the file without the paper on it. Use light strokes the entire length of the file to avoid buildup that would re-mar the surface.

[Jeff in PA]

If you hold the sandpaper/emery cloth in your hands (not on a file ), DO NOT wrap it around any fingers or your thumb if you are power sanding ( having the part spin )

If you put too much pressure on the cloth/paper it will grab and may suck your hand into the turning part.

A better way to hold the paper is to grab each end with only your thumb and forefinger of each hand. That way if it does grab, it simply pulls the abrasive cloth out of your hand.

[OTTO-Sawyer]

Very True.... I forgot to mention that option, wrapping the cloth around the part.

While that will blend in better in some cases, it won't get a shaft as 'dead straight' as backing it up with a file would, as it will conform to the shape and remove the same amount of material accross the width of the strip. Backing up with a file or even a piece of hard wood, will make it hit the high spots more than the rest of the shaft. But wrapping a strip around it Does allow you to blend smoothly into the radius or fillet without leaving steps in it. If you've already turned the shaft straight in a lathe, then either method will work OK. If using the file and/or file/paper method pay attention to what you're doing and don't get distracted if you're holding it anywhere near the throw of the crank ! You don't want it hitting and throwing it back in your face or chest. Use common sence and good safety practices no matter what you're working on. If you're not comfortable with it, Don't do it, just because someone else does.

Use a long enough piece of emery cloth to get your hands safely away from any moving parts that may hit you, and as I mentioned with the file/paper method, keep it moving to avoid loading up the grit in one location with metal filings that will end up re-gouging or scratching the shaft.

The only thing I would use finger pressure on would be steel wool or a fine scotch brite pad for super fine polishing where you're not really removing any material. And then be very careful when doing so.