Oil Pump Wont Prime.

Oil Priming a New Twin Cam Engine. You, me, we’ve all been there at some point –

Picture this, you’ve just completed a complete twink rebuild, the engine-gearbox has been re-installed back into the elan chassis, the body is back on the chassis, everything has been reconnected. You’ve arrived at the point where the oil pump and oil galleries need to be primed. The bearings and camshafts require a good oil feed during the first twenty to thirty seconds after start-up.

Alas, the engine is cranked and turned over with the plugs out for far longer than it should be, but the oil pump fails to lift and deliver that precious oil lubrication into the engine galleries and bearings, so, you decide to start and run the engine for just a short amount of time…. 20 to 30 seconds max, NO MORE, if you happened to use a top quality engine assembly lube, its OK to run the engine at low revs for just a very short time, without oil pressure – BUT be VERY, Very Careful.

But still the pump fails to lift and deliver oil flow to the engine.

Do you…

• Become irritated?
• Frustrated?
• Or not sure what to do next?

You should sit down and have a think about it for a minute or three, but the answer is pretty simple.

There’s basically only two or three things that will prevent an oil pump from working:

• Excessive clearance between the inner surfaces of the inner rotor and the outer annulus, and the pump body housing.
• The pump wont create vacuum – caused mostly by the pump sucking air… and just the slightest ingress of air will prevent the pump from making vacuum. (Creation of pump vacuum = atmospheric pressure that forces oil from the sump up into the oil pump)
• The “O” ring seal under the end plate of the oil pump has been known to compress with age and leak air on the Hobourn Eaton oil pumps.
• The drive pin in the pump drive gear has failed?
• On very rare occasions, the cast iron end cap can also develop a fine hairline crack which will allow air into the pump cavity, sometimes invisible to the naked eye.

If you enjoy a game of mechanical fault finding, a dicky oil pump can lead you an a merry dance…. and probably the main reason why most people throw out what at first seems to be a faulty pump, and buy a new item – (Hopefully a fault free pump) But like many mechanical items, the oil pump can be maintained to provide a very long life, if you have the engineering skills and machining equipment.

The First Fifteen or Twenty Seconds Of Engine Life!
One of the most nail biting instances of building a new twin cam engine is when the moment arrives to  fire it up, and you hope and pray that the oil pump is correctly primed and it will actually lift and pump  oil in those first ten or fifteen seconds of the new engines life.

You might have heard someone exclaim… “The Oil Pump wont suck oil up!” Well sorry, the oil pump doesn’t suck… it is the atmospheric pressure that forces the oil from the sump up the pick up pipe and into the pump… BUT, that will occur only if the pump can make vacuum, it is correctly primed, and only then will it deliver oil, at a predetermined pressure, the amount of lubrication required.

That was scenario faced by the writer during the days following the completion of the 1700cc twink engine rebuild… the frustration rose to a crescendo when the pump failed to lift oil… no matter how much careful pump packing and oil gallery priming was carried out.

The pump was also removed twice to check things out – (The pump removal is not that difficult in the writers S2)

On very close inspection, and I mean really close, as in using a magnifying glass to go over the pump body and the cast iron end plate. (This is an early oil pump where the end plate is held on with four short bolts.)

You would understand that the early pumps are of the cartridge type, but this particular pump has been converted to a later spin on type oil filter… but that’s another story in itself.

The first problem discovered was with the end plate “O” ring seal and the groove, the groove depth measured 3.22mm the “O” ring OD measured 3.19mm. Well, wouldn’t you know, it wasn’t hard to work out that was not going to work, was it?

There is every chance that’s where the air was coming in. Mind you, this occurred over a weekend… which meant there was no time to go get a thicker “O” ring from the local bearing shop.

The next best “over the weekend” solution was to machine a very thin .08mm steel packing washer to lift the “O” ring so that it would actually compress and create an airtight seal against the cast iron end plate.

The second discovery was that of a very fine hair line crack across the corner of the cast iron end plate… that was not a good sign either. Luckily, we have spare oil pumps, so a donor plate was had, but it also had signs of wear and scoring, so that was made like new again by face machining in the lathe… then reinstalled onto the existing oil pump.

When the machining work was complete, the end plate was made smooth with fine emery paper, the pump was assembled and re-installed, the rotors were once again packed with a mixture of Penrite Assembly Lube, light grease, (Vaseline works well) and gear oil, all of the voids in the pump were carefully filled.

Then the end plate was affixed to the pump body, the slightly raised “O” ring made all the difference to the sealing of the end plate, the compressing of the “O” ring could be felt while fastening the bolts, but the plate still compressed fully to the pump body to provide correct end clearance for the internal pump rotors.

So all things being equal, we were looking for a better outcome second time around.

Now for the big test, but there was still that lingering doubt, when the engine fired up, will the oil pump lift and finally deliver oil?

The custom made oil priming adapter was used to prime galleries and the bearings, and as luck had it, when the engine fired, oil pressure indicated on the gauge within a few seconds, and at about 1800-2500 RPM it was 60PSI, and once the engine was warm, at idle speed, the gauge indicated 40 psi, which is supposed to be normal twin cam oil pressure at revs! wow, what an oil pump we have

You might be interested to know that this particular standard twin cam oil pump has covered 80,000 – 90,000 Kms, but has been carefully overhauled, the internal clearances are still less than 003″, and the way we achieved these clearances with an old oil pump is another story again, and is certainly not a common practice.

This could be a story for another day. The very old Hobourn Eaton oil pump, employed on tens of thousands of small Ford engines, including the Ford Lotus Twin cam engine.

The photo on the left- The old canister style – The photo on the right is the same oil pump which has been converted to spin on cartridge, the pump also has the takeoff/return boss to feed oil through the oil cooler. A bit of re-engineering required to accomplish the conversion, but it works, this oil pump has lubricated two twin cam engines over a period of forty years.