CLEAN ENGINES (published The Javelin 1994 Volume 37 Number 2)
Jowett engines don’t have to be filthy. In fact they can be made to look healthy and happy with surprisingly little effort. I can’t get over the fact that an engine with twin carburettors coupled with aluminium crankcase, rocker covers and timing cover, add to this extractor type manifolds, can look so BAD!!
Let’s have a look at one of those areas we can easily address and, in a few weeks, I will comment on other ways to make your engine more presentable at those displays. We will look at oil leaks first.
Well, yes. Part of the problem IS oil leaks which may be difficult to stop. There is no substitute for correct engine assembly in the first place, and there is a very wide range of rubber impregnated cork, neoprene, nitrile and silicone, based gasket materials to make life easier than a few years ago. However, combined with these, certain areas benefit from modification of the original, without sacrificing originality to any degree.
Lets now look at rocker covers. The casting finish is generally mediocre, the cylinder head mating face bows with age (and over tightening) and the special rubber grommet at the securing nut distorts. This means you can’t get enough pressure on the self locking nuts to effectively seal the rocker cover gasket. Of course the grommets are so badly squashed that they can’t be reliably used again, and the rubber distortion from under the flat washer, definitely looks unsightly. Particularly on Jupiters where they are the first thing you look at.
Firstly, why not polish the rocker covers? It won’t cost you an arm and a leg to do, and keeping them clean and polished is a real pleasure.
Secondly, toss out the special grommets (that is if you ever use them) and get your friendly neighbour with a hobby lathe to turn you up some stainless steel washers similar to those in the drawing at the end of this article. (Not to scale).
An “0” ring under each special washer, chrome (or zinc) plated Nyloc nuts, with some new round section gasket material completes the job. Now you can tighten the rocker covers sufficiently to obtain an oil tight seal at the cylinder head mating surfaces, and not have to worry about the seal under the Nyloc nuts.
FROM THE JOWETTEER – AN ARTICLE ON VALVE SPRINGS (published The Javelin 1991 Vol 34 Number 2)
The Javelin and Jupiter engines have one valve spring inside another. One reason for this could be that the efficient opening and closing of the valves requires that strength of spring combination. The much talked about ‘Valve bounce at high engine revs” would be one problem if the valve springs did not keep the tappets on the camshaft, snapping them shut and keeping them there as required. Doug Hoyle has sent me a very dramatic picture of a valve where the groove into which the collets fit had failed due to “excessive dynamic forces”.
Put more simply, if the valve train (tappets, push rods, rocker arms, valve springs and valves) is not in good condition, then the valves will hammer against their seatings and fail. In the picture the valve head has come off, beaten up like a rotten mushroom. It doesn’t show the rest of the engine. Efficient valve springs must keep the valve closely following the camshaft or such things will happen. A pre-occupation with valve bounce can be misleading, because this sort of of damage clearly can occur below the danger level where true valve bounce should occur.
True valve bounce is caused by the spring vibrating in such a way that at the point when the valve closes, the coils of the spring are surging in the opposite direction, thus reducing the effectiveness of the spring and allowing the valve to “bounce” off the seating.
Various owners strongly defend their practice of omitting the inner valve spring, largely on the grounds that the outside one is strong enough and it is only a waste of power compressing the extra spring. There have certainly been cases where disastrous valve failure has been associated with omitting the inner valve spring. You might well say why not simply use a stronger valve spring? I have never heard the view that Jowetts only used two springs because it was the practice at the time, maybe but there were good reasons for it.
Firstly, there is the obvious one, if a valve spring breaks you still have the other to prevent catastrophe. Unlikely but not a bad idea.
Secondly, springs of different diameters will surge at different revs and hence reduce the possibility of valve bounce.
Thirdly, there is the fact that the inner and outer valve springs are coiled in different directions. Now I have to admit that I have based this on the engine on my garage floor. I do net know if this has original Jowett valve springs, but they certainly come from an early engine with standard bores and bearings. I know of no reference to this in the Jowett literature nor can I find it mentioned in any motor engineering books I have. The point is that if you compress a coil spring the ends tend to rotate in relation to each other. This produces a slight grinding action of the valve on the seating. That is unless the valve collets rotate in the collar. At one end valve wear takes place (eg. with unleaded petrol) or at the other the collets are worn and the valve drops into the cylinder. Fitting springs coiled in opposite directions will reduce this tendency to rotate the valves. However, I have to say here that I would like some comments!
Harry Brierley – Jowett Car Club (UK)