The Stag oil pump and oil pressure

The engine has a wet sump. British Leyland's service training notes describe oil being drawn from the sump through a wire filter to the pump and its pressure relief valve, then through an external full flow filter to the main gallery, which feeds the bearings and moving parts. The camshafts are fed intermittently from the idler shaft front bearing, and drain holes in the heads and the idler shaft tunnel return oil to the sump.

That camshaft feed matters. A Stag club technical article points out that because the camshafts are fed from oil that has passed the bearings, badly worn bearings starve the top end and the engine becomes noisy quickly.

The training notes describe an external pump secured to the crankcase by four bolts, sealed to the block by an O ring, and driven by the idler shaft through the distributor drive. Inside are an inner and an outer rotor. The relief valve is part of the pump base plate and consists of the valve, its spring, a spring location plug with an O ring, and a split pin that retains it.

The notes give these overhaul limits:

• Rotor end float in the body: not more than 0.004 in (0.10 mm).
• Outer rotor to body clearance: not more than 0.008 in (0.20 mm).
• Inner to outer rotor clearance: not more than 0.010 in (0.25 mm).
• The end plate must be flat and free of scores, and the chamfer on the outer rotor goes innermost.

The archive has found a factory source only for the rotor type pump. It has no source for a vane type pump on the Stag and makes no claim about one.

Replacement and uprated pumps have been sold by Stag suppliers, according to a Stag club technical article. The same article, written from an owner's experience, argues that a relief valve that sticks or does not seat fully in its housing causes much of the low hot idle pressure owners worry about, and that some engines have been rebuilt without curing it. The archive has not verified the specification of any current pump, so ask your supplier what the pump is and check the relief valve when the pump is off.

The factory figures are:

• Running pressure: approximately 40 psi (2.8 kg/cm²) at 1,000 rpm, from the Repair Operation Manual.
• Relief valve opens: approximately 50 psi (3.6 kg/cm²).
• Warning light goes out: 3 to 5 lb/in² (0.21 to 0.35 kg/cm²), from the General Specification Data.

The service training notes give 50 to 55 lb/in² at correct running temperature, with no engine speed stated. The copy consulted prints the imperial figure unclearly; its metric figure, 3.5 to 3.8 kg/cm², equates to about 50 to 54 psi. Both factory sources are given here rather than one chosen.

Owner readings collected by a Stag club put a warm idle at about 15 to 20 psi and a hot idle at about 10 to 15 psi. These are owner readings, not factory limits. A worrying reading is one that is well below these on a warm engine, one that has fallen noticeably from what your engine used to show, or any reading with a knock.

The full specification table is on the specifications page.

Because the light goes out at only 3 to 5 lb/in², and owners report hot idle pressures not far above that, a light that flickers at hot idle and goes out as soon as the revs rise can be a healthy engine. The way to know is to fit a mechanical gauge temporarily in place of the switch and read it hot at idle and at 1,000 rpm. The fault finder entry takes you through the test.

The standard car has a warning light worked by a pressure switch, not a gauge. To test a light that never comes on, disconnect the wire at the switch and touch it to clean metal with the ignition on: if the lamp lights, the bulb and wiring are good and the switch is at fault.

A gauge is an owner addition. A Stag club article notes that gauges are not always accurate and that pressure is set more by the relief valve than by the bearings, so read a gauge as a trend on your own engine rather than against a single number.

• The oil light comes on and stays on while driving: stop as soon as it is safe and switch off.
• Any knocking from the bottom of the engine: switch off and have the car recovered.
• The light does not come on with the ignition on: do not run the engine until it works.
• The oil level is low: top up before starting, then find out where it went.