MX5 into MGB GT engine swap

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Interestingly the existing heater control bowden cable was very easy to reroute from the right to the left - which is where the new stand alone heater bypass valve now sits.

The control works the new valve as easily as the original on the B series engine though the opposite way round- turning the control to hot closed the heater valve. These 'TX2' type heater valves are plentiful and inexpensive but although they were apparently made in both push to bypass and pull to bypass variants at one time, only one type is currently available- and of course this is the one which is the wrong way round for an MGB's heater control.

As I didnt want to disturb the cabin control or put up with cold really meaning hot I started looking at a way of modifying the valve itself. There is a moving lever across the front of the valve which rotates to open and close the valve. This has a 'pin' at one end to wrap the control cable around. It was straightforward to quickly tack weld a small spare bolt as an alternative pin on the other end of the lever. With the end of the inner bowden cable attached to this and the outer clamped to the valve body it now works the right way round to suit the MGB temperature control in the cabin. Two hoses into this from the engine and 2 hoses out to the heater. This pipework is approx 16mm and the MGB heater matrix needs 12mm so Ive used a couple of readily available 16mm to 12mm in line reducers for compatibility

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After some to ing and fro ing I decided to reuse the MX5 air cleaner assembly which incorporates an airflow sensor and fit this in place and function close to the way it was designed to be used on the donor car. It is mounted on the left inner wing by means of some bolts and self made spacers.
The filtered air output from this is taken care of by the donor car's large plastic hose which runs across to the right of the car, level with and just behind the top of the radiator. This fits to the front of the inlet manifold.

Aside from it's mounting, the airbox needed a couple of small modifications. The position of it's air intake on the left of the airbox worked in the donor car but in the B this would have been up against the side of the left inner wing. The solution was to blank off this side intake with a section of flat plastic and some appropriately gungy sealant (windscreen type stuff).
A large round hole was then cut in the front of the airbox as a new air inlet, below the level of it's airfilter. A convoluted plastic hose was then run through from here to the front of the car. As mine is an RB this needed to pass through the left hand radiator mounting 'cheek' panel. I used the time honoured method of drilling a ring of holes then joining them up and filing the rough edges smooth. By luck I found a handy piece of plastic trim to tidy up the engine bay side which was exactly the right diameter for the air hose to pass through. Though it looks purpose made the circular trim was actually part of an old TV stand. All that remained was to resite it's temperature sensor which would have clashed with the position of the B's sliding bonnet strut. This was no more complicated than drilling another hole of the same size about an inch lower, pushing the sensor into this and then filling in the old hole with a brass plumbing stop end fitting (preferable to a blanking rubber grommet here due to the amount of suction in the air box assembly).

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I knew from the outset that raising the engine slightly to gain clearance for the alternator against the right engine bay side rail was going to risk losing clearance between the top of the engine and the bonnet. The culprit being the VVT valve assembly on the centre of the camshaft cover.

I couldn’t find any suitable bonnet bulges that I really liked the look of so ended up buying a fibreglass replica C bonnet from Generation Glassfibre. Im quite pleased with it so far though know Im probably going to have to do some work to get everything to line up once I start on bodywork and paint.

No going back now as someone has bought my repaired steel B bonnet

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Carburettors have the benefit of a float chamber so can cope if there is a momentary interruption of fuel supply from the tank. I will be using the Mazda donor car's fuel injection setup though which needs a constant flow of fuel as well as quite specific pressure requirements so I needed to look further than the original SU fuel pump. After looking at what others had done I decided to use an in tank pump and spotted that someone had used one from a later MX5 (Mk3 /NC). This is an entirely different beast from the fuel pump that was in my Mk2.5 (NBFL) donor car in many ways and it offered the benefit of being more straightforward to fit and modify but also it has an integral 'swirl pot' (the plastic bucket casing you see here) which is a fine thing to have during cornering as it keeps some fuel available for the pump irrespective of the rest of the fuel being busy sloshing around the tank. This Mk3 pump assembly also usefully has an internal regulator so it doesnt need a separate return pipe to the front of the car - ideal as the MGB only has one fuel pipe from front to back hence one less thing to worry about having to make.

