None can stop the dreadful leak

Dealing with various leaks from the

Cam Pulley:    Cylinder Base Joint:    Rear Main Bearing:   
Valve chest, oil filler tube and sump:   
Cylinder Block/Crankcase Joint: Water Leaks from the Engine

Oil leaking from behind the cam pulley on coil engines can be most annoying and elusive to cure. 

The secret is in the fitting of the felt seal.  This seal must not rotate in the nose cone.  Reduce the seal in thickness to prevent it pressing hard on the rear of the pulley, then secure it in the housing with sealant, allow this to set then apply grease to the felt.  Water pump grease is ideal as this has more 'body'. 

Other reasons for leakage here, steel pressing loose in nose cone, steel pressing damaged, oil thrower damaged or missing from front of camshaft.  The oil thrower must not be 'holed' due to damage caused by knocking up or removal of lock washer. 

The cam pulley must also run true.  Lightly lap the pulley onto the camshaft to obtain a good fit as this is often the reason why they run out.  The fan pulleys are often split due to over tightening, a split pulley can cause oil leakage. 


Oil leaks from the cylinder base joint on both Magneto and Coil engines are well known, but occasionally on coil engines this joint is not the cause. 

Oil apparently leaking from the front of the cylinder block can be due to the camshaft locating screw being loose, but a more elusive cause is the badly fitting dynamo housing. 

Because the long dynamo fixing screw passes through an internal opening of this housing oil can find its way both up and down the length of the screw.  If the housing base joint is not sealed completely around the bottom of this screw, oil will seep out and give the impression that the cylinder base gasket is leaking.


Most engines leak a little from the rear main bearing.  Some leak a lot. The following which mainly applies to coil engines might be of help. 

The flange on the rear bearing housing must be in good condition and not distorted.  Avoid removing this housing with the two threaded holes provided as this will almost certainly distort the flange.  Heat the crankcase and tap it out from the inside. 

The cover plate must be in good condition, not distorted or cracked and the internal oil screw unworn and clean.  This housing will only work if it is a close fit on the flywheel.  There are two sizes, early 1⅛" crankshafts had a smaller diameter, so make sure you have the correct housing for the flywheel in use.  The drain hole in the bearing housing must be at the bottom, in line with the hole in the crankcase.  Enlarge both holes to 5/16" diameter.  The oil thrower must be in place and firmly trapped between the rear bearing and flywheel.  Replacement oil throwers have small indents on the internal diameter which get crushed on assembly to ensure a tight fit.  Loose oil throwers wear a groove in the crankshaft and don't do the job they were intended for. 

It is best to assemble the rear bearing, oil thrower and flywheel before finally fitting to the crankcase to ensure that when fully tightened the flywheel is tight on its taper.  Alternatively allow a .040" gap between rear bearing and flywheel when the flywheel is fully tightened. 

Assemble the bearing housing in the crankcase using the thin paper gasket and the cover plate with the thicker gasket.  Seal both with a sealant such as Blue Hermatite.  Allow the sealant to start to set after initial assembly, then finally tighten before locking the screws with the tab washers.  From inside the crankcase look up the 5/16" hole you have drilled to confirm that it is not blocked with sealant. 

The rear cover plate can be replaced with a modern lip seal with varying degrees of success.  These seals only work properly if the land on the flywheel is in good condition and runs true.  Also the seal needs to be square in the housing.  The oil thrower must still be fitted.  The seal with a double lip are best but offer more resistance.  When fitting apply a small amount of grease to the lips for initial lubrication. 

Another myth is oil from the engine getting on to the clutch linings.  This can only happen if the oil finds it way down the keyway and this is usually well sealed with the flywheel nut.  Oil on the clutch linings has probably come from an over filled gearbox or over enthusiastic lubrication of the clutch release bearing.


Preventing leaks from the valve chest, oil filler tube and sump can be a challenge, so the following might help. 

In all cases the good fit of these parts is essential if you are to stand any chance of success. 

Oil filler cap:  Oil seldom leaks from the oil filler on Magneto type engines, providing the cap has a cork seal under it and the clip holding it in position is sound.  Make a seal using the cork from the early valve chest cover gasket and stick it to the cap. 

