O Series (O1 - 2000cc) Feature Car

March 2010 (written December 2009)

Stephen picks up the story in his own words...

In theory, you could source a good reconditioned O Series engine for a lot less than a B Series engine, typical prices are around £100 or less.  Aim for the 2L which has factory figures of 105bhp at the flywheel, typically 10-15bhp more than the standard B Series.  However, both these figures are matched against bench tested units so expect a more modest figure of approximately 95bhp at the flywheel to begin with. 

Tuning wise, there is not a lot to gain with estimates of 15%.  Keith Dale has reportedly over 100bhp at the rear wheels approximately 120bhp at the flywheel.  For the comparative cost against B series tuning the O series seems the better bet.  However, in comparison with the B series engine, the B series head modifying would invariably allow greater volumetric air/fuel to enter the combustion chamber.  The head on a B forms a proportion of the engine's capacity.  Enlarging the head chambers increases the volume of the engine (albeit marginally).  However, on the O series the head is flat.  Therefore with the O series the combustion occurs entirely within the chamber.  To make this possible the pistons are dished.   People like Peter Burgess can get a bit more from the head.  This occurs from better gas flow rather than additional capacity.  The single over head cam (SOHC) non-cross-flow head is the limiting factor for further non forced (normally aspirated) gains.  It is possible to fit an SOHC EFI head to this block.  Although reducing the performance marginally, this would give better emission and fuel returns.   And it is plausible to fit a later a later twin cam head.  However, the machine work and costs associated don’t make sense when for 1/3^rd of the cost a whole new M series engine can be purchased.  Lastly, it is possible to fit the much coveted Garret T3 Turbocharger to the engine.  However, specialist advice is required to ensure that compression ratios and pistons are up to the job!!

To recap, the O1 engine in standard form is a straight-forward fit because:

• The sump is a similar shape to the B Series (note - this sump will not fit later OMT engines)
• The filter head on the oil pump is located on the side of the block (note - this pump will fit O2 engines to no. 20HAxxxx but will not fit the O2 20HB or 20HC and newer later owing to the different diameters of the later crankshafts.)
• The distributor is at 90° to the cam in the middle so avoiding complications with it fouling the bonnet or heater.
• The distributor and carburettors are easy to link up to the existing fuel and electrical feeds.
• The existing B series starter motor, flywheel, clutch and gearbox are used once a Sherpa back plate is sourced*. 

*The engine featured was mated to a 5spd LT77 box and the whole lot came out of a Rover SD1 2000 model.  It had been overbored to approximately 2100cc.   The other O series a 1.7 engine was from a Sherpa Van, so the backplate gave a link to put the 2L engine straight onto the BGT 4spd o/d box keeping things simple.  That said a B Series backplate could easily be adapted to suit the purpose. 

Before embarking on cutting up the MGB a series of trial fits and modifications were undertaken.  Step by step the slow process built up confidence that the engine would both fit and function.  This is not new MGB ground, however, there were elements of research and development which have meant that over the years the fit could be refined.  Below are photographs of the sequence followed during the fit into a B.

First the removal of the LT77 box and SD1 backplate and replacement with Sherpa backplate and B gearbox, remembering everything from the backplate rear is B.

Next trial fitting into a friends 1960’s scrap roadster (this was attempted in March 2008).  In this case the reference point for the whole assembly is the rear mounting points of the gearbox cross member under the car.  This allows the maximum space in the engine bay as possible as the front of the gear box will clear the bulk head.

With the addition of the existing 4-2-1 exhaust manifold the system seems to clear the n/s chassis leg.  However, the fit cannot be completed as the existing bend to the pipe clips the passenger foot well.  Modification of the manifold is required.

Supporting the engine without the front cross member in place allows an estimate of the engine height to be gained.  In this photograph the engine is too high.

Clearance from the front cross member to the engine sump is accounted for by the sump design.  Should fit nicely there.

Having made allowance for the engine height and fit, basic brackets are constructed from very thin steel to allow some degree of bending and further refinement.  Allowance in the bracket design has been made for later type r/b mounts which beneficially use only one central bolt and nut to hold them on the proposed chassis leg mount.   The new engine mount in the engine bay will sit on the chassis leg and to the inner arch.  (Confusingly in the photograph further rubbing down of a rust patch is shown on the n/s inner arch).

The corresponding engine mounts are shown in the photograph below.  The block allowed quite a few degrees of freedom in the fitting stage.  Mounts could be fabricated to various designs.  However, items such as the steering column and oil take off plate restricted the designer's flare.  To keep it simple the mounts were made from drilled bent metal and two triangular fillets.  Stephen made the first set, which were then manufactured for the princely sum of £60.  There are likely to be other manufacturing outlets who will surely beat this price, but the time involved and the welding seemed to push the price up!! (January 2009)

Up until this stage the engine had never sat on its (welded in place) mounts.  Once the initial trial fitting was completed time didn’t allow a full blown fit.  Blocks of wood and pulleys suspended the engine whilst the car was moved from workshop to workshop to have body work undertaken. 

The O series engine specification always appealed to Stephen as it wasn’t a wild highly tuned 4 pot or a V8.  It allowed a better torque curve, more power and added peace of mind than a B Series for motorway driving at a reasonable cost.  This particular O1 unit was purchased off e-bay in early January 2008 from Gravesend , Kent (quite a return journey from north east Wales ).  The unit had been totally rebuilt in 1994.  It was run for 5 minutes and then stored.  Why this occurred remained a mystery and Stephen didn’t believe the story until nearly £500 of receipts turned up in the post in late February.  Nevertheless, once the engine was stripped of the LT77 gearbox, power steering pump and auto choke carburettors the engine was stripped, inspected and rebuilt with new seals throughout by an ex-Leyland mechanic.  During inspection the news was promising that no corrosion of the bottom end had occurred from old oil.  And that hone marks could still be seen in the bores.  When doing this conversion you must remember to replace the gearbox spline bush at the rear of the crank.  The LT77 input shaft was slightly larger than the B series box and required a smaller diameter (bigger) bush.  With the backplate on, a new lightened flywheel was installed along with a stage 1 race clutch.  The thoughts were that 1) a smaller flywheel mass the quicker the engine would rev.  The draw back here is that the engine might prove to be lumpy on idle.  This, for the moment is an unknown. 2) A new Borg & Beck stage 1 clutch hopefully resembles a good quality old style Borg & Beck clutch making allowance for the reduction in build quality.  The potential draw back here is that here-say about build quality proves to be wrong and the clutch is difficult to live with. 

The photograph below shows the heritage of the 2L O1 series from the earlier 1.7L O1 series.  Note the blanking plate in the head on the rhs, allowance for a mechanical fuel pump.  The photograph also shows the inlet/exhaust, inlet/exhaust, inlet/exhaust, inlet/exhaust format for the cylinder allowing better breathing over the B siamesed inlets and central exhaust port.  The fit of the engine mounts described above can also be clearly seen.  Not clearly seen is the oil take off covered by a blue glove (October 2009).

The acid test is shown below with the engine sitting correctly in the centre of the car, upright and on its own mounts.  The chassis rail to engine mounting is now clearly shown.  Also the photograph illustrates the issue with the exhaust manifold which is yet to be resolved.  Potentially a brace bar will be required to stop the engine rocking over; however, this will be resolved during the next round of R&D.

Currently the wiring and brake lines are next in line for attention ahead of the engines final fit.  One minor issue with the last of 9 backplate and gearbox bolts also needs resolving ahead of its planned MOT in May 2010 (December 2009).

And just a cheeky preview of the final bodywork and home to the O Series...

Check back for more progress in future updates.