Sunday, October 19, 2008

MRI, Clearancing the Camshaft and Me


The x-ray mentioned in my last blog entry revealed nothing amiss. The pain is still there as is a monstrous swelling the size of a grapefruit that is its apparent source. But the swelling is all soft tissue; it offered nothing for the x-rays to 'see.' So I pulled up my pants and went home, accompanied by my lump and the pain. The next step was to schedule me for an MRI scan, which underwent on Friday.

MRI stands for Magnetic Resonance Imaging and takes advantage of two fundamental laws of physics. The first is the fact that every atom has a unique resonant frequency. The second is that fact that if you disrupt the local magnetic field, each atom will generate a small but distinct signal. The MRI machine provides a powerful magnet to disrupt the local magnetic field and a sensitive receiver to pick-up the signal given off by those disturbed atoms. (Yeah, I know... but I'm not talking to you, I'm talking to folks who don't have EE after their names.) That signal is fed to a computer and the computer puts the signals together to form an image. Of course, that image is only for a single slice of the target. To come up with a picture that makes sense you'll have to run the thing down the length of whatever you are trying to make an image of, then have the computer put all those 'slices' together.

The big advantage of MRI over basic x-rays is that everything generates a 'signal' of some sort. The signal from good tissue will be slightly different than the signal from bad tissue. That means you can now 'see' soft tissue, whereas the x-ray would blast right through them and not 'see' a thing.

I'm a ham radio operator (KA6HZF) and have a strong interest in stuff like MRI machines, which were invented only a few years ago, relatively speaking (they're about three years older than the Personal Computer, for example). Imagine a sewer pipe about as large as your shoulders. Rig a pair of runners on each side of the interior of the sewer pipe. The runners support a narrow table, allowing it to be rolled into the sewer pipe. You lay upon the table and get shoved into the sewer pipe, your shoulders touching the sides, your nose scant inches from the plastic surface. If you are even moderately claustrophobic, this is not the place to be.

An enormous coil is built around the sewer pipe and an equally enormous magnet spins around the outside of the sewer pipe. The magnet will cause anything within its field to give off a faint but distinct signal which will be picked up by the coil and fed to a computer. The computer performs a bit of mathematical magic on the signal and aligns that slice with the previous slice so that when you are done the slices will form a coherent image. The nice thing here is that the image will show soft tissue as well as bones and the odd bits of shrapnel.

This past Friday I reported to the Keeper of the Keys and she sent back to where Bill, ex-Air Force but a pretty good guy in spite of it, laid me down on a narrow bed and shoved me into the sewer pipe. Hit the big red button and the MRI machine commenced to hum and groan... okay, the groans were mine but the machine did hum a bit.

It took less than an hour for the MRI machine to slice me into electronic ribbons which were recorded by the local computer and stored. These would be processed by a more powerful computer which would combine them into an image. The image would be sent to the physicians and they would try to figure out what was causing the bulge on my hip.

So whats all this got to do with camshafts, fer crysakes!

Well... nothing, really. Except the camshaft in the picture is going to be installed in an engine fitted with a crankshaft having a throw of 41mm (stock is only 34.5mm). That means the connecting rods are sticking out 6.5mm farther than on a stock engine. And that means they are going to hit the camshaft.

Six and a half millimeters is 0.2559" - a full quarter of an inch. In the stock engine the connecting rods miss hitting the camshaft by about an eighth of an inch. In the big-bore stroker the connecting rods are about an eighth of an inch too long. So we have to grind away about an eighth of an inch to give them clearance. In fact, that's what this operation is called: Clearancing. It applies to the crankcase as well as the cam shaft.

The tricky bit is knowing where to grind away, and how much to grind away. In the case of the cam shaft, if you'll click on the image it will give you a blown-up view, allowing you to see where I've ground away metal from cam shaft. You can also get some idea of how much metal I've removed. If this is the first time you've clearanced an engine, odds are you'll go a bit too far. What you need is a gauge, something to tell you when you've gone far enough. (Remember, the camshaft must be strong enough to withstand the torque needed to open the valves. If you grind away too much metal... or leave a sharp edge that will allow a crack to get started, the camshaft will break.) Fortunately, you already have a gauge. It is the assembled crankshaft. That is, the crankshaft with all four rods installed. With the camshaft properly meshed with the crank (ie, with the dots properly aligned), rotate the crankshaft and observe the position of the rods as they swing past the camshaft. You want about sixty thou of clearance. Any more and you'll just weaken the camshaft whereas any less and you're liable to have a collision when the engine heats up.

-Bob Hoover
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