MX5 into MGB GT engine swap

[Not_Anumber]

The engine mounting plates are now made up and are welded to the rear of the sides of the crossmember. This places them slightly further back than the B's original mounting plates which were welded to the side rails.
they are based on what others have done but a bit more strength has been added to these in order to err on the side of them being intentionally over engineered.
The new plates, of 3mm steel are first attached along the top and one side to the crossmember. After seam welding them from the front the same was done from underneath, such is the advantage of working with plate this thickness. To strengthen the plate itself a length of bar was bent to shape and welded to the free edges from the rear and then a triangulation plate welded along the bottom. They have been primed with a couple of coats and already look like they were meant to be there. Next will be some seam sealer over the welds to prevent water ingress then more primer and some coats of chassis paint over them and the rest of the crossmember.

A new dual row alluminium radiator arrived the other day. Very tidy indeed but do be careful when ordering as I'd assumed it was from a UK seller and was surprised by a £ 40 additional charge for import duty.

I now need to start looking at where the alternator is going to fit. Some doing this conversion have narrowed the driver's side chasis rail slightly to create enough clearance for it to sit in the donor car's usual location. Others have mounted it on the opposite side of the engine. Now that seems a good idea but though there are a massive array of alternator adjuster brackets for sale nobody actually seem to sell a universal alternator support bracket for the bottom of the alternator. I'll continue to scratch my head on that one whilst I change the spark plugs, waterpump and timing belt on the engine.

[Not_Anumber]

The gearbox mounts are done now. With the way this is now being supported it turned out the ideal parts for the correct height were actually a pair of MGB rubber engine mounts - one is used to the right, supported on the MGB removable gearbox crossmember and the other further rearwards and to the left bolted to the gearbox tailshaft & down to the non removable crossmember. This will help to limit any potential torque twisting whilst working within the constraints of the available mounting points on the MX5 gearbox.

I successfully tested the engine and gearbox can both be taken out individually which is something I wanted to ensure for future maintenance.

Talking of future maintenance the intention is to stick with using standard parts wherever possible so anything that could conceivably wear out or break could be replaced with off the shelf MGB or MX5 parts. Well ok, there is also the matter of my modified MGF electric power steering column but it's motor, brain and sensor are also readily available.

The engine will go back in again this weekend and hopefully a start can be made on the modified exhaust manifold.
This has needed to be cut and welded to change it's angle as otherwise it would not be able to clear the inner bottom corner of the passenger footwell. Others have modified the bodywork here but I wanted to keep bodywork changes to a minimum. The manifold, though it does not look it, is made from a grade of stainless steel. As I dont have anything advanced such as TIG gear and will be using regular MIG Ive bought a small reel of 0.6mm stainless welding wire. My CO2 bottle had just run so I also took the opportunity of going to an 82% Argon,18% CO2 mix which people tell me is essential for stainless but is still perfectly fine for welding regular steel.

If time allows I may start taking measurements for the modifications needed to the propshaft. My current thinking is to have the MX5 prop shortened and either the rear flange changed for an MGB one or a flange adaptor made up.

In the donor car the MX5 running gear had been running with a 4.1 ratio rear differential. As Im keeping the standard MGB rear axle arrangement (at least for now) I wanted to swap the speedometer drive sensor to match the MGB's 3.9 ratio. Luckily Mazda used a 3.9 diff on one of their variants, a sensor of the right ratio came up on Ebay and was a doddle to swap over with just 1x 10mm bolt attaching it to the side of the gearbox.

[Not_Anumber]

Now seemed a good time to look at the clutch actuation. The first thought was just to use the MGB's current master cylinder with the MX5 slave cylinder, which several have done. Whilst some are quite happy with it others have reported that the pedal is quite hard so as I have struggled with hard clutch pedals on some cars in the past I thought I would look into.

The problem arises as the standard MGB clutch master cylinder has a 3/4" bore which moves slightly too much fluid for the MX5 slave cylinder which is usually coupled with a 5/8th" bore master cylinder. Rather than committing serious surgery to the MGB pedal box to fit an MX5 master cylinder that has offset lugs the simpler suggestion is to use a 5/8" bore Girling type cylinder which will fit with only minor modification. This size of cylinder was used on the Triumph Herald and several other cars and is quite easy to obtain. There are even Wiwood and AP racing versions of it available but generic will do fine for me.

I thought about replacing the mix of flexible and fixed pipework with a one piece braided hydraulic line. However the place i was recommended to try for this quoted the thick end of £ 70 which seemed excessive for a pair of standard fittings on a length of braided hose. So instead Ive bought a thread adaptor and will join the 2 sets of pipework as some others have done

[Not_Anumber]

The tubular exhaust manifold has needed some slicing and reforming so that the pipes will clear the bodywork around the passenger footwell. Its now mostly the right shape as a 4 into 2 into 1 and just now needs another stainless elbow to arrive and be welded together.