The filler tube on early coil engines is a push fit into the crankcase, being secured by a ΒΌ" whit screw.  Because of its position oil is constantly thrown at it from the rotating crankshaft and therefore it is necessary to seal this into the crankcase using a suitable sealant such as Locktite Multigasket.  Later screw in oil fillers have the same problem and a small amount of sealant on the thread usually does the trick.  But these later tubes are at an angle which allows oil to be thrown straight up the tube to the bottom on the cap.  Badly sealing caps are often the reason for what appears to be a leaking tube.  Reliant overcame this problem by fitting baffles in the filler tube, preventing oil being thrown directly up the tube. 

Early valve chest covers are the most difficult to stop leaking.  They are sealed with a flat cork gasket and rely entirely on the cover being a perfect fit.  Check the cover for 'flatness' by placing on a known true surface and seeing that it doesn't rock.  Twisting the cover can often correct this and gentle tapping with a light hammer will generally correct any other high areas.  When fitting to the engine make sure it is directly against the cylinder block and not 'held up' by being caught on the top of the crankcase or the lip under the exhaust manifold.  Later covers have a cork strip to seal them.  Currently the strips available are not of the correct section, being too large to fit neatly in the groove.  This is easily overcome by reducing the width of one edge only.  I have found that running it past the bench grinder does this very well and very quickly.  Fix the strip into the cover plate using Super Glue.  With both types of cover avoid fitting to the cylinder block with adhesives or sealants as this makes adjustment to the tappets rather difficult.  Although in extreme cases it might be the only solution.  A smear of grease on the joint helps the initial seal.  Place fibre washers, backed by steel washers under the fixing screws and check tightness regularly.

Sealing the sump is easiest when the engine is out of the car as it is much more accessible.  Before attempting to fit the sump check the condition of the fixing screw threads.  These, in conjunction with the screws have a major effect on the success of the seal.  The fixing screws are make of a relatively soft material and are easily stretched if over tightened.  Scrap any screws which show signs of being stretched.  The sump flange must be flat with no damage to the lip.  Gentle tapping with a light hammer will usually correct distortion, and if necessary weld up any damaged lip.  Having got the flange flat, gently tap each fixing hole away from the flat surface with a ball pain hammer.  This effectively provides a reverse countersink which will pull up when tightened.  Make sure both surfaces are perfectly clean by wiping with paint thinners before fixing with a new gasket and sealant. 

The most effective sealants are of the silicon type, but all of these are affected by petrol and to a lesser extent by oil.  For this reason don't be too generous when applying it.  The best sealant I found was in a pressurised container, unfortunately this was withdrawn due to the containers exploding while in stock!  Choose a silicon sealant which is oil resistant (from motor factors) and with oval and spring washers.  Gently tighten until the sealant starts to be squeezed out all around and then leave for period of an hour or so to let the sealant start to set.  Finally fully tighten all screws.  This job is most difficult on 'Nippys' or 'Specials' fitted with deep sumps as it has to be done after the engine has been fitted in the car!!


There are very few engines which don't leak from the cylinder block/crankcase joint at sometime in their life.  This joint is particularly difficult to permanently seal due to dis-similar materials held together with inadequate fixing studs in a flexible chassis.  The secret is to get a good fit between the cylinder block and crankcase first and this can only be done when the engine is completely dismantled.  Inspecting the top of the crankcase will show the imprint of the cylinder block.  The top is always low (worn) in the vicinity of the front and rear camshaft bearings.  I don't know why this happens, but it does.  Ideally this should be corrected by machining the top surface, but this luxury is seldom available locally and can cause serious problems if not done properly (see later).  It is possible to get a good fit by using the following method. 

First having completely dismantled the engine, remove all studs from the crankcase, including the oil jet cover screws.  Check that the block/crankcase stud threads are in good condition and correct if necessary.  The studs on the camshaft side are particularly troublesome and must be sorted out if damaged.  There are various ways of doing this, but fitting thread inserts or ⅜" studs here can be a waste of time as there is very little material in this area to start with.  Second remove all of the valve gear from the cylinder block, including the cam follower guides.  Lightly draw a file across the top of the crankcase and the cylinder block to remove any burrs. 

