The Engine

Coventry Climax engine  - Curing oil leaking into Bellhousing. 

Having built three Coventry Climax engines, all of them eventually suffered oil escaping from the engine into the clutch bell housing. They all had the original Coventry Climax seal i.e. a "Screw" thread on the crankshaft theoretically pushing the oil back along the crankshaft into the sump. This did not work.

Modern technology and engineering has provided a solution:

  • SKF now manufacture "Recovery" tube for shafts, which are worn in the oil seal area. These are hardened tubes about 5 thou thick with a flange attached for knocking onto the shaft.

Here is the unit:

The item No. 99267 covers a shaft diameter of 2.747" to 2.753" i.e. the Coventry Climax crank dimension at the "Oil Thread".

Here are the instructions of how to fit it:

The recovery tube is about 1 ½" long – only 5/8" is needed for the Coventry Climax engine. I made a mandrill 2.75" diameter and slipped the tube onto it, and used the edge/corner of a small triangular file to partially cut a groove into the tube. I then filled the screw thread with epoxy resin, hammered on the tube into its correct location. I let it harden off overnight and used the side cutters to remove the excess length of tube.

Ring fitted and reduced in length 1

The flywheel fits onto the crank by about 1/8" so I did bolt on the flywheel whilst the epoxy was still wet to get the ring positioned correctly.

I purchased an oil seal 2.75" ID x 3.346" OD (85mm) and used my lathe to machine the hole in the bell housing to the correct diameter. I mounted the bell housing on the tool post and used my old 'Trepanning" tool to cut the enlarged diameter hole. Perfect tool since you can adjust the cutting diameter by a threaded shaft – ¼ turn = 0.8mm increase in diameter of hole.

40 RPM, with oil, perfect cut. The "disc spacer at the rear keeps the bell housing vertical.

Here is the shaft "Tube":

Oil seal same place. All in parts cost £38.00 delivered.


Onto the next problem, replacing the three-fingered clutch with a Borg & Beck diaphragm unit with carbon pressure release bearing.

Loss of oil pressure

Not looking at the oil pressure gauge, I failed to notice zero oil pressure. The car was leaving a trail of oil which I didn't discover until the return journey. The cork/rubber gasket on the oil pump had fractured, no doubt spurting out oil at 50 psi. I stripped the pump, found the broken split between the high pressure port to the fixing bolt hole and then to the outside AND it looks like a bearing has picked up since I can't rotate the crankshaft a full 360 degrees. I took the plugs out and TWO of the plugs came out WET!! The head gasket had been leaking as well. Further stripping out revealed two wet cylinders - not a pretty sight.

After a days further stripping out - some of it not necessary as it turned out - I found the REAL problem. Here is an oil pump skew gear which decided not to rotate when the camshaft drive was rotating? I can't say why as the oil pump now turns freely and looks OK. So camshaft back and rotates freely so I will change to a spare oil pump and re-assemble complete engine.

My final thoughts are that there were three problems only identified when the engine came out for detailed inspection.

  1. The head gasket had been leaking water for some time into No.3 cylinder and a smaller amount into No.4
  2. The blow out of the oil pump gasket and loss of all oil happened next.
  3. To correct this I took off the oil pump plus external filter modification, made a new gasket and refitted the pump/filter.

I later noticed that the extended oil pump drive shaft was bent! (at the Woodruff keyway). I had to cut it to get it out of the pump body. Two hours later and a new shaft fitted. When I bolted up the pump it seized the camshaft. Just tight on the nuts and all rotates OK but full tightness and it VERY hard to turn the camshaft. Try as I might I could not resolve this problem so I fitted a spare standard oil pump and all turns OK. There must be mis-alignment somewhere which binds the shaft in the cylinder block inner bearing. I fitted this external oil filter after I had built the engine but before startup so could not/did not check its free rotation!

It does illustrate that attention to minor faults/occurrences can help prevent major problems.

Radiator repairs

Having consulted AJS experts upon how to solder a radiator without creating more leaks than you repair, I attempted the repair today. Not bad result since the drain plug boss was completely loose in the hole and the steel framework which holds the rad onto the car had also broken all the solder joints. With wet rag placed onto all the header tank to core joints I initially heated up the radiator with a blowlamp, then used a large old copper soldering iron to flow the solder into the various joints. The secret is to get the joint clean and the solder flows easily. Testing with 2psi indicated no leaks so onto cleaning off the old chrome on the read surround.

I was later told that plumbers stick solder and a mole skin wiping cloth would have made life a little easier.

General Specifications - Engine

The car was fitted with a four-cylinder side valve Coventry Climax engine with a capacity of 1018cc (60mm x 90mm bore and stroke) and an RAC rating of 8.92 h.p. which put it in the £9. 0s. 0d tax class.

  • 9 h.p. 4 Cylinder. Three bearing crankshaft.
  • Dimensions - 60 m.m. Bore - 90 m.m. Stroke - 1,018 c.c.
  • RAC Rating - 8.92 h.p. Tax £9.
  • Brake H.P. - 13 B.H.P at 1,500 R.P.M, 17 B.H.P at 2,000 R.P.M, 24 B.H.P at 3,000 R.P.M.
  • Ignition - Lucas coil and distributor. Firing order 1,3,4,2.
  • Cylinder head - cast iron, detachable.
  • Piston - Special design of lightweight alloy.
  • Crankshaft - S.26 high tensile steel stamping, dynamically balanced (three bearings)
  • Crankshaft bearings - High pressure cast white metal linings in gun metal shells
  • Connecting Rods - S.26 high tensile steel stampings with 3 per cent nickel steel bolts and high pressure cast white metal big end bearings integral with conrod.
  • Valves - Silico Chrome heat treated steel, arranged side by side.
  • Camshaft - S.14 steel hardened and ground with mushroom head tappets, driven by double adjustable chain.
  • Lubrication - Pressure feed to main bearings and connecting rods by means of gear pump.
  • Oil sump - Capacity, 6 pints. 

From the AJS Parts List - Oiling diagram

From the AJS Parts list - Oil pressure, Oil pump & Suction Pipe & Decarbonising

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Click here to go to "Clutch & Gearbox". 

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