Time to plan the rest of the exhaust. As Im intending to use the MX5 donor car's ECU it makes sense to have the engine management inputs close to the way they were on the donor which had pre and post cat lambda sensors. A mock-up to check clearances though showed the donor's catalytic converter would have sat too low on the GT adversely affecting ground clearance.
Following some basic research it became clear I wont need to fit the cat. Instead I have ground off and recovered the front, pre cat, lambda sensor's threaded boss from it ready to fit into a new section of pipe. I'l then do something similar slightly further back for the 'post cat' lambda but following recommendations this one will use a spacer to allow it to detect the heat and gases without being directly in the full flow of the exhaust, a known to be successful workaround to keep the ECU happy.

[Peter Cresswell]

By doing this you could lose your VED Historic Tax privilege, and your MOT exemption. It depends a bit on how far you go with the work, but the Mazda engine will count against keeping the status as it is not an engine fitted in period by the factory. You will need to look into both aspects - the Historic Tax with the DVLA and the MOT exemption with the DVLA. I would warn against contacting them directly as this is likely to result in a Q plate, but there is plenty of information on their respective websites, and there was a lot of discussion on here about this topic a few years ago. A search should bring up the threads.
Pete

[Not_Anumber]

Thanks Peter. People who have done these engine swaps have taken a variety of interpretations. One being that because British Leyland no longer exists as a manufacturer of new B series engines it is reasonable to repair an MGB using an alternative suitable engine of similar capacity so long as such change does not materially change the structural integrity, passenger capacity or appearance of the vehicle.

[MGB281]

The MGC used a 5/8” clutch master cylinder, I fitted one to go with my RX8 gearbox. I would like to know why despite using the same gearbox (albeit slightly upgraded) MG chand both master and slave cylinders to smaller bores. This looks like an excellent conversion, personally I have no problem with losing the classic car status and I took part in the consultation where I said all classic cars should have an annual MOT. There are so many MGB’s left that another non original won’t make a difference. I believe that in the event of an accident the insurance company will always look for way out of paying up, the MOT shows that on the day of testing it was roadworthy. What happens after an accident if you have done all the maintenance yourself for ten years and no test?

[Not_Anumber]

A bit of time was lost trying to persevere on the exhaust manifold with a welder that wasnt really up to the task. I had a bright idea it would be sensible to keep 0.8 mild steel wire in my pal's Snap-On 150 welder for general jobs and then use my early SIP Migmate 100 with stainless steel 0.6 wire for the exhaust manifold.

Although this is smaller diameter wire and Im also now using 82% Argon rather than pure CO2 this welder needed to be run on full power to cope with joining stainless steel tubing. And that uncovered this welder's Achilles heel. On full power the wire feed stutters just when you need it not to. A few minutes on Google revealed this as a known limitation and there is a tried and tested workaround to add a separate power supply to run the wire feed so it isnt dependent on the main arc transformer. That sounds sensible so Ive bought a power supply and now just waiting for a 24volt relay to actuate it from.

For the time being the much better Snap On welder has been pressed into exhaust duties and it made short work of welding on the threaded boss for the front lambda sensor. Just a few more welds to do now to finish the manifold which is now quite a sizeable thing, incorporating as it does the B's original 2 into 1 Y piece at the end of the modified MX5 4 into 2. Importantly it can still be removed and refitted by one person working from the engine bay which was my main concern.

The B series engine had the benefit of being able to simply shut the heater valve without adversely affecting the engine cooling. That isnt the case for the Mazda engine which is designed to have continuous water flow through it's heater matrix (with air flaps then isolating it from the passenger compartment). The continuous flow is to ensure proper cooling of the head around the rear two cylinders so this mandated I install some sort of bypass. I searched a few sites for diverter valves, envisaging the need for some monstrous trombone like pipework to make it happen. Surprisingly though it turned out there is a commonly available, cable operated, 4 port, valve assembly known as a TX2 heater valve which is simplicity personified: 2 hoses to the engine, 2 hoses to the heater matrix, cabin heat one way, bypass the other. I just now need to find a longer solid Bowden cable to run over to the heater control.

[Not_Anumber]

My propshaft has just come back from Dave Mac Propshafts in Coventry. MX5 slider at the front, MGB flange at the back. It was away for 2 weeks but they have produced something which looks like it was made by a motor manufacturer.

It can only fit into the gearbox one way due to the shape of the splines. Trying to fit the propshaft yesterday evening on my own, on low axle stands in a high temperature proved tricky and frustrating as the angle the propshaft had to be held at wouldn’t then allow enough rotation for the splines to align and slide into the Mazda gearbox. Next time I will give myself more of a chance by using dabs of paint to help with alignment.

[Not_Anumber]

The propshaft is now in. My pal dropped by and pointed out that there would be much more clearance if i raised the rear of the bodywork NOT the axle. Ah yes, it went in within 5 mins.

I am doubling up on electric fans- for belt and braces with separate circuits rather than to double the airflow.
Turned on it's side, the Mazda fan with it's cowling fits nicely on the inside of the rad held on radiator through mounting ties. Whilst there is no reason to distrust the engine temperature sender, the ECU or the fan it made sense to me to also refit the original MGB electric fan back where it belongs in front of the rad. This will be operated by a temperature switch at the top of the rad, similar to the standard arrangement. I went for a slightly higher temperature range so that it shouldnt overlap with the main fan in normal use.