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The "PUSH" Side of the Chain

(thanks to Don Mallinson for showing me his cam and helping me think this through) - new 9/18/01

After the Peoria Ford Show Don Mallinson showed me a sample of a failed cam sprocket. Damn I used to be able to see, I had to take off my glasses and put the shaft on the end of my nose to get an eagle eye.

We are both trying to understand what the mode of failure is. As you may know the sprocket is slid over the cam tube, then the tube is expanded and holds the sprocket in an interference fit. The inside of the sprocket has splines (illustrated as lines) that run left-right in this view. They help prevent the sprocket from turning on the shaft (which is direction of primary load) but do not prevent the sprocket from moving left or right.

The shaft of the cam is turned down slightly in five places which become bearings that run on plain bearing surfaces machined out of the head. The bearing caps have an oiling groove. The bottom bearing surface has a very narrow "waist" to accommodate the valve shims and valve springs. Don & I both took one look and simultaneously concluded: "good engine to run synthetic oil of the recommended weight". These bearings will continuously weep oil at a high rate given they neck down to almost no width at the bottom. Of course the caps hold the cam down compressing the valve springs so in fairness the bottom half of the "no bearing" – bearing has very light loads.

On both heads the rear most cam is driven by a long chain driven by the crankshaft and then this master cam is harnessed to a slave cam by these secondary cam sprockets. All four cams share the same sprocket design and all four of the cam sprockets been known to fail.

You may notice the center of the sprocket is not centered on its hub that mates to tube. If you think about this, a chain only works under tension, It CAN'T push. So the load on the sprocket is not balanced. In this illustration the top of the sprocket is being pulled out and the bottom is on the slack side of the chain. This puts a side load on the sprocket – in a direction the splines can not resist.

It is easy to see in this drawing but hell to notice in the flesh. The tube has a series of grooves (not threads) between the secondary sprocket and what I suspect is a thrust surface. The grooves have a common minor diameter but the top of the tube has a taper as you can see from the drawing. What I am told is the dealership tried and tried to start the car after the owner dropped it off. The car ran on one bank while one camshaft was frozen. The sprocket slipped and cut the taper after it initially failed. That would not be important except it indicated the mode of failure. The asymmetrical sprocket with the asymmetrical load puts a side load on the splines in an axis they can not resist. Maybe some of the material is the wrong hardness, maybe an industrial adhesive was used, we could not find any trace of it. 

We have reports of 19 sprocket failures (in a group of less than 1000 SHO owners) so far out of a "bad batch" of 30.How is it the one 99 and all the 96 SHO with the same failure are in the same batch of 30?

In truth I suspect what we have is a failure prone design and not a bad batch. I am getting my sprockets welded ASAP. It is as simple as this, there IS NO push side of a chain the load is not symmetrical. If the sprocket was symmetrical over the hub it may last a lot longer but it doesn’t.

Happy motoring,
And thanks for coming along for the ride,

Timothy "Buford T. Justice" Wright

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