Oil Pressure

Before looking too deeply into oil pressures it should be noted that the dash board gauge is not a precision instrument and the reading it gives should be considered more as an indication that an exact measure.   It is worth noting that in 1965 Rileys gave the following advice to an owner who was worried about low oil pressure on his 2½ car at tickover speeds.

"Oil pressure readings on the 2½ litre engine at tickover can vary from 2-3 lbs. psi  to approximately 10 lb. psi.  The lower readings may not register on the gauge fitted in the facia and it is advisable to check the actual tickover pressure with a master gauge.  Providing nominal pressure is registered, then you should not cause any damage to the engine by running it in that condition.  Once the engine speed is increased the oil  pressure should rise until it is reading approximately 40 lb. psi at 40 m.p.h. in top gear."

 The Workshop Manual and the Driver's Handbooks give the following information with all readings being taken with the engine at normal working temperature:

RMA/RME Engine

12 psi at tickover
19 psi at 15 mph in top
22 psi at 20 mph in top
26 psi at 30 mph in top
30 psi at 40 mph
35 psi at 50 mph in top
oil pressure relief valve opens at 50 psi

RMB Engine

12 psi at tickover
40 psi at 50 mph in top
oil pressure relief valve opens at 60 psi

RMB2 Engine

12 psi at tickover
30 psi at 50 mph
oil pressure relief valve opens at 50 psi

In his book, Riley Maintenance Manual 1930 -1956, S.V. Haddleton gives the oil pressure for all 2½ litre cars as 30/40 psi and comments that 30 psi at 40 mph is a good pressure. For 1½ litre cars he gives an oil pressure of 40 psi.

All engine have a by-pass valve for the oil filter which opens if a high pressure is detected within the oil filter due to the filter becoming block. This allows unfiltered oil to circulate around the engine and will affect the oil pressure.

The oil pressure at tickover usually reveals most about the condition of an engine. A pressure which is so low that it barely registers on the gauge might be due to an inaccurate gauge as noted by Riley or it might be due to a worn engine. It could be due to a piece of dirt trapped between the oil relief valve and it seating. This will prevent the valve from closing properly giving a low pressure at lower speeds. It might also be due to an oil pump problem and to check this remove one of the rocker box covers and check if there is a good drip of oil from the rockers. If there is plenty of oil at the rockers and the pressure rises quickly with increasing engine speed there is nothing to worry about.

In contrast, if the tickover oil pressure is high and the bearings and the pump have not been rebuilt recently there is plenty to worry about. This is almost certainly a sign that the oil ways in the crank shaft are blocked with hardened sludge. This will prevent the oil reaching the bearing surfaces and soon lead to the dreaded big end bearing failure. Cleaning out the oil ways in the big ends involves dropping the sump and unscrewing the blanking plugs in the big ends. Unfortunately these are usually very difficult to unscrew and often have to be drilled out. Drilling out cannot be done with the crank shaft in place so the engine will have to be removed from the chassis and stripped down.

Low oil pressure at running speeds is usually easily cured by cleaning and adjusting the oil relief valve. A neglected and blocked oil filter can give a low pressure as oil by-passes the filter.


The Oil Relief Valve

The oil relief valve on all 1½ litre engines is on the right hand side of the block and can be adjusted easily and removed for cleaning. The seat itself can be pulled out with a bolt after a thread has been tapped through it. This is rarely necesary as it can be cleaned with a piece of rag over the end of a pencil or similar rod.

On RMB engines it is on the left hand side behind the exhaust front pipe and is again easily cleaned and adjusted although the presence of a hot exhaust pipe can be a problem.





On the RMB2 engine as fitted to the last of the RMBs and the RMF, the oil relief valve is part of the oil pump and this is inside the sump. It is designed to be non-adjustable but the operating pressure can be raised by putting a spacer such as a washer behind the spring.

Some Thoughts

Apart from the above figures there is very little informtion about oil pressure although there are many reports of engines running well and for a long time on very low pressure. It is generally believed that the oil pressure registered on the gauge has little impact on the oil pressure at the bearing surfaces. To look at this in more detail I undertook an exercise in 1985 to try to model the oil flow in the bearing mathematically. The analysis and results are shown here. I would not recommend ploughing through the sums and since I had to make assumptions and simplifications I do not offer the results as being accurate but the overall trends are interesting and may be near to the truth.

To sumarise the results:

1. Provided there is enough oil being provided to the bearings to keep them topped up the pressure at which it is supplied to the bearings is not significant. All the calculations in the analysis assume that this pressure is 0 psi.
2. The pressure generated within the bearing due to its rotation is measured in hundreds of psi which is very much greater than the pressure shown on the oil gauge provided enough oil is supplied to the bearing  to it to keep it topped up.
3. The minimum oil film thickness in the bearing varies little with the nominal bearing clearance i.e. bearing wear makes little difference.
4. Lower loads give greater minimum oil film thicknesses.
5. Larger bearing clearances give greater oil loss from the sides of the bearing but the temperature rise in the bearing is less.
6. The optimum radial bearing clearance is 0.003 to 0.004 in.               

Overall Conclusions.

I would hesitate to draw too detailed conculsions from the results knowing the assumption which have been made but assuming the results are somewhere near the truth then it seems that larger clearances than those recommended in the Manual are acceptable and indeed beneficial in terms of lower oil temperatures and less power consumed by the bearings but at the calculated optimum bearing clearances the RMB oil pump may have little spare capacity and this may show as a reduction in the recorded oil pressure. The pressure developed by the oil pump is much less than the pressure generated in the bearings by their rotation so provided the oil pump has the capacity to keep the bearings topped up the recorded oil pressure probably has little significance. The minimum oil film thickness increases with RPM and decreases with load suggesting that it is better to climb a hill in a lower gear at higher RMP than to slog up a hill in a high gear. Finally, since larger clearances give  lower temperatures and less power loss, a well worn engine showing perhaps a low oil pressure may run sweeter than a newer engine.