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Re: Crank advice

Totally agree Ian. I have to say I have never run a big end on a splash fed engine and I've given them more stick than most.
What is not generally understood is the actual mechanics? of big end lubrication. The oil from the jet, or pressure supplied oil is not what creates oil pressure in the bearing - that is done by the relative movement between the crank pin and con rod. This is of the order of several hundred if not thousands of lbs per sq inch. As long as there's an adequate oil supply the bearing will create its own pressure. In the case of an oil jet it's easy to see that is what happens. With pressure feed, you need about 10psi to overcome the centrifugal forces acting on the oil in the crank oil ways. If the pressure is t high enough you can stall the oil flow and still show pressure on the gauge. That's why you need pressure, otherwise just volume would suffice.

Re: Crank advice

Correction That 10psi should read 10psi per 1000 revs.

Re: Crank advice

As a matter of interest, what oilway configuration is adopted within the b.e. for racing with jet feed? Are the drillings in the conrod retained? Is a trench across the top of the bearing as original mid 1930s retained? Is a pocket provided at the dividing line?
Long ago, after reading of the bad effect of interruptions in bearings where heavily loaded, I dispensed with both drillings and trench. Seemed OK to medium rpm. However the oil spray must be very dispersed and probably considerable enters via the drillings.
Has anyone fitted a window to a scrap crankcase and attempted to observe the spray?

The flycut machining of the original crank pockets seems curious. I wondered if it was to avoid a trapped cushion of air. But the success of Phoenix style pockets and the Villiers mod to a Bugatti seems to dispel that notion.

Location: Auckland, NZ

Re: Crank advice

Don't bother fitting a window in the crankcase in an attempt to observe oil spray, you won't see anything Bob. Some of the early Doxford marine engines with a top speed of about 120 RPM had windows in the crankcase with internal lights and you could see what was going on at 30 RPM. However there was so much oil flying around at 50 RPM you couldn't see anything.