Oil pressure on OHC Velo’s

Oil pressure on OHC Velo’s.

The scope of this article is limited to reasons why an engine might not be generating oil pressure and to offering suggestions how this might be rectified before a deeper investigation that would require the engine to be dismantled.

When Percy Goodman designed the oil pump that was fitted to the 1925 K, its performance was so good that it remained in use and unchanged for the duration of OHC engine production, including all racing models.

I believe that it was this innovation that made a K the first modern motorcycle. By that, I mean that the rider turned on the petrol, started the engine and rode away. No priming, drip feed or sight glasses to keep an eye on. Its capability was proven the following year when it lubricated the engine that won the Junior TT.

I think that in the ‘high pressure’ installation, its capacity far exceeds what is required for excellent lubrication and it was more than capable of meeting the higher flow rates of the post Mkv KTT racing engines 

The slight problem with the early ‘open rocker’ engines was to balance supplying enough oil to the top end without too much escaping. I should add that with everything in good order, these engines are remarkably oil-tight.

We should bear in mind that any OHC engine was made between 101 and 75 years ago and quite a lot might have happened to it since. Thus, it is critical to know how things should be and how they actually are on the engine in question.

Over the years, I have encountered an array of problems, some of them very surprising, that have convinced me of  the need to check everything.

The problems under discussion are typically on rebuilt engines that have never established oil pressure. This is a good place to start :

Here is a photo of a gauge to check the distance of the ball seat from the spot face on the crank case. It comprises a ¼” ball attached to a rod with tape to mark the depth of the oil pressure valve drilling. The photo includes the screw, lock nut, spring and ¼” ball. The spring is 1.375” long, .1875” outside diameter with 20 coils of .O24” wire.

This is the drawing for the pressure adjusting screw.

There is a second ball and spring valve in play whose sole purpose is to prevent oil draining into the crank case when the engine is stopped. They are located in the hollow timing side main shaft and retained by the left hand threaded cap nut. This spring is much lighter because it only needs to resist the weight of oil in the tank. When the engine starts, oil pressure in the lower bevel housing lifts the ball off its seat to feed oil to the big end. I once made a test rig and found that the ball lifted at 2-3 lb psi, a considerable margin below the operating pressure in the system. 

The instruction books provided by Veloce recommend disconnecting the oil drain from from the upper works of the engine to check that oil oil is being delivered. If it’s reaching the cam box, the big end is also being fed.

Once it is established that all of the essentials are in good order, it is well to understand how the system works before any further steps are taken.

Oil is delivered to the ‘feed’ side of the pump by a gallery in the crankcase, tapped ¼” BSP. Parallel to this gallery is a second one with a cross drilling to unite them. The second gallery is tapped ⅛” BSP and it is here that the pressure regulating screw with a lock nut is fitted. The screw acts upon the spring and a ¼” steel ball. Held in position by spring pressure, the ball occludes a small hole into the lower bevel box.

Photo showing the oil feed to the left and the oil pressure adjusting screw to the right.

Oil enters the bottom of the pump and is carried by the gears until it is disgorged through the rectangular hole in the pump end plate, directly into the lower bevel box. Pressure builds until it is sufficient to lift the ball off its seat, thus setting the running pressure. The pressure is adjusted by increasing or decreasing spring pressure on the ball. Over-pressure oil is returned to the feed side of the pump via the small hole.

Pressurised oil is fed through the hollow main shaft to the big end and up through the vertical drive shaft tube to the upper bevel box and thence to the cams, from where it is scavenged to the crank case.

Oil gathered in the sump is returned to the oil tank by the scavenge gears of the oil pump.

An adequate quantity of oil must be supplied to the pump, the original pipes were ½” bore and though the crank case fitting represents a restriction, the large bore pipe holds a sufficient volume to resist cavitation at the pump. It is essential that there is sufficient oil in the tank to cover the strainer and to provide a ‘head’. If the oil level falls below the top of the gauze oil strainer, it’s possible that air can be drawn in, reducing oil flow. Note : early engines 1925-8 had a ⅛” BSP tapping in the crankcase, with a smaller bore than the ¼” BSP fittings later used. Care should be taken to eliminate any unnecessary constriction.

Note : some bikes will have had the oil tap at the oil tank replaced by an anti-sumping ball valve assembly that was used on spring frame push-rod models. Care must be taken to ensure that the feed line to the pump is fully primed with oil or the ball will not lift off the seat and no oil will be delivered to the pump.

With a pressure gauge installed at the upper bevel box, the ball, spring and screw can be installed. A good starting point is to have two to three threads showing at the locking nut. With a good pump, an incremental gain of about two pounds per revolution can be expected, once the bevel boxes are full and pressure is apparent. It takes time for sufficient oil to return to the sump to be scavenged back to the oil tank. Even so, the flow will appear intermittent with a mixture of oil and air because the scavenge gears are greater capacity than the feed gears. 10 lb PSI is a good pressure that will circulate sufficient oil.

If satisfactory results are not obtained, there are a number of possibilities that range from damage to incorrect assembly of other parts that might require the engine to be dismantled to investigate. These include :

Oil retaining shim on the timing side main bearing absent or damaged.

Cavitation at the oil pump

Insufficient oil in the tank

Damage to the ball seat

Inefficient or damaged oil pump

Poorly fitting oil pump – damaged crank case.

Ball and spring in the main shaft missing.

The most puzzling problem that I have found was a cross-drilling out of position, so that the screw occluded the drilling as pressure was applied to the spring. Pressure started to build and then died away, the solution was to turn some threads off to allow oil to pass.

If the ball valve assembly is all in good order and oil pressure is not achieved, there is an interim step that can be taken before stripping the engine. Removing the outer and inner timing cover will expose the lower bevels and the oil pump. With the oil pipes still connected, a anti clockwise rotation of the driving ‘tongue’ should cause oil to be discharged from the rectangular hole in the cover. If the rotation is smooth and with some resistance, a simple tool can be made to rotate the pump with an electric drill. Blocking the hole with a finger should generate pressure. While it’s not possible to measure the pressure, it can be a reasonable indicator that the pump is working. If not, it must be removed, stripped and inspected but under no circumstance must this be attempted in situ. The engine must be removed from the frame and stripped.

I have written another article to deal with oil pressure bench testing at the point of engine assembly. The article includes the option to install a lipped seal to prevent oil draining into the sump, eradicating taps in the oil feed line. It can be found on page 3 in the ‘Articles’ section of the website.