The average big end clearance is what, .001 ?, not much oil is going through a .001 gap at 60 odd PSI.
Right, that's what keeps oil pressure and hydrodynamic lubrication up at low flow rates (low rpm/idle).
I agree that the hole makes sense, otherwise the BSA engineers wouldn't have bothered to change the design, but I'm not sure if it is a good idea to fit it on both sides.
Oil is fed to the big ends through the timing side bush, this being the first point to reduce the oil pressure, the second one being the rhs big end. The residual oil pressure has to be sufficient to enable proper lubrication of the lhs big end. Therefore I think they incorporated the hole on the lhs to increase flow to this point, which is farthest from the feed pump, reducing the pressure on the rhs bearing and improving lubrication on the lhs.
If you increase the flow rate on the rhs big end, you automatically reduce the residual oil pressure and therefore the feed rate to the lhs bearing. Although, with an oil pump in good nick, I would consider this discussion academic.
Yet I don't see the need for additional oil flow on the rhs? Even if the relief valve is closed, the camshaft gets the oil draining from the rockerbox (coming from the return line), flinging it up to the cylinders, and the timing side bush is the first bearing to get oil from the pump anyway.
Unfortunately I don't have enough long-time experience to give an answer to your sludge trap question, but I would also think that with a good oil filter and modern oils (which are designed to keep particles in suspension to deposit them in the filter), sludge build-up should be almost zero.
I'm sure someone will be able to tell us (Trev
).
Cheers, Markus