How to fit the B Series 5-Bearing 1800cc Engine into a Z Magnette

This article draws on the experiences of a growing number of Z Type Magnette owners who have successfully fitted the 1800cc 5-bearing engine. We are grateful for their willingness to contribute a useful record of their experiences via the Register web site and the bulletin board. With the advent of bulletin boards on the internet, the exchange of knowledge has improved exponentially in recent years and whilst some of the pioneering work dates back to the days of “36,000” and  “Magnettics” , the MGCC Z Magnette Register’s magazines (no longer published), it is largely the internet that has brought the refinements and the details that will spare the reader from those head-scratching moments or worse, the feeling that it has all gone wrong.


Soon after the introduction of the MGB, the factory realised that with the increased power output generated by 1800cc the original crankshaft design was a weak-spot. It had only three main bearings, which meant that there was more scope for the crankshaft to flex in use because there was no main bearing between each pair of cylinders. That is why the designers at Longbridge decided that the engine had better be strengthened. The change was probably prompted by actual failures of the 3-bearing 1800cc engine. A new crankshaft was therefore designed with five main bearings to give better support to the crankshaft . This meant that the spacing of the cylinders changed, along with the degree of off-set in the connecting rods. The benefit was a much tougher engine that has proved capable of considerable power tuning. Whilst earlier B-series engines with three main bearings will drop into a Magnette engine bay with relatively few headaches, they lack the refinements and improvements that the later design offers. The 5-bearing engine brought a number of design improvements that were radical departures from what had gone before. This engine was used in a number of other BMC/Leyland vehicles, so the MGB is not the only source of parts.


It is fair to say that anyone who has done this modification will admit that it was more complicated than he expected. So why persist with the 5-bearing modification? The simple answer is that the 5-bearing engine is significantly stronger than the 3-bearing and for the owner that wants more performance, the extra two main bearings bring a lot of reassurance.

The main areas of “challenge” in this modification are:

  1. The bottom of the block has a different profile, so the sump bolts are in different positions and the flange is different. The Magnette sump is not a straight swap, nor is there an easy off-the-shelf alternative. In the Magnette, the main part of the oil pan is in front of the cross-member, whereas the 1800cc applications generally have it at the rear.
  2. The oil pump, although fundamentally the same component as on previous engines, protrudes further below the sump flange and has different intake and strainer arrangements. Only the Magnette strainer pick-up is suitable for the Magnette sump but the strainer, when fitted to the MGB oil pump will foul the bottom of the sump unless the intake pipe is shortened. There are no lugs on the block casting to fit the strainer legs to. Significant alterations have to be made to the block to install the Magnette strainer and move the pick-up position to the front of the engine where there is space for it in the sump.
  3. The 1800cc engine water pump has a longer nose-piece, which takes the fan very close to the Magnette radiator.
  4. Unlike the 3-bearing engine, the 5-bearing has no alternative pre-drilled hole for the dip-stick, so a bespoke hole has to be drilled and tapped to take it.
  5. The 5-bearing back-plate carries the starter motor in a lower position which means that it clashes with the steering column on RHD cars. If you want to fit a 3-bearing back-plate to overcome this, it has no housing for the crank-shaft rear oil-seal on the 5-bearing engine.
  6. On LHD cars, the steering column is in the way of the exhaust system. There is little point fitting a 5-bearing engine then strangling it with a standard Magnette exhaust, so you have to devise an exhaust manifold (header) that gets the pipe past the steering column and behind the cross-member, unless you are prepared to take on a modification that involves cutting the inner wing.
  7. The increased output of the 5-bearing engine makes it desirable (though not essential)to fit a stronger gearbox than the standard Magnette component. This can involve modifications to the body around the front end of the transmission tunnel and a custom-made lever to get it into the right position in the cockpit.




There are two possible approaches to this. The first is to cut the top of the 5-bearing sump off and mate it to the bottom of a Magnette sump. The second is to fabricate an adapter plate that overcomes the problem of mating the different profiles. These options are described below:

Cut and Weld. Experience has shown that there is a risk in using this method: the welded joint in the modified sump may be porous and oil is drawn out by capillary action, leading to persistent irritating drips.This is the method:Cut the top flange from the Magnette sump with about ¼” of the sump wall all round. Keep the cut as clean and straight as possible. Then cut the flange from the 5 bearing sump at exactly the same level relative to the flange face. The lower part of the Magnette sump is then matched in shape and carefully welded to the 5 bearing top flange, ensuring that the same overall sump depth is retained (important or it may foul the cross-member). To avoid oil leaks it is important that the flange face remains perfectly flat and is not distorted by the welding heat. The easiest way to do this is to bolt the flange onto the up-ended crankcase, having removed the gasket, and to complete the welding with it in position. Some heat damping is probably advisable to avoid excessive heat transfer into the casting. If you have to send the sump away for this work to be done, ask your welders to make sure the flange is trued up before it is returned to you. These are the relative dimensions:

