Displaying items by tag: 420

Ben Stafford had another problem with his 420. There was generally very poor oil pressure of only 20 to 25 PSI when hot and cruising on the open road that dropped back to almost 0 at idle. The engine did not sound "clapped out" and the oil consumption was quite reasonable. The electric oil pressure gauge was suspected but substitution of a hard line gauge only confirmed that the electric gauge was telling a true story.

I suggested that before he did any thing drastic like an engine overhaul he should check the condition of the oil pressure relief valve. I have had two occasions in Jaguars where low oil pressure was traced to relief valve problems.

Ben found that the oil pressure relief valve spring had been bent. This resulted in the valve itself sitting at an angle on the relief hole and a groove had been worn into the face of the valve. He used a lathe to clean up the face of the valve and subsisted a straight spring for the bent one

The results were fairly dramatic with an improvement to 45 PSI on the open road and 15-20 PSI at idle when hot. In his own summation of the situation he "was surprised that such a small gap in the relief valve had led to such a dramatic loss of oil pressure."

Whither goes thy oil pressure? (continued)

MY Mk2 3.8 Auto has had a slightly low oil pressure and I was a little concerned that the engine may have been "loose" i.e. getting worn. I took a trip to Ben Stafford's to use his hoist and get easy access to the pressure relief valve.

Upon removing the relief valve it became immediately obvious that the valve face was not seating properly. A quick trip in the lathe to face off the valve resulted in an increase of 10 pounds per square inch over all of the operating range.

However I now believe the real problem lies in wear on the upper flutes of the valve which allows it to sit in the bore of the valve body at an angle and the final solution will be to replace the valve itself with a new part and possibly even re-sleeve the valve body.

I have been intrigued by an on going problem in the 1960s model Jaguars such as MK 2, S type, E type, 420 etc. The main area of concern is the oil pressure gauge. This is an electric device that was apparently Smiths first foray into the domain of electrical transmitted oil pressure. Apparently the Jag Enthusiasts Club in England has also been following up on this problem.

To say that the Smiths electric oil pressure gauge is an accurate instrument would be taking the truth very lightly. It is one of the most Heath Robinson devices I have ever encountered in any instrumentation. Normally an instrument transmitter contains a device a bit like the variable resistor used in volume controls in a radio. These are generically referred to as "potentiometers". As the temperature or pressure changes the variable resistor feeds more or less voltage to the gauge in your dash panel to make the indication. Simple enough?

However Mr. Smith put a whole new twist on the scene by using a system in his oil pressure transmitter that works as follows. The flexible diaphragm in the transmitter has an electrical contact on the diaphragm. When oil pressure expands the diaphragm this contact connects to another contact on the end of a bi-metallic strip. This makes a circuit through a heating wire wound around the strip which heats up until the bi-metallic strip bends and breaks contact with points on the diaphragm.

The bi-metallic strip then starts to cool down and straightens out until contact is made again and the process repeats. The more oil pressure there is the longer it takes for the strip to heat up and therefore the more electric current runs through the system. This current is measured and shown on your oil pressure gauge as pounds per square inch!

The bi-metallic strip system is well known and is used in the majority of "flasher" units in directional indicators in motorcars. However they do not last forever and eventually they fail due typically the points burning out. It is an electro/mechanical device and as such will never be a stable source of information for a gauge system.

Why I am making this point is that an old oil pressure transmitter can give quite erroneous readings. One club member was really quite concerned with his oil pressure in an XK series motor in a MK2, which only ever got to about 40 PSI cold and was flat out making 20 PSI when hot. The engine was generally in good order and was not rattling or giving any typical signs of distress associated with clapped out Jag engines. On my suggestion he obtained a direct reading after market gauge and coupled it up in place of the electric system and was delighted to find that all of the above pressures were just about doubled.

I have also run across this problem and have had widely divergent readings from different transmitters. In fact I trust the older Bowden Tube direct reading oil pressure gauge used in early Jags like the MK5, 7, 8 and MK 1 any day against one of these later electrical devices.

The good news is that a company by the name of CAREBONT in England have purchased the rights and tooling for early Smiths gauges and have redesigned the oil pressure gauge for these 1960s Jags. This kit apparently consists of a completely new transmitter and gauge using modern techniques but looks just like the original. It is not cheap at around 70 pounds or about A$200 [Australian] dollars but the current price for a new oil gauge transmitter to fit the old system is A$130 so it would be worthwhile going the extra dollars for something new, reliable and accurate.