Morris Minor Owners Club

my 1098 engine has a side breather and valve cover breather is there any advantage to fitting a breather type oil cap as well

No -you will just get an oily mess on the rocker cover -and nasty carcinogenic smells in the cabin....

Martin,

The 'breather' oil filler cap is used on cars with the closed circuit breathing systems. For 'in-line' engines these were the later 1098 engines in Minors, A35's, Midgets etc., and all 1275 in Midgets.

so what is a closed circuit system

The later 1098 engines have it. There is an oil separator on the front tappet chest cover, no vent connection on the rocker, a PCV (Positive Crankcase Ventilation) valve in the inlet manifold and a vented oil filler cap.

The system works by the PCV valve creating a partial vacuum, and sucking in the fumes from the crankcase through the oil separator. Fresh air is drawn in through the oil filler cap, and a slight negative pressure is maintained in the crankcase, which makes sure that any fumes are kept within the engine and drawn into the inlet manifold, as well as helping to keep the oil the right side of the rear crankshaft scroll.

thank you

And it is by far the best system - most 'moderns' have something similar - although not with a vented oil cap!