Fitting Gib Keys

This is certainly one of the more important topics regarding the Lister CS engines and other high mass flywheel engines that use the Gib Key as a fastener. We have all heard the stories of a ‘Run Away’ flywheel, and the damage to man, beast, or building they can do.

I remember having a a good conversation with an oil man that told me about a field engine that was missing a flywheel when they came to perform maintenance. Sure enough, they were able to follow the marks on the ground and found the wheel one mile away across open and flat country.

Of course losing a wheel is not the only danger of a poorly fitted Gib Key, flywheels have been broken in half with the root cause being an ill fitting key. Our DIYer community knows the importance of safety, and this is one important item.

With that said, I’ll turn this page over to Jack Belk, he’s certainly comfortable with this old design, but being a gun smith by trade, he’s got the background to explain it properly. My thanks to Jack for writing this up for us. – George B.

I Sure Learned Something Today!

By Jack Belk

I started (but got side tracked) to work on the bottom end of the new Power Solutions- Jkson kit engine this morning and wanted to be able to rotate it with a flywheel, so I had to make a gib key fit a *little* bit, anyway.

My *assumption* was that the Lister flywheels came as a right/left pair with tapered keyways in the wheel. At first glance it sure looked like they were not tapered, but after cleaning the gooey paint and assorted fibers out of the keyway and filing the paint off the face of the hub it’s clear the flywheel keyways ARE tapered.

The gib keys used in the Jkson kit engine are not standard to anything that I can find. British standard doesn’t call for a 9/16” key in any conformation.

SO, what follows is an amalgamation of information extrapolated from information gleaned from the 17ed. of The Machinery Handbook. On page 880, Table 3 it shows the ½” key having the same 5/16” height as the key supplied with the Jkson but having a width of .561 (nominal 9/16”) instead of .499-.501 for the 1/2” key.

My pair of keys were milled exactly right for width and are a sliding fit in hub and shaft. The thickness or height is .335″ at the head end tapering down to .305 at the tip.

I fit my keys with at least half inch of ‘pulling gap’ between the gib and the hub. Two keys took an hour to do right.

What follows is a crash course in using the “Expedient Milling Machine”, the common file, to fit steel parts accurately and easily.

I welcome any questions and if I can get a photographer lined up, I’ll get pictures, too. This is all two handed work.

With these keys, and many more parts that need fitted, it’s important to keep flat surfaces that are square with each other. There are ways of doing it without a milling machine or surface grinder, too.

You will need:

1) A small, flat, India or med aluminum oxide stone for de-burring and flattening. I like the combination sharpening stones by Norton. Six inch is a good size.

2) Degreaser. Acetone or spray brake clean is good.

3) Thin (.004″ or so), two sided carpet tape….looks like masking tape with a white paper cover.

4) A sharp scribe.

5) At least one of each of the following: 8 to 10 inch second cut file, smooth mill file, and smooth pillar file. Cheap ones are better for the time being. Indian surplus files are cheaper than the scrap iron they’re made from and will work for a long time.

6) At least a pair of HSS lathe bits at least 3/8″ square. Get ’em as big and as long as you can and rectangles are especially nice. When you see the use for them, you’ll agree.

7) A mounted bench vise in pretty good repair. The condition of the jaws makes no difference as long as you can get the old jaws out.. Be sure the vise is STABLE. There’s nothing like an avalanche of forgotten goodies from four shelves up on the fourth hard stroke of a big file!!

Set-up to cut STRAIGHT, FLAT, PARALLEL surfaces with a file

Use the stone and degreaser to flatten and deburr either the existing, smooth (rare) vise jaws, or remove the jaws and de-burr and degrease the socket. The socket more likely to be flat and square anyhow. Once both jaw sockets are smooth and burr free and *plumb* clean, (‘plumb’ is a Southern absolute), put a layer of two sided tape on both sides and trim the top flush with the vise.

Degrease a pair of tool bits and dry them. Holding them together and flat across the top, (you’ll curse yourself later but you *could* stick the bits to a magnet to hold them together right.). Now squeeze the lathe bits in the vise so that the top of the bits protrudes about .025 above the top of the vise and forms new ‘hard jaws’ being held to the vise with tape. Clamp in down one grunt tight when you *know* they’re straight and flush with each other and leave it alone over night.

