Phil,
The small drilling in the front barrel is to aid lubrication of the upper side of the front piston. Due to the flinging action imparted by the crank, the oil gets thrown down into the front barrel and up into the back one. The small drilling does virtually nothing for the big end lubrication.
If it is assembled correctly, the lubrication system draws oil from the tank and meters it through the sight glass. From there it is piped to a non return valve on the top front of the crankcase. From there it runs into a gallery inside the top of the crankcase, and drips onto the revolving crankshaft - and spread around the engine, aided by another non return valve in the timing side of the crankshaft. This valve also provides an escape route for crankcase gases, as well as providing mist lubrication for the timing gear. The gases then escape from the timing chest via a pipe that is usually routed so that any remaining mist oils the primary chain. The valve in the timing side of the crankshaft is a ball and very light spring behind the timing pinion retaining bolt. With the timing cover removed, the valve should 'fart' as the engine is turned by hand - if it doesn't, the spring is probably too strong, and the build up of pressure is preventing the oil from draining to the engine. If you have just rebuilt the engine, it pays to open oil regulator and allow one pumpful of oil into the crankcase prior to starting. Any excess of oil will be discharged through the timing chest, onto the primary chain and eventually the road! Also, bear in mind that these 2¾ engines have a very crude oiling system, so bigend clearances need to be fairly slack to allow oil to get into them - 2 or 3 thou should be fine. Also, make sure the ends of the bigend shells are flush with the surfaces of the con rod and cap, so that the keep plates are sandwiched by both the shells and the halves of the conrods, otherwise the shells will move or the bigend bolts will eventually slacken and undo. I usually tin the outside of the shells, then fit them over a suitable sized end of bar, and fit the conrod halves without the keep plates, then heat the whole assembly so that the shells end up soft soldered into place - the ends can then be accurately trimmed flush with the joint face of the conrod. With the keep plates reinstated, the shells can then be scraped to obtain the correct running clearance on the crankpins.
Hope some of this helps,
Regards,
Eddie.