It is not uncommon to see oil dribbling down the exhaust side of the engine and for there even to be a smell of burning oil under hard driving. Under close inspection you may find that the oil seems to be coming from the head gasket. To rectify this problem you will probably not need a new head gasket but you will need to remove the cylinder head.
So from where is the oil coming? With the cylinder head removed you will see a hole in the block face half way along the block and very close to the exhaust side of the block. There is no corresponding hole in the head gasket. This hole was drilled by the factory in order to drill the oil way from the centre camshaft bearing down into the main oil gallery. You can see this oil way in the oil system picture below. The drilling above the camshaft bearing subsequently serves no purpose and needs to be blocked off somehow. Riley's method of blocking it was to use the head gasket. This might have worked well had not the hole in the block face been so close to the side of the block. As it is, the gasket eventually gets tired of holding back the oil and allows some to escape to the side of the block. Just how much escapes depends on how tired the gasket is. When it gets very tired it is possible that under hard driving some of the oil may reach as far as the exhaust system and hence the smell of burning oil.
The cure is simple. Tap a thread in the top of the drilling making sure to keep a lot of grease on the tap to prevent any swarf from dropping down. Now use a short piece of studding with the same thread as the tap (a piece cut from a screw will do) and cut a screw driver slot across it. Apply some gasket cement to the threads and screw the piece of studding into the drilling until the threads lock up making sure that the top of the studding is below the face of the block to prevent damage to the head gasket. This will block the hole and prevent further oil leakage.
Clean up the old head gasket and check for any damage. If it is a copper/asbestos one and is undamaged it can be used again provided it is greased lightly on both sides. The purpose of the grease is to allow the copper to slide slightly against the head and block faces to get a good seal. Unlike a new gasket it will compress very little in use and will only need to be checked once. If a new gasket is used it should be torqued down correctly, the engine run in the garage until it is hot and then torqued down again. It is not uncommon to find that a further quarter of a turn is needed on each nut. It should be checked again after 100 miles and preferably rechecked after 500 miles.
Note also that oil reaches the centre exhaust camshaft bearing from below only unlike the front and rear bearings which are fed from above and below. Oil flows around the outside of the centre bearing bush in a grove in the bush to reach the feed hole in the top of the bush. This grove is often very shallow and with little oil flowing through it is often blocks and restricts the oil flow. This can cause seizure of the bearing onto the journal. If this happens removal of the bush locating pin from the side of the engine will often allow the centre bush to remain stuck on the camshaft as it is withdrawn from the engine. With the bush removed from the camshaft it is often found that the damage to the bush and the journal is not great and can be cleaned up easily so that the bush spins freely on the journal. The grove in the journal should be cleaned out and if it is very shallow is can be made a little deeper.
While you are looking at this picture, note how the oil is fed from the centre of the big end journals to the surface of the journals. This is easily seen on the No. 1 cylinder big end. As the big end rotates the oil in it is centrifuged and heavier sludge is thrown outwards but it cannot escape from the oil way in the middle of the journal as the feed to the surface points inwards. Over time the sludge builds up inside the big end journal until the oil way is completely block and the oil flow stops - result, great oil pressure and ruined big end bearings! The cure is simple - drill another oil feed hole in the journal which points outwards to let the sludge escape into the oil filter.