May I add a few thoughts?
1. Nearly all modded TR engines have excessively heavy valve springs fitted.
Some you could use as road springs. Remember most engines are only going to
around 6500rpm, not the 9000rpm these springs will cope with. (OK - it's
diffferent for 1300 Spitfires). In a TR-4, you will marvel at the way your
skew gear disappears as the camshaft bends under load, especially if you are
unlucky enough to have one of those uprated alloy oil pumps on board. The
6-cylinder just sheds its lobes, not helped by certain cam profiles which lift
too fast for a 0.8"follower and run off the side of it... We see engines where
the valve cotters are belting the tops of the valve guides, and guess what,
the cam people get the blame again for a failure.
2. We now grind cams on chillcast iron blanks, and for some years have used a
modified Hillman Avenger follower, both on original hardenable and chillcast
cams. Diameter is as original, material is also chillcast.
Sorry, don't know the hardness, but I'll find out.The diameter really should
be as large as possible in order to use fast accelerating profiles. I use a
large follower in the TR-6 race engines for this reason - it's not too hard to
bore out the block to suit. I have also had a large batch of OE TR-6 followers
made and nitrided, but NOT shortened or mucked about with. These go nicely
with any cam.
3. A loose crank grind gives more oil throw-off onto cam & followers - it's
their only source of lubrication.
4. Follow break-in procedure rigorously and don't ever use thin oil.
Returning to my first point: the greatest proportion of failures I have seen
has been due to overspringing.
You should be able to turn a freshly built motor, plugs out, with no more than
30ft lb. on the crank nose. That's my rule of thumb, and I'm sticking to it.
Jon Wood
The British Lobe Office
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