Adapter Plate. This option has the advantage that it not only mates and seals the Magnette sump to the 5-bearing block but it also adds depth to the sump to accommodate the strainer and provides lugs to fit thestrainer legs. It is relatively expensive because the manufacture is best carried out by a workshop with a plasma cutter. We are indebted to Allen Bachelder for evolving this solution during his own conversion, though I apologise to anyone else who came up with it earlier and also deserves credit. Allen’s own separate article provides an excellent description of the process. Click here  to view it. The photo shows an example of the plate that Allen developed. The plate needs to be about ¼” thick and is designed so that it accommodates the profiles of both the 5-bearing block and the Magnette sump. There are four sump bolt positions that are common to both configurations, so this enables you to bolt them together and observe the differences. New holes have to be drilled and tapped where they do not exist on the 5-bearing block and the obsolete holes plugged. The Magnette sump provides a drilling jig for the new holes so with it held in place by the four common bolts, the new holes can be made through the plate and into the block with the certainty that they will line up. Where there is block casting outside the width of sump flange, you need to flatten the edge of the flange and weld on enough new metal to accommodate the new hole. The plate will also need drilling so that the dipstick will pass through it. (See below) Finally, it has been found from bitter experience that the oil-drain sleeve inside the rear of the Magnette sump will probably foul the 5-bearing crank webs, so remove it. It serves no useful purpose in the new set-up. When fitting the sump, you put a gasket on both sides of the plate.

The increased sump depth created by the plate and the extra gasket will probably take the shallow part of the pan very close to the top of the cross-member. In order to preserve the original clearance you can raise the engine with spacer packing under the rubber engine mountings. Take care with this because changing the engine angle may also affect the clearance of the propeller shaft above its restraint strap below the transmission tunnel.


You use the original pump body from the 5-bearing engine but replace the top plate with the Magnette component, so that the strainer take-off is correctly positioned. Check for wear as per the workshop manual and carry out any reconditioning that proves to be necessary.

If you have used the cut-and-weld sump modification, you will need to cut and shorten the intake pipe so that the strainer head is at the same level below the block as it is on the Magnette. Needless to say, this weld needs to be perfect with no leaks. The Magnette sump strainer is supported by four set-screws located in holes tapped in the crankcase flange. These holes are provided in most 3 bearing engines but not in the 5 bearing unit. It will be necessary to make a couple of 16 gauge mild steel brackets. Attachment presents a problem but one way of solving this is to drill and tap 1/4" UNF holes in the lower faces of the first and second main bearing support webs and secure the improvised brackets with set screws and tab washers. Make sure that sufficient clearance is provided for the dipstick to pass through when the new brackets are in position. Drill a hole in the bracket if necessary.

If you are using the adapter-plate solution, you can incorporate four lugs on the inner edge of the plate to take the original support legs. Just mimic the position and spacing of the four equivalent holes on the 3-bearing block. Allen Bachelder’s article shows very clearly how this is done. The thickness of the adapter plate and the two gaskets should be enough to incorporate the depth of the strainer head but minor easing of the sump floor with a hammer and dolly may be needed.


First remove the dipstick guide tube from its present position using a wide-bladed screwdriver or a strip of sheet steel in the slots on its top edge. The 5 bearing engine does not have the tapped hole for the alternative dipstick position used on the 3-bearing engine  but there is a blank boss in almost the same position. Using the Z Type sump as a guide, carefully calculate the angle required to bring the lower end of the dipstick onto the stop-plate which is welded to the sump floor. Drill through the boss using a 8.75mm or 11/32" drill and tap 1/8"BSP to accept the dipstick guide tube. You may want to entrust this to a machine shop.

If retaining the original dipstick it is important to ensure that the bottom is locating on the stop or a dangerously false reading might result. Once the engine is re-assembled and in situ, you can check the dipstick calibration by filling the sump with the known maximum capacity and checking how this relates to the dipstick MAX reading. If it is not close to being right, the dipstick will need to be modified to bring it into the correct relationship with the oil level. Possible solutions include:

  •  brazing an extension onto the stop-plate
  • adding new markings to re-calibrate the dipstick
  • brazing a retaining collar on the dipstick stem to hold it at the right level in relation to the sump bottom.

 Note:  All references to drilling and tapping assume that the engine is on the bench and will be flushed clean before reassembly. You are strongly advised not to attempt this with the engine assembled in situ.


The MGB engine employs a pump which is about 5/16" longer in the nose, bringing the fan blades much closer to the Magnette radiator. The radiator should be moved as far forward as it will go in its mounting  to ensure maximum clearance. The mounting holes may need to be extended to achieve this. Even so, the clearance is reduced and it is important to check the fan blades and correct any run out or imbalance which might threaten the radiator. Always renew engine mountings in these circumstances because any softness will certainly allow the engine to move forward under conditions of heavy braking and contact may result.

Allen Bachelder mentions that an early 70s double-pulleyed pump fitted to USA cars with the “smog” pump system has a shorter body and provides a good solution.

