The tappet or cam-follower is the barrel shaped item that runs on the camshaft ‘lobe’ and into which the push-rod fits. Like the majority of ‘new overhead-valve’ engines of the 1930s, the XPAG is a development of a previous side-valve engine. It is uncommon for completely new engines to surface; it is far cheaper to develop a current engine so that existing machinery can be used. The side-valve engine required something to fit between the rotating camshaft and its lobe, and the base of the valve stem. The ‘tappet’ carried out this function and on the better quality engines it had a threaded adjustment in it to set the correct valve clearances. Once the engine was converted to an ohv, the adjustment was fitted to the rocker above the push-rod. It was also very common for the designer to save money and leave the camshaft lobes and tappets to be lubricated by ‘splash-oil’. That simply means that he did not bother to feed oil under pressure to the area.
Designed into the lobe to tappet contact area was a simple system that caused the tappet to rotate. This would increase the wear area and extend the lobe and tappet’s life. To get the tappet (cam-follower) to rotate was done by simply running the tappet off the lobe slightly to one side of the lobe’s centre. That meant the tappet was being ‘wiped’ from underneath more to one side than the other, so it rotated a little. On a partly worn tappet this shows up as very faint scratched circles on the tappet’s face.
But within the tolerances of a mass-produced engine sometimes the cam lobe ended up directly under the centre of the tappet. The tappet went up and down OK but did not rotate. The same bit of the tappet was rubbed by the lobe every revolution of the camshaft. Eventually the bottom face of the tappet became concaved; it wore into a hollow. The very high stress of the tip of the cam rubbing the same area causes the surface heat-treated steel to begin to craze and crack. The cam lobe becomes very worn on its forward ‘ramp’ (the sides of the bump on the camshaft) and the cam and tappet become very noisy. Some owners try to adjust the valve clearances to cure the clatter. Alas it is not the ‘gap’ that causes the noise but the tappet literally taking-off like an aircraft from the tip of the cam lobe, then crashing down again onto it that produces the noise, so they are wasting their time. This accelerates the wear. More noise is produced by the tappet wearing its hole oval. As the cam rotates it forces the tappet to one side. Eventually the tappet not only goes up and down but sideways as well. You cannot adjust this clatter out either. The cure is to ream out the hole another ten-thou and fit oversize tappets (many BMC Gold Seal-exchange XPAG engines have over-size tappets).
You end up with a noisy clattery engine so that when you inspect the tappets lobe-face, it looks terrible. Add to this the fact that today the supplier of the tappets may not carry out the heat treatment properly, you end up with very short-life components. Also you need to accept that their life is not very long even in good condition as the lobe to tappet face is the highest stressed bit in the engine and it only gets the odd splash of oil.
Once the lobe wears it ruins the camshaft lobes, you find you need to adjust the gaps more often. Eventually you notice that one or two rockers seem to hardly move as they open their valve. The camshaft has been worn right through its case-hardening and soon there will not be any lobe at all. Remember, it was a mass-produced engine and not selectively assembled (blue-printed) and add to that after-market cams and tappets, some good and some not so good. Today a camshaft has very wide lobes running on a huge tappet face where as the 1930s standard narrow lobe and small diameter tappet meant shorter life.
As an aside, it is worthy of a mention that the rockers also eat their way up into the underside of the rocker-shaft very quickly. This adds to noise and the difficulty of getting an accurate gap and can be identified by loosening off the rocker adjustment and ‘rocking’ the rocker side to side’.
Time someone sold an ohc kit for our XPAGs? The only quiet XPAG is one that is not running. With today’s hydraulic cam-followers eliminating all tappet clatter you need to get use to vintage machinery. To hear a tappet is far safer than not to hear it. Silent gaps on XPAGs lead to burnt out valves, that 19 thou (or later 12 thou) is required to permit lubricant to get between the various wearing faces and is to suit the camshaft-lobe design. There are four ‘joints’ to your ohv so that is only just 5 or 3 thou each! No gap means no oil can get to where it is needed.
Article supplied by Neil Cairns