Convert the MG Magnette ZA (BP15GA) engine to full flow oil filtration








The original 1489cc B series engine, installed in the MG Magnette in 1955, was in one major respect a downgrade in the XPAG engine it replaced. For reasons unknown the engine was produced with only a bypass type oil filtration system. This resulted in the lubricant being filtered on a piecemeal basis through a very small bleed from the main oil gallery. It is estimated it took around 10 miles for all oil to be passed through this orifice and filtered.


While this was just about acceptable in the 1950s, modern engine oils incorporate detergent chemicals which suspend foreign matter within the oil rather than allowing it to settle out into the bottom of the sump. This results in any debris continually recurculating through the bearings multiplying the damage and wear.


With the improvement to the BP15GB model, and subsequently all B series engines, this system was abandoned in favour of a full flow filtration system.


It is possible to retrofit the full flow oil system to the earlier engine, this article details my method and experience in carrying out the modifications in my own shed with limited tools. This will also allow the installation of an oil cooler if required.

 Before you start


The parts required are as follows:

A spin on adapter from any type B series engine and a new oil filter.

I used the upside down version from an 18V engine because it was all I had in the shed.

If you wish to use the original ZB and MGA type separate filter and bowl then you will need all parts from that engine including the bolt through the filter bowl which does not have the bleed orifice.

The special adaptor from the MGA engine (Moss Pt No. 1H922) and three ¾” copper crush washers.

Either the original steel oil pipe from the donor engine with it’s banjo fitting


make your own pipe with some 15mm copper or brass  water pipe, two 90 degree 15mm to ½” BSP tap fittings and a brass 90 degree ½” BSP male to compression fitting adaptor (all from the local DIY store).

½” BSP tap and a ¼” BSP Tap (preferably taper and plug).

A 19mm Drill and an 11.8mm drill (I used a 12mm)

The taps and drill were obtained very cheapley from


The Modification

Firstly, a short note on the task in hand. Please carry out all the work in a safe manner with due regard to normal safe workshop practise. Check the safety information on the bar to the left.

I did this work on a removed engine which had been hot tanked and rebored. On reflection, it would have been better to do the mod first and have the tanking done afterwards to further reduce the possibility of debris in the oil system.

Locate and remove the oil pressure capilliary and the threaded adaptor from the rear right side of the engine.

Block the cross drilling from the pump pressure relief valve to the main oil gallery to prevent swarf & debris falling into the oilways. I used a small ball of oiled rag.

Pict_004Using the 19mm drill carefilly drill out the casting approximately 15mm deep (Same depth as the threaded portion of the adaptor + 1mm). Take care the drill doesn’t wander, particularly as you will be drilling through the side of a brass welch plug to the rear of the main gallery.

I put a compressed air supply into the oil filter housing to ensure the swarf was blown out of the hole past the drill bit, rather than into the engine.

Pict_011Carefully tap the ½” BSP thread ensuring all swarf is removed. A magnet was helpful for this but unfortunately the brass bits were quite difficult to remove. I found I also needed to carefully drill down the cross drilling to slightly widen the hole to accept the end of the adaptor, Several trial fits are the best approach, Don’t install the copper washer until you are able to screw the adaptor fully home.


Pict_014Next drill a 12mm hole in the adjacent  unused casting to take the oil pressure capillary adaptor.

It should be quite easy to see the alignment of the main oilway through the large hole. Try and aim for the centerline.

Thread this hole to ¼” BSP


Pict_013Carefully clean out the oilways ensuring all swarf is removed. A magnet was helpful for this, unfortunately the brass bits were quite difficult to remove. Don’t forget to unplug the cross drilling, I pushed my rag plug up from the pressure relief valve hole with a stiff piece of wire.


Install the two adaptors. That’s all the hard work done. The next part depends on your choice of filter adaptor and whether you wish to add an oil cooler.

If you have managed to obtain all the parts from a donor engine then the rest is just assembly work. If you are adding an oil cooler then just connect the oil gallery adaptor and the new oil filter connection to your two flexible pipes from the oil cooler (Note: If the cooler is mounted with one connection higher than the other, the pipe from the oil gallery adaptor must go to the lower connection.).

As the picture at the top shows, I used the upside down spin on adaptor which was used on the later 18V type engines. This does make filter replacement much easier, if a little messy!

The steel pipe which should connect from the oil gallery adaptor to the filter adaptor was missing on the donor engine so I decided to make my own pipe.

Pict_016This pipe is very simple and is just soldered plumbing fittings from my local DIY store. There is a single bend of around 60 degrees and two standard 90 degree kitchen tap comnnectors (the type which have a little fibre washer in the package). To join the filter adaptor I used a brass tap fitting with a 15mm compression fitting. 

I have now run the car for around 2000 miles with this modification and it has held up very well. I have around 70PSI oil pressure and there is no leakage. I am certain the design is sound and it is filtering the full flow of oil.

Now I have carried out the work, I believe it would be possible, with care, to modify an engine in situ. the access is quite good in this area and the only major component that would need removal is the distributor.

I hope this article is of some use. If  anyone needs any further details or clarification, I can be contacted on the MGCC forum or as PaddyR on the Magnette Technical website


Paddy Reardon

22nd January 2011


#1 Bruce Hall 2017-02-03 20:31
Anyone know where I can purchase an original inverted spin on oil filter conversion for an MGB?

You have no rights to post comments