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Engine Upgrades
Mike Arnot's Feature Car
Written by Mike Arnot:
When I bought my first MG a 1976 rubber bumper GT I read with a great deal of interest the O series conversion as written by Roger parker of the MGOC. I did a lot of research into the history of the design of the engine and its intended use in the MGB.
Scouring the local exchange and mart I bought an engine from a local man who restored commercial vehicles and had bought an engine to go into an early Sherpa. The engine came from a 1982 SD1 and was fitted with twin SU’s. Just the job ….. That was some ten years ago and the engine sat in my garage until early this year.
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The O series engine in this guise as many will know was the engine that would have been fitted to the MGB had production continued. The previous owner said the engine had been rebuilt and was in good condition, so the first job was to confirm this.
We took off the sump and examined the bearings and crankshaft. The man was as good as his word and the bearings and journals were brand new with no signs of wear.
I took an old B engine back plate and re-drilled to fit the O series engine, as although a rover 5 speed box came with the engine I wanted to keep the B gearbox as I like playing with the overdrive and had the romantic idea of being as close as possible to the original MG design.
The spigot bearing in the end of the crank shaft was changed as it was a different size to the one used on the B. I fitted the modified back plate and used the original B flywheel, a new clutch kit was fitted at the same time.
We then spent ages trying to fit the engine to the gearbox while the gearbox was in situ. Even with taking off all of the auxiliaries this was a no go.
Out came the box and we fitted it to the engine first try!
The engine and the box were lowered into place in one lump; this is where the fun really started.
This engine has a timing plate fitted at the back of the crankshaft pulley wheel which wants to sit on the front of the B cross member, Lifting the engine high enough to avoid this means you can’t shut the bonnet. We took the timing plate off and transferred the timing marks to the pulley wheel and slid the engine back in.
This time the pulley wheel was tight up against the steering rack. Off came the pulley wheel and we machined the front lip which then gave us around 10 mm clearance.
We then lowered the engine back into place and we now had 20mm clearance between the top of the engine and the bonnet.
A big sigh of relief, I had just painted the bonnet and did not fancy cutting out cross members or even worse fitting a bulge.
Once the engine was settled into its new nest we set about fabricating engine mountings. Cardboard templates were formed before manufacture of the real things from some 7 mm thick plate. The original chassis mounting on the offside chassis leg was kept but we moved the nearside mounting back 150mm to allow for the oil filter.
The engine sat there nice and square and started to look the part. We then started to fit all of the bits and bobs you need to get an engine running.
Modifying the original bracket the alternator was fitted in almost in the same place as it would be on the B, however remember the crankshaft pulley wheel? It’s so close to the steering rack that we had to loosen off the rack to fit the fan belt.
The exhaust gave similar problems as it fouled the nearside foot well. We chopped the 4 – 2 -1 manifold and added a couple more inches to the two down pipes, we then custom made an exhaust pipe to fit an MGOC sports back box.
As stated earlier the engine came from an SD1 fitted with two HIF44 Carburettors and a FASD when the inlet and exhaust manifold were put in place modifications were made to the air intake of the FASD this enabled the fitting sports air filters.
The fuel lines more or less followed the same route as the B no problems there then, the coolant hoses were another matter, however scouring scrap yards and the pile of spares I have accumulated over the years brought out pipes of the correct diameter and conveniently with bends in the right places.
The more parts we put on made the appearance of this conversion look as though it had been factory supplied.
The oil pressure and temperature sending units are different on the O series engine and without a fair bit of machine work the dual temperature gauge originally fitted to the B would not fit. The route I chose was to fit a later model temperature gauge and an oil warning light. This actually works pretty well and does not look out of place.
The day came when we tried to start the engine and from the first turn it was obvious something was wrong. Fuel mist was bellowing out of number one carburettor. A pressure test showed no compression on number one cylinder. I had seen this before on a Jaguar and knew we had head trouble.
Off came the head and sure enough several valves were bent. On the O series cam shaft pulley there is a mark, this mark is not TDC but 90 degrees before TDC, at the crankshaft pulley wheel there is also a 90 degree marking, lining the two points up makes the engine ‘safe’. At some time these marks had been misunderstood and the cam had been set at 90 degrees (safe mode) when the crank was at TDC.
While the head was off the bores of the engine were inspected and found no wear what so ever, the head was rebuilt and fitted with new valves, valve guides and seats. A few days later we were ready to start again. The head was fitted everything else put back on and then I turned the key.
The engine fired up but ran on only two cylinders, I took a look at the points gap but was sure I had set them correctly, one lobe on the cam showed the points closed another showed them at about 30 thou, further investigation showed that the shaft in the distributor was bent giving no or uneven sparking depending on how you looked it
We dismantled the distributor straightened out the shaft and the engine fired and ran a lot better, but still would not run correctly, bonnet down and kettle on I sent off for a reconditioned unit.
While I was waiting for this to turn up I had another go at the FASD, some suggestions had been made that if you can get over the initial lumpiness of the start up you do not need the electronic delay in this unit, however I wanted to try and eliminate all possible causes of uneven running. I bought a control unit, found a web site that gave me the information I needed and wired the thing up.
During this time the batteries were continually letting me down and so a single 12 volt battery was installed in the front, at the same time a large electric cooling fan was fitted, the O series does not leave enough space for a mechanical fan in a chrome bumper car.
The day finally arrived when the new distributor turned up; it fitted exactly as the manual said with the rota arm turning from the one o’clock position to three o’clock as it clicked home.
A turn of the key and the engine ran as sweet as a nut; we did not even hear the starter motor engage before the engine fired into life.
Lots of smoke filled the unit we were working in and some fear did creep over me when it did not disappear as fast as I thought it would, however once the oil and paint burned of the manifold everything was fine.
The engine is a lot smoother and quieter than the original B with masses of torque, the exhaust note is deep and throaty and sounds more like a V8 than a straight 4.
This car now out performs my MGTF! What a shame BL did not go ahead and fit this engine as planned.
O Series Power