Apply coarse grinding paste to the top of the crankcase and proceed to lap the block and case together.  Continue to do this until the bottom of the block has a uniform matt surface.  Inspecting the top of the crankcase will show all the low points, and you can continue with the lapping until these all disappear.  Keeping both surfaces well lubricated during the lapping operation makes the job less tedious.  In practice getting rid of all the low areas is extremely difficult, but try to reduce the low spots to no more than .0015", test this with a feeler gauge.  Pay particular attention to the area adjacent to the oil pump.  This job doesn't take as long as you think, but it seems a very long time when you are doing it!  When you are satisfied with the fit, clean both surfaces and carefully wash all traces of grinding paste from the crankcase before assembly.  When fitting the oil pump cover plate, check that this does not protrude above the crankcase top as this will hold the cylinder block up at this point.  Refit all of the valve gear to the cylinder block.  VERY lightly counter sink the crankcase stud threads to prevent these pulling up, and fit the studs.  Only use new studs of known parentage, they must be good quality, usually made from EN16T.  If the supplier doesn't know what they are made of don't buy them.  Ideally do not fit a gasket.  Fitting a gasket will ensure that this joint is never really tight.  But there are problems here.  If you leave the gasket out (or if the crankcase has been machined) the pistons will almost certainly protrude above the top of the cylinder block, a situation you can only put right by further machining. 

I leave the gasket out on all race engines and some sports engines, but fit it on most of the others.  Clean both surfaces with paint thinners to remove any signs of grease, before sealing the joint with Locktite Multigasket, applying this to both sides of the gasket if fitted.  This adhesive only sets when in contact with metal and the air is expelled.  Make sure that all holding down nuts are tight.  If the gasket is fitted, continue to check the tightness several times during the engine assembly as the gasket continues to 'give' as it is compressed. 

Separating this joint at a later date can be very difficult, but it usually remains oil tight.  Just one last point, when fitting the dynamo check that there is back-lash in the drive gear.  Removing material from the crankcase top brings these gears closer together and they can 'bottom'.  This is the main problem if the top is machined.  Even worse if the machining is not true to the crankcase centre line!


Now for something different.  Water leaks from the engine. 

These often occur from the water branch joints.  As always, the joint faces must be in good condition and any irregularities should be corrected by light filing of the surfaces. 

Early top branches usually have the fixing thread either damaged or missing.  The same applies to the side branch fixings on all blocks.  How you repair this damage will depend on how bad the damage is, but if only the thread is stripped then thread inserts (which are stainless) work very well.  The alternative is to bridge the gap with a 'T' section trapped on the inside of the water jacket.  I personally avoid welding or brazing the cylinder block, but 'tack' brazing the 'T' piece in the cylinder head does hold it in place while tightening.  Providing the joint surfaces are in reasonable condition, don't fit a gasket, seal with a suitable sealant.  This is particularly important with the early top branch which has the annoying habit of moving when fitting the radiator hose if a gasket is fitted.  I have found the best sealant is one used by glaziers to seal windscreens into frames.  This is not silicon based, is black, and completely water resistant.  As with all sealants allow to harden slightly before fully tightening. 

Side water branches should be secured by bolts, not studs, with fibre washers backed by steel washers under the heads.  Apply a small amount of sealant to the fibre washer.  Beware, if these bolts are too long they will damage cylinder wall when fully tightened. 

Water leaks from head studs are particularly annoying.  The water could be coming from the head stud thread, or an adjacent water hole.  Often leaks from threads quickly cure themselves as they self-seal, putting a little 'Rad-Seal' in the water will accelerate this.  To avoid leaks from the threads when assembling the engine initially, apply a small amount of pipe sealant to the thread.  I avoid other sealants such as 'stud lock' as you might want to remove the stud in the future. 

If the leak is from an adjacent water hole this is probably due to corrosion around the hole, coupled with a badly fitting gasket.  This can usually be avoided if both head and block are resurfaced prior to assembly. 

Head gaskets should be smeared with grease only and tightened evenly using a torque wrench.  Tighten to 20-24 lbs initially and retighten after a short period of use. 


This was a series of articles written by Vince Leek which originally appeared in Focus Jan 2003 p18; Feb 2003 p25; Mar 2003 p25; Apr 2003 p25; May 2003 p24 and Jun 2003 p24.