Engine Autopsy

Danny’s Riley RMA engine strewn acorss the workshop.

The 1½-litre and 2½-litre Riley RM engines are the final incarnations of the famous twin-cam design that originated in the 1926 Riley Nine. Their most memorable feature is the placement of those camshafts – they’re not overhead, but high up in the sides of the engine block. Large valves are inclined at 45° and the combustion chambers are hemispherical, improving power output significantly.

Hugh Rose refined Percy Riley’s concept when he created the 1½-litre 12/4 engine for the 1935 season. The 2½-litre Big Four engine appeared a year later, bringing further changes. Both engines survived the Nuffield takeover and reemerged with various minor improvements to power the RM from 1945 until 1954. The 2½-litre lived on in the Pathfinder until 1957.

[A] TIMING GEAR Here, the two large camshaft timing sprockets sit either side of the duplex chain. Below the chain is the crankshaft sprocket and to the left is the chain tensioner assembly. The idler sprocket fits over the large brass boss attached to the lower plate of the tensioner assembly. A second plate (not photographed) goes on top. Splines in the top plate mesh with the small splined adjuster to the left. The doorknob-like item is the oil cap.

[B] CYLINDERHEAD The famous ‘PR’ (for Percy Riley) cylinderhead is what gave this family of engines its spirited performance. The hemispherical combustion chambers, near-central spark plug and large inclined valves offered excellent gas-flow. In the centre we can see a ‘hot-spot’ conduit to carry warmth from the exhaust to the inlet side, helping to vapourise low-octane petrol. This is now usually removed to improve cooling, as are the two we can see below in the block. They're replaced with core plugs sold by the RM Club.

[C] TIMING CHEST This aluminium casting has various roles. It covers the timing gear, provides locations for engine mounts (the large paired holes) and supports the distributor on its upper left surface, as we see it.

[D] PULLEY, THROWER AND PIPES

Below the timing chest is the crankshaft front pulley (left) and an oil thrower (right). The thrower sits inside the timing chest. It has a long hollow nose with a left-handed thread on the outside, which protrudes through the hole in the chest and mounts the pulley. The short pipe between them feeds oil from the pump output at the base of the block to the filter (not shown). The longer pipe lives inside the sump and takes oil from the front main bearing to the other two.

[E] ENGINE BLOCK A high-quality iron casting that’s not as heavy as it looks. We can see a vertical bulge in which the oil pump’s shaft resides, driven from the top where it meets the inlet camshaft. To the right are the fuel pump boss and a coolant tap. Further down there are two bolts that hold the oil filter, which takes oil from the feed visible to the right of the base of the oil pump shaft. The adjustable oil pressure relief valve is the protuberance at the bottom. After filtration, the oil is sent into the hole in the block adjacent to the front main bearing. The brass plate shows it was a factory replacement engine.

[F] SUMP The finned aluminium sump is quite shallow but the capacity is slightly higher than that of the pre-war 12/4, being ten pints instead of eight. [G] VALVES, SPRINGS AND CAPS The RM’S valves are generously-sized and have long stems. The concentric springs help to move all that mass back into place smartly.

[H] ROCKER SHAFTS AND PUSHRODS This engine has the pre-1952 style of cam follower, pushrod and adjuster, with mushroom-shaped rather than hemispherical heads and cups. The ‘nose’ of each rocker shaft takes an oil feed from a hollow stud in the head.

[I] CAMSHAFTS We have the inlet cam above and exhaust below. The inlet has a scroll drive for the oil pump shaft and an extra cam to operate the fuel pump. Its longer nose drives the distributor via the gear on the opposite page, just above the left cam sprocket.

[J] PISTONS These look like Hepworth pistons of a type that came in during the Seventies. The original type had no ring below the gudgeon pin. The pre-war origins of these engines meant that bore was reduced as far as possible to keep below a road tax threshold. In 2½-litre form the RM had the longest stroke of any post-war British car at 120mm.

[K] CRANKSHAFT All RM engines share an odd shortcoming. The crankshaft’s hollow big ends accrue a hard sludge that eventually blocks the oil hole. There is a 5/16 BSF plug that can be removed from each journal to allow the sludge to be scraped out.

[L] MAIN BEARINGS The 1½-litre and 2½-litre engines have a major design difference. In the 2½, the crankshaft is fitted from the bottom in the usual way. In the 1½, shown here, it’s threaded in from the rear of the block. These huge bearing carriers are lined with white metal and then assembled, line-bored and dismantled again before installing the crankshaft. A common mistake is to fit the centre main bearing back-tofront; it is essential to align the oilway in the carrier with the feed from the oil supply pipe (shown opposite) via a banjo and hollow bolt. Our engine’s front main bearing is still in the block.

[M] OIL PUMP The pump picks up oil through a coarse wire mesh and sends it directly out of the connection in the side of the block (see opposite) and into the filter. The top end of the shaft above the skew gear rests in a bush in the top of the bulge in the block. n