Engine Rebuild

Most people will know that my riley was recovered from being in storage for 35 years and was used for spare parts. The engine, called a “12/4” because it was 12horsepower / 4 cylinder, had a large hole in the side that had been poorly repaired at some time but I repaired that, it was coaxed into life and was driven for a couple of years in its as-found state. Finally in mid 2020, the oil pressure would drop to near zero after a 45 minute drive which was its way of telling me that it was time for a rebuild. I covered this in an earlier post and since October 2020, the engine was been on holiday…

Now vintage cars are simple in design but it probably comes as no surprise that technology has moved on. The crankshaft, conrods, camshafts etc all have bearings. But in a vintage car – you cant simply ‘replace’ the bearings like you do in a modern engine. The bearing is made of white metal (or babbit as it is sometimes called) it is a tin based alloy that is melted and whilst in liquid form it is poured into a mould around the part that needs the bearings so basically the white metal bearing becomes part of the actual component. Obviously it is then machined to size and made smooth etc.

Machining tolerances in the 1930’s were bigger than they are now so throw in 90 years of wear and those tolerances are somewhat generous – but what was originally acceptable? Similarly with the oil pump, any wear has to have the worn components replaced by fabricating them in a machine shop – you cant ‘buy’ a new oil pump. So all things considered, whilst I am perfectly capable of building my own engines – having someone with vintage Riley experience is fairly logical and that is where Steve comes in. He owns several Riley specials that he competes in and he has many years of experience rebuilding Riley engines so that they perform better than they did new.

Riley engines when released from the factory were hugely under-stressed, but their fundamental design was way ahead of their time – it had 2 camshafts, overhead valves and a hemispherical head design. Many people cried out for the factory to build in more performance but the Riley company had no interest in performance or racing (as a manufacturer) so private owners made the modifications themselves and in a number of cases – they could exceed double the engine power! A number of Rileys competed at LeMans so you can sense there was some potential to be gained.

But I digress: Step 1 – the 12/4 crankshaft was worn and needed new white metal bearings. There are a couple of companies that manufacture a replacement crankshaft which can take replaceable bearings but at £3000 that requires deep pockets – granted they are stronger than the original so they can take more power but thats a lot of money. Step in Steve – he had stumbled across a complete engine, gearbox and ancillaries from a later Riley RMA (1950’s), he stripped the engine down and found that the crankshaft and conrods bearings were in as new condition but there is no real money to be made rebuilding RMA engines so he advertised it for sale as-is.

In Riley circles – The RMA crankshaft will with a little bit of machining fit the Riley 12/4 engine block but it is bigger and stronger than the 12/4 version so it can comfortably handle more power and rev to a higher level than the original. The cost for the entire collection of RMA parts was less than a quarter of the cost of buying a new crankshaft plus I could sell all the parts i didnt want so a deal was struck.

Although I have shown this before – below is the crankshaft when it was taken out of the RMA engine

My 12/4 engine block was stripped down revealing a number of horrors which I will touch on as i go along. First thing to do was send off the engine block and cylinder head for acid cleaning to get 90 years of grunge out of the oil galleries. It then had a 25 thou rebore to smooth out wear in the cylinders, the top face of the engine block was lightly machined flat. The Cylinder head had a skim and it was machined to take larger valves. A previous owner had simply fitted bigger valves in the space where the smaller valves were. That was pointless without machining the cylinder head to take them! Steve took the opportunity to replace a few of the head bolt threads which at some time had been bodged and were now not only worn and loose but the wrong size too. The engine now has correctly sized head bolts so the clamping action is uniform across the head.

Below is the re-engineered Crankshaft inserted into my 12/4 block – looking very nice and strong. The oil pump (seen centre left) had a lot of end float and was found to have significant wear internally – luckily Steve had the experience and knowledge to be able to machine replacement parts and ‘refurbish’ it. With the worn bearings and a worn oil pump replaced – that should mean an end to the poor oil pressure.

The Riley Register manufacture replacement pistons, and the decision was made to go for the high compression sports type. The cylinder head has a hemispherical shape to it which creates an efficient gas flow but the original pistons have a flat top. That combination wastes a lot of the compression space. The new high compression pistons have a dome on the top that fills that space. Higher compression means more power is created each time the fuel is ignited and that in turn results in more engine torque.

Below you can see a couple of the repaired threads and 2 of the domed pistons which have small cutouts to prevent the valves hitting them.

In the image below you can see the new timing gears which link the crankshaft to the two camshafts. The ones on the engine had badly worn teeth and the engine idle speed wandered up and down a fair amount. More importantly it is these gears that control the rotation of the camshafts and hence the inlet and exhaust valves. If the timing of the valve opening/closing is wrong against where the piston is – you lose power output. You can also see that the crankshaft has a number of holes drilled into the outside face – these appear to be random but are there to balance the crankshaft, 5000rpm is a lot of rotations per minute and if the rotation is out of balance you get vibration which if not controlled stops the engine rotating any faster and destroys the engine over time. I had all the moving parts of the engine balanced including the flywheel and the very heavy clutch.

Below are a number of images of the engine coming together which hopefully will be started for the first time this week (June 2021). What you cant see is the competition camshafts that lift the valves more than standard which in turn increase the speed of the combustion – more efficient means more power.

It is very close to coming home now. Exciting? You bet it is…

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