It is essential to use the MGB pulley with the MGB pump so as to retain fan-belt alignment. Check it with a straight-edge to ensure the bottom, fan and dynamo (or alternator) pulleys are in perfect alignment or fan belts will wear rapidly.

It is possible to dispense with the fan blades by fitting an electric fan in front of the radiator, but this involves removing the bonnet catch platform and securing the bonnet by alternative means such as external clips. (See Cooling System Modifications section).

Do not be tempted to retain the Magnette water pump with the 1800 engine. It has a smaller capacity and the impeller will not fit snugly in the block, leading to inadequate water circulation and over-heating.

In order to preserve the correct alignment between the engine thermostat housing cover and the radiator connection, the original Magnette elbow should be used. Others will fit but risk inducing a kink in the top hose or insufficient "give" to accommodate engine vibration.


To some extent, the solution to the lower starter position will depend on your choice of gearbox.

If you decide to retain the original Magnette gearbox, you can use the Magnette back-plate but it will need modifying a) by being machined to accept the MGB crankshaft rear oil seal and b) by having the oil drainage channel on its inside face filled with braze and finished flat to avoid leaks. There are mixed opinions as to whether the Magnette gearbox is up to the job of handling the output from the bigger engine. There are examples of this combination lasting for years trouble-free but if you enjoy driving “enthusiastically” or want a competition car, then prudence suggests using a gearbox designed for the job.

If you decide to fit the MGB gearbox, then you will retain the 5-bearing back-plate but you will need to make a modification to avoid a clash between the starter and the Magnette steering column (RHD cars only). The recent development of smaller, geared starters has eased this problem, thanks to their more compact size. You may still need to remove some metal from the back-plate and the bell housing flange where it passes closest to the steering column. The later MGB engines had a pre-engaged starter and it has been found that a Marina starter with the solenoid on top will fit in a Magnette conversion. If you fit this starter, you will have to fit the MGB inverted oil filter and unless you are fitting an oil cooler, you will need to obtain a suitable pipe to connect the filter head to the oil connection on the block. The original Magnette pipe will not fit but a length of flexible pipe will bridge the gap. Moss USA can supply one off-the-shelf but the Moss Europe catalogue does not carry the same part. Many suppliers will make up pipes to your design.

If you plan to fit one of the gearbox conversion kits that allow you to fit a more modern 5-speed gearbox (e.g. Hi-Gear Engineering kit that uses a Ford T9 gearbox or the Riverside kit that uses Datsun), then it is best to ask the kit supplier for best advice on choice of backplate. See also the separate article on fitting one of these kits in the Gearbox Modifications section.


The tappet covers are on the side of the engine block, each retained by a single bolt and sealed with a cork gasket. To ensure correct appearance and to avoid clashes with the exhaust, fit the front cover from the Magnette engine. The ZB type is best with its upswept breather pipe. Clean the pipe before refitting. Do not be tempted to overtighten the bolt or the cover will distort and leak. On some engines you will need to fabricate bracketry to hold the pipe because on certain B engines the blanking plate that covers the (unused) mounting point for a mechanical fuel pump was deleted, so there is no stud to mount the bracket on. If you are having trouble with the breather pipe obstructing the exhaust header the front cover with the breather can be switched to the rear.


You can retain the original Magnette exhaust manifold (header) but it does not offer the best solution and to get most benefit from the new engine, an exhaust system with bigger bore size is desirable. If you are fitting a bigger exhaust then it would be false economy to retain the original manifold. The MGA manifold offers better gas flow and for RHD cars it has a better angle at the bottom joint, allowing a specially-made front pipe to get past the gearbox with a smoother flow. (The original pipe design was used on both LHD and RHD cars but the front pipe has a kink that is only required on LHD cars, to route the pipe round the steering column, which is on the same side).

Alternatively, if an exhaust specialist is being employed to fabricate a new system, he could also be asked to make a long-centre-branch manifold to mate to the new system, though this will be pricey. The problem is compounded on LHD cars because you have to get past the steering column as well as the bell housing. John Perkins' solution (HERE) was to fabricate a header that is angled forward initially to go down past the chassis leg quite near the front of the engine before turning to the rear so that it does not have to negotiate the cluttered gap at the rear of the engine bay.


The fitting of the 1800cc engine may prompt other modification, such as carburation and ignition but they are not essential. All standard Magnette accessories will fit the 5-bearing engine if you want to retain the look of originality under the bonnet.  Alternative  options are considered in separate articles on the site.


#1 Trevor Jones 2015-01-27 17:30
The adaptor plate shown in the above article is available in the UK , Allen Batchelder very kindly allow me to reproduce his design in the UK, it makes the fitting of a 5 Bearing MGB engine so simple.Thanks to Allen I still have a few left, happy to ship them anywhere. email me on . The full "Derrington exhaust system is still available at £425 complete,plus carriage see exhaust section. Trevor Jones

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