RREC home page

20 hp Gear box oil leakage – a cure!

by David Else


In November 1992 I bought GKM30, a 1928 20 hp Binder bodied saloon from the Real Car Company. During the next seven winters my wife completely re-upholstered the car while I gave the mechanics a total overhaul. I noticed a continual loss of oil from the gear box. In the winter of 1998 I carried out a major overhaul of the engine and gear box. I found nothing wrong with the gear box and apart from cleaning and renewing a couple of bearings it was re-assembled as found but the oil leak continued.

I constructed a gantry to make removal of the gear box as easy as possible. See photo 1. The propeller shaft only requires disconnecting at the forward end and then moving and tying up towards the offside. The rails are fitted on top of the chassis. The lifting plate replaces the gear box forward cover. With the gantry in place the weight is taken by the screwed rod. Once the bell housing studs are removed the gantry and gear box are moved backwards. See photo 2. The gear box is then carefully lowered on to a piece of old carpet on the ground turning it as necessary to avoid the drive shaft, exhaust pipe and propeller shaft etc. The car requires to be jacked up about 3 inches (75 mm) at the rear and 5 inches (125 mm) at the front. The gear box can then be slid from under the car by pulling the carpet. Replacing the gear box is the reverse of the above taking care to make sure the clutch release fork is correctly engaged in the clutch release bearing housing. This rig has been used successfully on another 20 hp.

Photo 1. Lifting gantry and rails.

Photo 2. Gear box being moved to the rear.

I had covered 47,500 miles in the car and the oil leak averaged a pint every 7,000 miles. (1 litre every 20,000 km.) This winter I was determined to cure
the leak. I removed the gear box and mounted a ¾ HP electric motor to drive it (Photo 3), powered through a frequency converter to give variable speed. Tests soon showed that oil was leaking through the input shaft seal. This consists of a ring on the shaft with a square four start thread having a clearance of about 15 thou. (0.4 mm.) to the end cover. This leakage only occurred in top gear and was worse when the oil was cold and thick. The gear box holds 2.2 pints (1.3 litres) of oil to the correct level which is to the top of the filler. Varying this made no difference.


Photo 3. The test rig.

The reason oil leakage is only in top gear is that in top gear, the third gear driven gear, acts as an oil thrower and throws oil to the oil collector and at the input shaft bearing The only way that oil, which passes through the bearing, can go is back through the bearing or dribble out via the seal. My first attempt at a cure was to fit a bearing with one seal. This naturally worked but put quite a drag on the input shaft and I felt this would affect the gear change characteristics adversely.
It would possibly lead to quicker changing up but more difficulty changing down. This approach was abandoned.

Steve Lovatt gave me an end cover from a scrap gear box with which I could experiment. My second approach was to mill a groove 3 mm. deep by 15 mm. from 4 mm. below the seal to the lower bearing cover. This can be seen on photo 4. This would give the oil that passed through the bearing easy drainage. This was an immediate success and no oil leaked when running in top. I was disappointed when I returned to the test rig after half an hour to find that a very small amount of oil had leaked out while it was stationary. I repeated the test with the same result.

Back to the drawing board! I then machined a groove in the end cover round the seal area to direct any oil that ran down the end cover round this groove to the previously milled groove. This can also be seen in photo 4 and the cross section diagram. This gave a complete cure. I ran the gear box on test, stopping, changing gear, doing the equivalent of 55 mph (88 km/h.) down a 1:6 (17%) hill. After covering the equivalent of 300 miles (480 km.) not a single drip of oil leaked!

Photo 4. End cover showing final machining.


All the tests were carried out with an open bearing. Since I was replacing the input shaft bearing, I fitted one with a shield to the forward side, - a belt and braces approach. Shielded bearings do not seal or put any drag on the shaft. 

I feel that most 20 hps suffer from this problem. The oil drips from the bell housing through a hole near the hole where the sump oil from the rear crank shaft seal drips out. It is therefore not obvious that the gear box is losing oil here. Loss from the gear box will not be of significance if one only does a modest mileage and is topped up during annual maintenance. It only becomes noticeable when one does a higher annual mileage. On a couple of rallies I have been asked to check 20 hp gear box oil levels and have found them virtually empty!

One other place that oil leakage occurs but on a much smaller scale is where the brake operating shaft passes though the top of the gear box. This can be reduced by replacing the existing washer on each side with Dowety washers with the neoprene seal cut from the outboard side. This gives a seal on the shaft and a seal against the end of the shaft bearing. One final point, once a gear box has been refitted any oil leakage from the seal will take quite a time to show up owing to the dammed area in the bell housing in way of the front brake operating shaft having to fill up first.