Thanks jack , i haven't built the engine yet , just working on the various bits and want to get it right first time and not have to rework anything, i wonder does the oil passing through the tappet block holes cause an obstruction to air passing down into the crankcase ? I will plumb in a couple of small hosetails in the rocker box caps and link them up to the main crankcase breather.Bon, the return side of the pump is larger,that is true, but only slightly,and it dumps into your oil tank with only enough restriction to back some of the oil up to the rockers.I've had clear line on a rocker feed before and never saw enough air in the line to say it was significant. The pressure in your rockerboxes is Crankcase Pressure. It is the same pressure everywhere inside the motor that is connected to the c/c. Relieve it one place, it is relieved everywhere. The notion that air from the return side of the oil pump is causing leaks at your rockerboxes is IMHO wrong. Jack
Gravity..How do you think the oil from the rockerboxes gets back down to the sump?
Thanks for the helpful replies everyone. Jack is there a specific valve i should be looking out for ? A commonly available one from a car or truck or machine or something ?Bon, Haven't built the engine yet? That changes everything, if you want to do it right, research one-way valves. If plumbed correctly,you can get neg. c/c pressure and that equals less leaks and more power.It may not be much in the power increase,but every bit helps and it's "free". Jack