1952 Sunbeam S8 – Resurrection




Part Two of Four – Engine, Gearbox, Rear Drive


The use of heat to ease the dismantling and re-assembly of an engine is more than a useful tool. It is often essential to avoid permanent damage. I use heat a lot because it contributes to the longevity of the castings and numerous components. I use it to varying degrees and in a variety of forms, including an oven, a hot air gun (not a hair dryer), a flame and even boiling water. If you try to press out a valve guide without heating the cylinder head casting first, the guide will drag metal from the head casting, which will reduce the interference when the new one is pressed in. The amount of interference is critical. If you try to press or, worse still, knock the timing pinion onto the rear of the crank-shaft without heating it, you will apply too much force to the end of the crank, which can damage the pinion or even bend the crank. The engine will vibrate horribly after that and bearings will wear rapidly. Cylinder liners must never be removed or pressed-in cold, for the same reason as in the case of the valve guides. Gudgeon pins are usually easy to push in or out with just a little heat from a hot-air gun and it avoids the need to use an extractor or, much worse, a drift. I even heat the head casting before attempting to unscrew the studs, to avoid tearing metal from the threads. These engines are old and deserve to be treated sympathetically.




I now switched my attention back to the cylinder head. The inventory of parts contained three good valves and some that were not so good, so a new inlet valve had to be purchased to make up the set. All four valves were lapped into their newly re-cut seats and the effectiveness of the seal tested with a small amount of petrol poured into the ports. I lubricated the valve stems and completed the re-assembly of the cylinder head, fitting a new camshaft (the original one was worn beyond use) and the refurbished rocker shaft assembly.

When re-assembling engines, I use a special oil that is specific for the purpose. It clings to surfaces and is a persistent lubricant that obviates damage during the initial seconds of the first start-up. I use it on big-end bearings, main bearings, gudgeon pins, camshaft journals/cam lobes and valve stems. Engine oil is sufficient in other cases. The one I use is an oil and not a grease. The thicker assembly lubricants can block oil ways and delay the initial circulation of oil, which seems to me to be self-defeating.

Another thing that I like to do during re-assembly is to clean the threads on studs with a die and clean the threads in castings with a tap. It not only avoids crossed threads but it makes assembly quicker and avoids the frustration of nuts that won't easily pickup on their studs owing to dirt or minor thread damage. Another useful precaution is to ream the stud holes in the engine block and cylinder head, to remove dirt, debris and any burrs. The head will drop into place more easily and it will lift off much easier next time the engine is dismantled.



The next job was to finish-off the assembly of the engine, timing gear and clutch. Before putting the clutch together, I spin the engine by hand from the flywheel, slowly at first but then more quickly. The purpose of that is to check that everything moves freely and smoothly, with no tight spots or any sign of noise from the big-ends or from piston slap. In this case, I fitted a Borg&Beck friction plate – the type with the tangential damping springs. They make no noticeable difference to the feel of the clutch but I have an unproven theory that they may reduce wear on the main-shaft splines. Incidentally, I always lubricate the splines very lightly with molybdenum disulphide grease during assembly. I also lightly lubricate the pressure springs with copper grease, to eliminate that strange squeak that Sunbeam clutches often make when disengaged.




I imagine that the purpose of drilling the sump pan was to increase the surface area to aid cooling. I doubt that it worked. But it succeeded in weakening the casting.

Those who are familiar with the internals of a Sunbeam engine may notice that a previous owner of this bike liked to play. A number of modifications have been executed or attempted. Two of them are shown here. Most of the mods that I see in Sunbeam engines are either pointless but harmless or sometimes pointless and harmful. The exception is the lightening of the flywheel, in this case by extensive drilling. There are easier ways to achieve the same result but I know from personal experience that a lighter flywheel helps to smooth out gear changes and doesn't seem to have any detrimental effects if it is done right. If it isn't done right, it will shake out your fillings.



Removing metal from the face of the flywheel does the same job with less risk.



It was now time to make a complete gearbox from the parts that were available from the inventory and source any parts that were missing or in need of replacement. The case, which had been vapour blasted and cleaned, was completely empty. The internal components were mainly inside another carcass and their integrity was unknown. So the first job was to remove the front bearing plate and extract the gear cluster, selector forks and bearings and inspect them for wear or damage. Then the case needed to be checked for signs of damaged threads and anything else that might impede assembly.

The kick-start mechanisms and the selector mechanisms were removed from the two side-plates that were in the inventory and their components scrutinised so that the best candidates could be chosen from which to build a new set. Cracks in the kick-start return spring are common and so it was here. One of the gear selector cam plates was also cracked – a sure sign that somebody had forced the side-plate into position without checking that the selector forks were properly engaged with the cam plate.


All of the oil seals in the gearbox casing were replaced with new ones and the internal components of the clutch thrust bearing were renewed.

Sunbeam gearboxes are pretty tough and it is unusual to find damaged or worn gears or shafts in them but worn selector forks, worn bearings and loose bearing holders are less unusual.

In this case, two new bearings and a usable selector fork were required before the complete gear cluster could be trial-fitted into the casing, to make sure that the shafts run free and to see whether any re-shimming is necessary. In this case, some adjustment was needed to achieve free rotation.

A good, leak-free seal of the side plate relies not just on the mating faces and the gasket but also the seal where the kick-start mechanism attaches to the side plate (a common source of an annoying dribble) and the integrity of the counter-sunk holes for the fastening screws. In this case, it was necessary to clean-up the counter-sinks on the milling machine, otherwise oil would seep past the screw heads.





I always use solid copper cylinder head gaskets unless a customer particularly requests the use of a composite gasket. The composite gaskets are single use only and, in my opinion, they are less reliable than the solid type. Also, the solid gaskets are available in a variety of thicknesses, which is convenient to compensate for a cylinder head or engine block that has been skimmed. It reduces the risk of valve/piston collisions and helps to restore the full range of adjustment of the timing chain.