Now open and close the jaws several times and learn to take the ‘slop’ out of the vise so that the jaws line up flat with each other every time. You might find it’s easier to take out the slop while setting the ‘jaws’ position the first time. It becomes second nature to nudge the movable jaw into alignment each time you tighten after a while. 🙂

Now you have a vise with flat, smooth, and HARD jaws that a file will scar up some but the jaws are the same hardness as the file and the file teeth are smaller so they lose the battle in the long run. Most solvents dissolve the glue in the tape and 200 degrees will loosen it too, otherwise it’s surprisingly hardy for holding the jaws. ‘Smooth and clean first’ is the key.

Fitting the key– Step One

Spray the sharpening stone with WD-40 or any light oil and stone the flat sides of the key. Look for bright edges or corners that contact the stone and gently stroke the part on the stone pressing straight down to maintain flatness with the stone. *Down* pressure should be about the same as de-bugging a windshield. Let the part find ‘flat’ for you. You’ll feel it. You know it’s right when you see the mill marks in the beginning of a mirror shine. All you want to do is make sure the surface is flat. Its’ already a bit too small. Degrease it and lay it aside.

Clean the keyway in the hub and the shaft and pick, scrape, file, and worry every last bit of paint, putty, swarf, scrap, grease, and crap out of both of them.

Use the EDGE of the mill (as in saw mill) file, which has a curved end just for this reason, to deburr and highlight the high spots in both hub and shaft keyways. Be sure to notice bad corners. When tools dull the corners are the first to go. If there’s a corner that’s *really* screwed up, don’t fret it. Just file the key to match, don’t try to repair a bad keyway with a file.

Break the sharp edges of the crankshaft keyway. All it takes is .010 to take away the terrible cuts to come if you don’t.

The edges of the crankshaft keyway is a natural for draw filing about four strokes to take away the razor edge with a flat angle that catches the light and look cool when running. IT’s also the very best way I know to do a self-amputation of digits with the OTHER sharp edge. BE CAREFUL!

Fitting the Key Step Two—

You don’t want the corners of your key to be what holds it in the hub or shaft. For this reason, file all the edges to a 45 deg. x .010″ or so chamfer. Just go on and get ’em out of the way so we can concentrate on the real fit.

Now the key is flat and smooth and the sharp corners are off. Will it slide in both keyways? It should, but just ‘slurpily so’. If it does, look very closely at the bottom of the key. Is it contacting all the way across or is it ‘bridged’ between bad corners? If bridged, draw file the full length of the key corners to relieve them enough to let the key ‘sit down’ in the keyway flat. Prussian Blue was made for this type of sliding fit fitting jobs. Think of the key as a racing slick not twin buggy wheels.

– “DRAW” filing is miss-named. You push the file to cut, not draw it to you.

It’s easy to get a ‘sway-back’ when drawfiling. Rub the part on the stone and it’ll tell you what’s high and low so it can be corrected before the part ‘dies’. J

Fitting the Key Step Three—

Mount the wheel on the shaft with the keyway at 12 O’clock. Remove ALL steel hammers from the room. J

Dry the key and the keyway (don’t panic, this is not for keeps) with degreaser…

Dry files allow the chips to fall from the teeth. A fingerprint has enough oil to let chips stick in the teeth and make DEEP gouges across the part being filed. Old timers rubbed chalk in their files, or dunked them in lime to keep them dry and ‘free’. Brush files often to remove chips. ONE chip can create more work…..!!

To keep my files dry I prefer a smudge pot to Prussian Blue for marking purposes. Both work well. Smudge pots are less hassle and is a one handed operation. J

Fire up the pot (diesel works but with aroma, kerosene is better) and wave the part until the shine of the steel is dimmed but not hidden. That soot layer is about .0003 inches thick…as in thirty millionths(!) thick.

Push the key into the keyway until it stops. Be careful to push the key straight down the keyway with the bottom flush. Pull it straight back out and examine what has hit the hub to stop forward motion.

Mine contacted harder on one side of the top than the other and the ‘snout’ hit before the butt end. That tells me the taper of the key is less than the hub, but there’s no way yet to tell how much, yet. The corner that was dragging only took three strokes of the draw file to take it down far enough to try again. I just lightly took enough from the ‘snout’ to see how much different the bore was from the key. A light ‘zing’ to just remove the bright patch and leave the soot around it unscathed is the goal. It takes skill to do it.

Re-smoke lightly and try again.

