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Notes on Pre-War Cars

by Bert Ward, 1978

20 HP


The first 3 speed 20 HP chassis did have trouble with the 1st gear sliding, in that under normal use, a tooth would just drop out of the gear, just as if each tooth was fitted into a socket on the gear hub. It was lucky that when the tooth dropped out, it fell to the bottom of the box and did not cause other damage. However, when this gear had to be replaced it was necessary to replace the pair, as the modified 1st sliding gear would not operate other than with its matched gear. Of course, this gear did get a lot of work, due to the small H.P. of the engine, and with the chassis weight of 23 cwt, plus the heavy weight of some bodies fitted to this little chassis. This modified 1st gear was made a standard fitting on later model 3 speed 20 HP chassis. The 20 HP 4 wheel brake 4 speed gear box model had the same trouble with the 3rd speed sliding gear, which was subject to a lot of work. The trouble with the 3rd speed sliding gear was the same, a complete tooth would just drop out. A modified complete 3rd speed gear set, which comprised 3 gears was introduced, and fitted as a replacement in cases of failure. It was standard in late model 20 HP chassis.

20/25 HP

We know that the 25 HP chassis was produced during the "0" series. In that chassis, No. GXO10 was produced as a 20 HP, and GXO11 was a 25 HP chassis. All that had been carried out was that the engine of GXO11 had an engine with a 3�" cylinder bore, whereas GXO10 was produced with a 3" bore engine, which was the standard size of the 20 HP engine.

The 20/25 HP chassis was modified on chassis GLR1, which was 3" longer. With GLR26 they increased the compression ratio to 5.25 to 1, fitted a new exhaust manifold, and flexible engine suspension. Chassis No. GXT22, had a synchromesh gear box, compression increased to 5.75 to 1, a high lift camshaft, heavy exhaust valves, and stronger clutch springs. Then in "Z" series, the gear box was fitted with a silent 2nd gear. However, many more modifications were carried out to the 20/25 HP chassis during its production until, with the extra length of the chassis, plus extra weight bodies, it was found that the chassis was underpowered. On the last 20/25 HP chassis K2 series, Rolls-Royce started to incorporate some of the 25/30 HP chassis features, in that 11 chassis, starting with No. GTK42, were fitted with Borg & Deck clutches, hypoid rear axle and Marles steering boxes.


One of the main troubles with the 20/25 HP chassis was the clutch. This trouble did not start until the synchromesh gear box was introduced. With this type of gear box, engine oil was recommended, and as no special oil seals were used, the light oil leaked into the clutch housing. With the heat generated the oil vapour entered the clutch, and if not kept washed, the clutch linings would get saturated and slip. Although stronger springs were fitted, slip still did occur, and in the end the clutch would have to be relined.
A modification did help in that the heat and oil vapour could get away. This was to remove the inspection plate from the clutch housing, held by 4 %" BSF nuts and washers, cut out the centre of the plate, which is aluminium and not hard to carry out, and fit strong wire gauze over the hole. This allowed the heat and oil vapour to escape. This is also a good modification to carry out on the 20 HP chassis, although they do not suffer to the same extent with this trouble like the 25 HP. Some owners might find an extra washer fitted under the 20 HP, and early 25 HP chassis clutch springs to give more tension to these springs, but R.R. Ltd. stated that the washer must not be any thicker than .100".
On some 25 HP chassis circa 1933, Rolls-Royce produced and fitted a spring drive centre specially clamped clutch plate. The design and manufacture of this type of plate was an excellent idea, being similar to the present spring loaded Borg & Beck clutch. However, it was not a success, and we had troubles with rattles and shuddering. This plate was built around the old 20 HP plate so we just fitted a standard 20 HP plate complete with hub as a replacement. This type of plate was also fitted to the first 3� litre Rolls-Royce produced Bentleys, and we had the same trouble. The plate was deleted by Rolls-Royce Ltd. on "G2" series, and the standard 20 HP plate used until the Borg & Beck clutch was introduced.

25/30 HP

This chassis was produced from the last 25 HP chassis, except that the engine bore was increased to 3.5" and fitted with a down draught Stromberg Carburettor. The Hypoid rear axle had a 9 x 41 ratio. This chassis, fitted with the larger engine, was a perfect car. With the extra power it made a lovely car to drive, however it had its troubles.


The early 25/30 chassis gave trouble with over oiling of the engine, and piston knocks. It was necessary to dismantle the engine, expand, refit and lap the pistons in the bore until a good bedding was achieved. A modified type piston ring, of "L" section, was fitted to overcome the over oiling. This piston ring was fitted as standard on chassis GRM72 onwards.


This unit was very fierce. To rectify, the clutch had to be removed and the clutch plate linings (Borg & Beck) removed and replaced with Thermoid linings. This type of lining was a much softer material than the original. This type of lining was available from Smith & Rees, Wentworth Avenue, Sydney. The Thermoid linings, however, were fitted as standard on GGR29 onwards.
Another trouble which we had in NSW on this model was overheating of the engine. As usual, everything was checked for the cause, but the trouble would disappear for a period, then return. After a lot of checking it was found that the water pump was the cause. When the car was stationary, this would operate, as one could see the water being pumped into the radiator, but on the road the pump stopped operation. The cause was that the shaft where the impeller or rotor was fastened would fracture. At slow speed, there was enough friction for the impeller to revolve, but as soon as the engine speed increased, the shaft would turn but not the impeller. This happened to a few 25/30 HP, and we also had the same trouble on one PIII.
There has been a lot of talk about cracks in the cylinder heads of 25/30 HP engines, and that this is a fault with them. This is not a fault, and I will go further and say that not one 25/30 HP delivered in Australia has this trouble unless the water system has run dry and the engine overheated. The only cars that have trouble with cracks in the head are those cars that have been purchased in England second hand and brought to Australia. Apparently at some time or another the engine has been subject to the water system freezing. One of this model car was purchased in England and brought to Australia. Soon after arrival in Sydney, water started to leak into the engine sump, and it was found on dismantling, a piece of the cylinder wall approx. 3" in diameter came out of the water jacket. A new cylinder block was necessary.
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