Greg,
Oil pressure is important! Yes, it is true that much of the engine is
lubricated by splash, but the bits that REALLY matter, your bearings, have oil
supplied to them from the oil gallery at the full gallery pressure. If the
pressure drops, you don't get enough oil to the bearings, the oil film will
get too thin and your bearing will wear rapidly. This is not good. I should
know, I analyse engine bearing perfomance for a living!
It sounds like your pressure is too low once your engine gets warm. This is as
a result of oil viscosity dropping as the oil gets hotter. If you are using
new-ish, reasonable quality oil and your pressure is dropping like that then
there is a problem with your engine. Likely causes are either/or of your oil
pump being worn, and not pumping effectively, or your bearings being worn, so
there is too large a gap between the journal (the bearing section of the
crankshaft) and the bearing shell. I have not done this job on a Spit, so I
don't know exactly what's involved, but I reckon you might as well replace
bearing shells and the oil pump at the same time. You may need your
crankshaft re-ground to match new bearing shells.
I'm not sure what the details of the camshaft lubrication in the Spit engine
are, but it is possible that worn bearings there could also contribute to low
oil pressure.
Better get a quote on the engine work - I have no idea what it would cost in
time or money. However, it is reasonably true that the only parts you are
doing any serious damage to are the bits that would be replaced/re-ground
anyway, so delaying for a short while shouldn't cause too much problem.
Sorry not to have better news,
Richard & Daffy
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