This time the contact on the snout was much broader and signs of scuffing was more than half way up the shank. The corner I filed down is still dark but smeared. The bottom is nearly shiny from solid sliding contact and the sides retain very little soot anywhere. Now it seems the top surface is geometrically correct and the angle of the key and hub are very close to the same, but the overall height of the key is too much so the key can’t seat deeply enough. We need the top surface to magically ‘sink’ a few thousandths but keep it’s flatness and relationship to the bottom surface.

Re insert the key and scribe a line on the sides flush with the face of the hub. That’s a reference to measure back to.

Step Four—

Place the key in the hard vise jaws and keep adjusting until you can feel a consistent .002 sticking up above the jaws. You can feel it with a fingernail, but barely. Now draw file the key until you feel and hear the ‘zing’ of the file on hardened steel. Use lighter and lighter strokes (to save file teeth) until the file no longer grabs soft steel anywhere up and down the key. De-burr the top corners, they’ll be razor sharp, and smoke the top and re-try the key. Again scribe a line at the level of the hub. Measure between the line to find out how much progress .002 made.

Check out the contact—is there a place that’s obviously tighter or binding quicker that others? *What* stopped forward progress of the key? Adjust accordingly.

Once the top is fitting to where there are scuffs evenly distributed across and up and down you can calculate how much steel off the top of the key it will take to sink the key to final depth. Be sure to measure four more times to be SURE, then take half of it and re-measure again.

To set a certain amount to file off the top of a key use a feeler gage laid beside the part on top of a vise jaw. A fingertip can usually feel a difference of .0005” between two parts IF the edges are sharp.

Step Five—
A tapered part fits in it’s socket with a ‘chunk’ sound. Several things affect the sound the parts make. The harder the socket and the taper are and the smoother they are and the steeper the angle of the taper is all determine the sound and feel of a taper seating correctly. This gib key will not be a solid thunk. The taper is too ‘long’ and the steels too soft. It’ll feel like a progressively tighter fit (like the tractor pulling sleds) over about 3/8 inch of travel, then stop. Figure on seating the key another 1/4 inch past thumb tight (make a mark) to hold the wheel for keeps but first it has to be prepared.

Figure out FIRST where the wheel will be set. Some are so close to the engine case removal of the cam cover is an exercise in Houdini magic tricks and bodily contortions to accomplish… inch past the bearing seal to give more work clearance won’t hurt anything, but figure it out first and mark it

I use a self-acting center punch for marking parts. It’s one handed operation and doesn’t cause grit and chips BUT the dimple has to be stoned out on sliding parts, like the crankshaft ends. That makes them hard to see. Make three marks side by side or in a triangle to make them more obvious. In this new kit engine I’m marking each part as its’ completed with a little (HJB) initial logo stamp I use for gun work. For a personal touch its’ hard to beat for about $25. (ENgraver Corp, no affiliation)

Scrub the shaft with solvent and steel wool. Remove any little flecks of rust or paint an any burrs there may be with a light stoning. DON’T use abrasive paper or cloths. The surface needs to be smooth and not have fresh ‘scarred’ metal. If you need abrasives to clean it, do so, but steel wool afterward to ‘deaden’ the surface.

Wipe dry, then dry again. Blow off all steel wool remnants . Apply chalk powder or lime with a dry rag and scrub the steel until it squeaks. DON’T touch it! Fingerprints are likely to pit the shaft and hub severely. Give the inside of the hub the same treatment. Don’t leave powder on the parts. Blow it off, don’t wipe. These parts are now oil and grease free. If not set together now they’re likely to rust over night. (seems a lot of trouble to go to, but this is how they did it when the original Listers were set up.)

Hoist the flywheel up and on the shaft and slide it to your mark. Give the key another wipe (you did scribe your name and date on the back of the key, right?) and set it in place to your mark with a brass bar and ball peen hammer. It’ll ring and let you know it’s there for the ‘duration’. I’m not going to attempt to tell you how hard to hit the bar, but it would have sure hurt if I’d missed.

Don’t worry about rust under the hub. It’s free of moisture and oxygen and sealed against both. That joint will likely be the last thing to rust on the entire engine. Now you can oil what you can still see to prevent the rust likely to come to whatever air can touch.

It’s a good idea to run the wheel by hand enough to know there’s no bad wobble and knock or weird feel or sounds. The fly wheel should *act* as a part of the shaft and sound like it.

I’m going to make key lockers for mine. The sledge hammer marks on Indian set keys makes me wonder if there’s another secret somewhere I haven’t thought of.

-Jack Belk

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