I usually anneal the gaskets by heating them to a bright cherry red with a gas torch and I then clean off the oxides with a wire brush before fitting them. I also check that all of the holes line-up with the studs and oil ways before chamfering them on both sides to eliminate any binding at the base of the studs.



I find it convenient to place the seven lower cylinder head nuts with their washers into their wells in the cooling fins in the engine block in preparation for lowering on the head. The brace plate for the front engine mount needs to be positioned under the lug at this stage and aligned so that the front cylinder head stud can pass through it. Omit the brace plate at your peril. If it is not fitted, the eventual but inevitable result is that the engine block will crack in a place that is extremely hard to repair. I have seen a number of otherwise perfectly good engine blocks effectively written off by omitting this apparently insignificant component.

I positioned the new gasket onto the engine block and lowered the cylinder head into place. I engaged the nuts with their studs and turned them until finger tight before checking that the head was sitting down onto the gasket everywhere. Then the four rear nuts needed to be fitted, including the two that sit inside the cylinder head. Once all of the nuts are finger tight, the process of tightening them incrementally, in the recommended sequence can start. The ideal is that the tension on each of the nuts is increased gradually and in small steps, with them all being at the same tension throughout, until they are all fully tight but not strained. If it is done properly, the cylinder head will not be distorted and the gasket will have a long life.




Then the crank-shaft must be set to the TDC position so the camshaft sprocket can be fastened to the camshaft. If the preparation has all be done correctly then the valve timing will be correct and the cam chain tensioner can be fitted and adjusted.

As another aside, I always use a magnetic probe to position the two nuts and their washers inside the rear of the head and onto their studs, minimising the chance of dropping one of them in the sump. I also use the same method to remove them safely, so using stainless steel fasteners there is a definite no-no. In fact, using stainless steel anywhere inside an engine is a serious error for a variety of reasons that I won't go into here. It is a common mistake made by amateur mechanics.

Now that the head was fully fastened down, I was able to set the valve clearances to their recommended setting of 0.018”. I once read, in a post on the SOF forum, that performance can be improved by reducing the valve clearances. It can't. Doing so will deliver no benefit whatever and there is a risk of collision because the valve-to-piston clearance is minimal, even with the correct clearance, particularly if the flat-topped pistons are fitted to your engine,. I have dismantled many engines where tell-tale crescent marks on the piston crowns have shown that contact has occurred. Several of them have had bent valves and even cracked guides where the impact has been particularly heavy.





It was now time to unite the engine and gearbox. That's an easy job if you have centred the clutch friction plate accurately. It can be quite frustrating if you haven't. Getting the lump ready for lifting into the frame is then just a question of fitting the components for the engine mounts and the generator but I find that bolting on the exhaust stubs provides a useful hand-hold for lifting the engine. I nearly always lift the complete engine/gearbox, with cylinder head and generator, into the frame single-handed unless my son happens to be passing by.

I used the word “generator” instead of “dynamo” because the inventory contained a new Hawker alternator kit instead of the original dynamo. These kits are out of production now but similar devices are available from other suppliers. It comprises a standard Lucas 12V, 10A stator and permanent magnet rotor housed in a spun aluminium cylindrical casing with a detachable front cover.

An adapter allows the rotor to be attached to the front of the crank-shaft, in place of the original armature, without any necessity for any modification. Similarly, the casing and the stator are bolted to the engine casing using the original dynamo mounting points. Even the electrical connector block is compatible with the original dynamo connector. Some small changes need to be made to the bike's electrical wiring and the original regulator must be replaced with an electronic rectifier/regulator, all of which is straightforward. The shape, size and general appearance are similar to the original dynamo but the diameter of the casing is slightly larger.

I must confess that I like these alternators. They are simple and reliable and they are built around standard Lucas components that are readily available and cheap. I have an uprated 16A version fitted to my own S8 and it has proved very successful over a number of years.



Setting the engine aside, my attention focused on the rear drive. Again, one good casing had been blasted and cleaned but most of the internal components came from another, less appealing, housing. Cleaning them and checking them for wear or damage started the process. All seemed pretty good, including the worm wheel, except for a nasty indentation across one of the teeth. It must have been the result of a considerable impact and had caused the area around the wound to bulge. It would hardly have been right to consign such a valuable part to the rubbish bin without trying to find a way to restore it. I used a magnifying glass and a hand-held electric multi-tool with a fine burr to carefully grind away the swelling around the damaged area, to restore the profile of the tooth. It looked good but the proof would come later, after assembly, and then later still when it would be tested under load during a test ride.





Assembly of the rear drive went smoothly but some re-shimming was needed to achieve the correct end-float on the worm-shaft and the worm-wheel in their new casing. Rotating the worm-shaft while dry showed up no problems with the repaired section of the worm-wheel and, jumping ahead somewhat, the test ride confirmed that the repair had been successful, even under load.

The rear universal joint was worn beyond use so the needle roller bearings and spider were pressed out and replaced with new ones. The loose paint was cleaned away and the whole assembly was painted in gloss black then a new grease nipple fitted to restore it to its original condition.

All of the brake linings were worn and contaminated so the old linings were removed, the shoes cleaned and painted, and new linings glued and riveted to the shoes. The front brake shoes were fitted with over-sized linings, so that they could be mounted on the lathe and turned-down to the exact diameter of the drum. They were then perfectly cylindrical and concentric, to maximise the contact area between the shoes and the drum. This is a simple and effective way to optimise the performance of the sometimes barely adequate Sunbeam (BSA) 7” brake.



In Part Three, I describe how the cycle parts were repaired, refurbished and prepared for their future life on the road.