MASCOT April 1991

THE ROADSTER FRONT - The Oil Pressure Release Valve

Bill Haverly

Most Annual General Meetings are traditionally stiff-necked affairs where only dedicated members put in an appearance. But surprises can occur, and those who came to last September's ASCO AGM were given a rare treat in the middle of otherwise routine business. In the less than formal ASCO atmosphere, when discussion had somehow wandered on to the production of oil pump gears, John Horne entertained us all with an impromptu talk on the workings of an oil pump. It was totally unexpected and livened up the afternoon considerably. His main point was that renewing the gears alone was not enough to restore a pumps efficiency, as oil pump chamber, gear spindle housings and base plate had all taken their share of the wear. All needed to be replaced or repaired if a pump was to regain its former output. In other words, one part doesn't live in isolation from the rest of the assembly - and the best answer is inevitably a thorough and more expensive one!

Johns talk on oil pumps set me thinking about oil pressure generally, and the way in which the relationship of many parts in and around the engine contribute to the reading on the gauge, but in particular to the oil pressure release valve, which, like oil pump gears, is sometimes naively seen in isolation - in this case as an adjuster which can be screwed in further to magically raise the oil pressure. It will, of course, but only if incorrectly set to less than the recommended pressure. Screwing in the valve any more will only give an artificially high reading when the engine is cold. When the oil warns up and becomes thinner, the gauge will then reflect the real pressure, governed mainly by the bearing clearances in the oil pressure system and the oil pumps ability to deliver.

In operation the release valve bypasses oil from the main gallery to the main bearings back into the sump when pressure exceeds the setting of the valve assembly. This happens mainly before the engine warms up when the oil is thicker and reluctant to be forced into the bearings by the pump. The resultant back pressure releases the spring loaded valve. The same thing happens at times when high revs generate more pressure from the pump than the crank bearings can cope with and oil is again fed back into the sump.

Common problems with this release system are worn and leaky valve seatings and foreign particles which cause loss of pressure or the gauge needle to oscillate. Singers recommended settings for the release valve on the 9 Roadster are generally given as 30/35 lbs at 30 mph in top gear; and for the 4AD 30/35 lbs at 30-35 mph in top gear. Best done before the engine warms up and oil pressure starts to fall.

All Roadsters up to Engine no. A 1890S in 1949, Bantams with 1074cc engines and Ten Saloons were fitted with the simple but well proven ball bearing and spring pressure release valve. The 1939 Bantam Parts Book shows the diameter of the ball to be half an inch.

When Singers post-war SM 1500 Saloon arrived in 1949 both its new engine and the 9 Roadster unit were supplied with a plunger valve. Spring, adjuster, plug, locknut and washer remained as before. The same assembly continued to be used in the 4AD Roadster in 1951 as this car was also powered by the SM 1500 motor. The benefit of this new shouldered plunger valve was to give a bit more support to the spring, which, under the ball stem tended to buckle and chafe on the housing walls when the valve was in action - eventually resulting in a worn and weakened spring.

Plunger no. 1's drawback was that being made of a softer metal than the hardened ball bearing, more wear took place on its seating. Plunger no. 2 was a further development fitted in the slightly modified SM 1500 unit which served the Hunter Saloon between 1954-56. This 'half a ball on a shank' seems to be of a harder steel than its predecessor, though judging by the wear on those I've seen still not as durable as the ball bearing, and in appearance something of a compromise between the two earlier valves.

Standardization of parts between cars is always a good idea, and Singers saved on tooling by making all the components of the oil pressure release valve interchangeable between the 9 (1074cc), 10 and SM 1500 engines.

What's found in the engines is another matter. From my experience all sorts of incorrect fittings get added to cars and engines over years of different ownership, and it's naive to assume, as I once did, that what comes with your car is an original item.

Following a total rebuild of my 4AB engine back in '88, the oil pressure was very disappointing. After discovering blockages in the cooling system, things improved dramatically and restored some of my faith in the engines rebuilders. But pressure was still not up to expectations and attention turned to the pressure release valve. There I found, after checking the parts books, that an oversize ball had been fitted - and the spring was a foreigner as well. A correct ball now sits exactly on the narrow valve seat with a new spring in place, and oil pressure is far more stable than it has ever been.

Whether Singers developments on the oil pressure release valve were worthwhile is not for me to say, but it's a good subject for discussion among the engineers over a few pints in the pub. My feelings are that when wear on a plunger valve takes place, it's a whole lot easier to drop in a hardened ball valve as a replacement.

Pressure release valve kits consisting of ball bearing, original spec. spring and copper washer are available from me at £1.50 plus post.