Tuesday, June 26, 2007

Crank Basics - II




(Be sure to read Crank Basics - I

http://www.runstarcn.com/2007/06/crank-basics-i.html )


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As you can see in the drawing, the journal for the #3 main bearing is adjacent to the flange for the throw of the #2 connecting rod. Since the #3 main bearing is a full-circle bearing rather than a pair of shells, it is trapped on the crankshaft by the cam’s driver-gear and the distributor’s scroll gear, both of which are a shrink-fit. Pulling the gears off the crankshaft is a basic maintenance chore. The procedure is shown in the following photos.

I’ll be using a portable screw-type puller specifically designed for this task, the first step of which is to remove the pulley, dynamo rotor or whatever happens to be on the pulley hub. The oil slinger and #4 main bearing are then slid off the nose of the crankshaft. You’ll need a pair of special pliers to remove the snap-ring.

The #3 bearing should be free to slide back & forth on its journal. Doing so will reveal a gap about a tenth of an inch wide between the bearing and the cam gear. I you use an hydraulic press, the press-plate must fit into this gap. See the previously posted drawing for dimensions.

Whatever method of removal you elect to use, you most protect the nose of the crankshaft. The usual method is to simply thread a pulley-bolt full depth so the head of the bolt contacts the crankshaft. Pulley-bolts come in a wide variety of lengths and head designs. You want to use the short type so that the stress will be transferred directly to the crankshaft and not to the threads.

With a suitable pulley-bolt in place, fit the puller into the gap under the cam gear and pull the gear using a suitable wrench.

When the gears are seriously tight you may have trouble holding the crankshaft whilst turning the puller. If you have a large vise with padded jaws (ie, lead or copper pads) you may find it handier to hold the crankshaft in the vise. If you plan on building more than a few engines you will probably make up a fixture to support the crankshaft. If you make it sturdily enough it will serve to hold the crankshaft while pulling the gears.

The first thing I do after removing the cam gear is to apply grease to its bore. This surface is usually free of oil. If left unprotected it’s liable to have developed a haze of rust by the time you are ready to re-assemble the crankshaft.

-R.S.Hoover

AV - Found in a Barn



More of a shed, actually. On top of a cabinet. Under a carton of magazines tossed up there about twenty years ago, now glued into a solid lump of water damaged paper.

It was a pad of fabric, at first unrecognizable as such. Like the magazines, it was glued together and I tossed it into the growing pile of trash. Later, loading the trash into garbage bags, a layer of the fabric came free revealing a brittle chunk of masking tape and the figure ‘15' that triggered a flood of memories.

It was the summer of 1968. I’d recently returned from Vietnam and was helping a friend recover his Cub. We were using Grade ‘A’ cotton because he’d been given a whole roll of the stuff. We were working in one of the old wooden hangars at Brown Field, under a bit of pressure because the space was borrowed, as were most of our tools.

He’d never done any fabric-work and my skills were stale at best but we got the job done, right through the final coats of Cub Yellow, a glistening black lightning bolt and twelve-inch hull number.

There was some fabric left over and Witt gave it to me. I rolled it around the tube and hauled it home. A couple of years later I took it to the local EAA swap-meet, tagged with the length: ‘15'. But no one wanted fifteen feet of Grade ‘A’ cotton and I hauled hit home again.

I forget how the tube got crushed, bent in the middle. I unrolled the fabric, folded it up and... put it somewhere.

The Cub is long gone, off to where ever old Cubs go. So too is my friend, his death in the mid-west discovered by accident long after the fact, too late for cards or condolences, remembered only as a laughing, smiling fellow Chief Petty Officer with whom I’d shared a couple of tours of duty. And a passion for flying.

I stood there with the hunk of fabric in my hand, the garbage bag waiting. But you can’t throw something like that away.

I washed the fabric carefully in cold water with just a dash of soap. It was pretty bad. The mildew had eaten holes and imparted a leopard-like pattern of stains. But enough of it remains to make a shirt or two; something good enough to wear while working around airplanes. And remembering old friends.

-R.S.Hoover
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