The pump assembly needed some modification in order to fit the MGB tank as it was too tall as standard. The rods at the side were cut down and the length of the spring reduced. Then it's fuel sender was taken off and mounted further down (the float rod was suitably shortened too).

After rinsing out the MGB fuel tank a hole was then cut in the top for the new fuel pump and it was trial fitted.

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A thin reinforcing ring was made up from a bit of ordinary mild steel to reinforce the edge of the hole around the pump and was welded around it's circumference to the tank to help provide a more rigid surface for the seal of the pump to fit down to.

Focus then moved onto the pump retaining ring. This was a standard Mazda Mk3 part intended to fit on the top to hold the pump down to its original tank. I also needed something to go on the inside of the pump as an anchor for the retaining bolts so I got hold of a second retaining ring and trimmed away the central part away leaving just the outer ring of bolt holes.
A slot was cut so this so it could be passed through to the inside of the tank, the top ring was put in place at the top and after some careful marking, bolt holes were drilled through the tank with the bolts then trial fitted to tie the upper and lower rings together with the top of the tank as the middle layer of the sandwich.
The lower ring was removed and the bolts were welded to it. After some coats of fuel resistant laquer and sealant it was put back inside the tank with the bolts pointing upwards through the holes.
The top ring was then trial fitted as shown here. Thats it for now. So it now just remains to take the top ring off, fit the pump with fuel proof sealant and refit the top ring.

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I needed to cut an access hatch in the boot floor above the new pump but wanted to limit the height this needs to be in order to avoid compromising space for the spare wheel. The work around was to discretely lower the fuel tank by approx 3/4".
Others have lowered tanks for a variety of reasons with no issues. The base of the tanks wont sit any lower than the after market high capacity touring tanks so there will be nothing to worry about with ground clearance.

The spacers are tack welded to the tabs of the tank - the eagle eyed may have spotted they are made from shortened old wheelnuts.

Nine 40mm M8 bolts with nylock nuts will now be used to secure the tank to the car. These replace the original 5 nuts and bolts and 4 fixed studs which were not long enough. For completeness Ive used larger steel washers in the boot for those bolts which have replaced the 4 studs.

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Lowering the tank slightly has allowed the fuel hose to exit under the boot floor rather than within the boot space. The lid over the cut out area now does not need to be as high which has the benefit of allowing the spare wheel to sit almost flat.

The sides of the square cut out area were edged with some hefty angle iron to avoid any loss in rigidity of the boot floor.

Though the fuel consumption should be the same or lower the fuel pressure will be higher due to fuel injection. The original 5/16" stainless steel front to rear fuel pipe is fine to be reused but it's no longer ideal to rely on the original method of rubber hoses being pushed and clamped onto it. Looking around Ive managed to source a pair of 5/16" compression to 5/16" barb fittings which meet fuel injection requirements.https://www.ebay.co.uk/itm/800352922556.

I will then be using R9 type reinforced hose at each end to connect these to the filter and fuel pump at one end and the fuel rail at the other using these type of fittings.https://www.amazon.co.uk/dp/B0FSWHGNPT? ... b_ap_share

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Mx5 throttle cable is now connected to the MGB pedal.
This was straightforward other that the need to change the 'pedestal' on the horizontal part of the bulkhead directly above the pedal. Access to this from the engine bay proved fiddly on late cars like mine with dual circuit brakes but given some time I managed to get the original conical one off and replaced with a more tubular shaped one that correctly supports the different end shape of the outer cable.

A bracket has been made up and fitted to hold a clutch switch as the donor car's ECU required this as an input. This is held in place using the bolts which go up into the side flange of the pedal box so avoided drilling any new holes.

[Paul Scott]

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