Camshaft clatter

Fate caught up with my ’37 Mk2 Ruby one late afternoon. A harsh machine gun-like rattle occurred at various revs and loads and sounded a bit like small end knock – but an Austin Seven doesn’t suffer with that problem especially when the pistons and small end clamping bolts are new. It didn’t sound like the centre main bearing; that too was new and I’d had previous experience of that noise. It was happening under load, so it was unlikely to be big end problems. With the engine out for a major inspection and investigation, the elusive problem was identified. With the block off, and the rods disconnected and removed, I heard the same metallic clatter as I rotated the camshaft timing gear. There was excessive end float somewhere on the camshaft. I considered the alternatives - removing the end float at the rear camshaft bearing seemed a possibility, using a wavy shim washer to take up the excess clearance or to put a shim of the right thickness to take up the clearance between the front camshaft bearing and the timing gear. The final consideration, and the one suggested by Woodrow in the Austin Seven Manual (page A247) is to lap the taper of the timing gear on the camshaft until there is a clearance of 0.002” between the timing gear face and the front face of the bronze bearing and that was the chosen method, because it used no extra components.

In lapping the two parts, there is minimal wear to the case hardened camshaft, but the timing gear bore, being softer becomes enlarged and the gap between gear and bearing reduces. Measuring the gap produced a staggering reading of 0.020”, TEN times what is required. No wonder the camshaft was rattling violently back and forth as the cams repeatedly bashed against the tappets, aided and abetted by the skewed timing gears.
Repeated rubbing down with first coarse and later fine valve grinding paste produced the fine grey finish to give the .002” clearance. Hold the camshaft between pieces of lead sheet to produce soft holding jaws in the vice, and remember when finally assembling the camshaft components to fit the woodruff key that holds the timing gear in place relative to the shaft. If you don’t, it will be probably be impossible to get the engine to run!

It’s not a difficult task, but a considerable amount of dismantling of the engine is necessary, so allow plenty of time to do the job. When reuniting the camshaft assembly with the crankcase it’s worth checking that the square headed screw that secures the bearing to the crankcase is a good fit because any wear between screw and bearing needs to be corrected or the rattle will still be there!

Bob Kneale
With thanks to the Cornwall A7C newsletter