MX5 into MGB GT engine swap

[MGB281]

When Ruth and I went to OZ and NZ we used the Off Topic forum on the MG Ex to keep our friends and relatives updated, although none of them has recently owned a MGB they were able to follow us and be updated without us sending countless emails. We got quite a few comments from those in the USA as well.

[Not_Anumber]

Hospital visiting for a family member, work and the poor weather has limited progress but I am a stage closer towards lifting the engine and gearbox out of the MX5.
I have been labelling connections on the wiring loom whils disconnecting to make it clearer later on when I need to combine the MGB and MX5 looms. Current thinking is to keep the B's original wiring for the lights, horns and wipers as the loom had already been modified to work with the MGF stalk switches, necessitated when I fitted the MGF EPAS steering column. Other wiring decisions are yet to be made but theres more than one way to skin a cat..
Im closer to understanding how to fabricate new engine mounting plates and now need to source some 1/8" thick sections of steel plate or box section. When made up these will be welded to the sides of the B's crossmember rather than to the rails as the original mounts were.

[MGB281]

It makes a lot of sense to use the crossmember, initially I tried using the chassis rails but couldn't make it work and went the crossmember route using BMW E30 engine mounts. It's a common solution when fitting American V8's. make sure that the gearbox output is in the centre of the tunnel when you fit it, with the RX8 gearbox the gearlever ends up about 20mm closer to the driver.

[Not_Anumber]

Thats a very good point- to focus on centralising the output shaft rather than the gearstick.
Trimming back the edge of metal on the tunnel cutout and modifying the console to accommodate a slightly relocated gear lever is very little work to avoid unecessary strain on the propshaft UJs.

Its interesting to see how different people approach these things. Geoff Rawlings' is selling an uprated K series engine & Ford Type 9 gearbox in the Parts For Sale section here as a B transplant kit for £7.5k . I wish him luck selling it though it's not a route I could afford to take for an engine swap.
I'll be surprised if I end up much more than a tenth of that out of pocket by the time Im done: Selling my B series engine, OD gearbox and starter brought in £ 100 over what I had to pay for a good MX5 donor car. So far Ive recouped another £ 250 on unwanted MX5 parts and have really only just got started on parting it out.
Im expecting the largest costs will be having a propshaft made, an exhaust and a new radiator that fits. When I get a bit further into the build I may not be able to resist optional spends such as a stainless exhaust manifold and an after market ECU though.

[MGB281]

That's the way to do it, when I bought my S type I gat the engine for free after selling the unwanted S type parts for more than £700 more than I paid for the car. A new prop shaft is quite expensive but I was lucky in that my GT axle had a MGC flange on it, I have just acquired a Scimitar GTE prop shaft with the MGC UJ at one end, I have been quoted £120 to fit the RX8 UJ and balance it. There are bound to be other cars from the seventies running MGB sized flanges.

[Not_Anumber]

The chap I mentioned who is developing parts to be able to use most of an MX5 diff and IRS assembly has finished his prototype and is starting road trials so if thats sucessful I may end up having the end to end length of a Mazda prop shaft altered rather than changing the flange.

Im still waiting for a sufficiently dry day to remove the engine and gearbox from the donor car despite now being almost disconnected and only an hour's work away from being able to lift out. At this rate the donor car may remain as a garden ornament for some time before I have removed everything I need to keep from it.

[MGB281]

Keith Tanner of Flying Miata’s put MX5 front suspension on his MGB GT but his donor car was a Chevrolet Camaro.
https://www.britishv8.org/MG/KeithTanner-MGBGT.htm
It’s a good idea to keep the donor car for as long as possible, it’s surprising just how many little bits can still be retrieved.

[Not_Anumber]

The donor car's engine and gearbox are out and now sitting in the garage next to the B. Im currently working through some maintenance and preventative maintenance jobs on the engine such as cambelt and waterpump, rear crankshaft oil seal, clutch plus teh inevitable clean and paint, spark plug change etc.

In between work and near incessant rain much of the donor car's wiring has now been disconnected, unclipped and it's connectors labelled. I worked inwards from the engine bay then the back and underside of the car first and I will just need to devote a free day to get the lumpier bits such as the ECUs and main harness connectors out from behind the console and dashboard.

The jury is still out on whether I will do the rear suspension but in case I do go the transplant route Im most of the way towards getting the IRS and rear suspension removed from the donor car. The setup was held on by many bolts which have largely been removed or cut away and it's currently only held to the body by just 2 bolts, chosen as their threads have free movement and they are easy to access when i need to separate it all.
The propshaft isnt as easy to separate as I'd expected, at least not when working under axle stands as one side is blocked by a lightweight front to rear girder like reinforcer and there’s the exhaust on the other - the type that was once supplied and fitted in several sections which now stalwartly refuse to come apart. Im hoping I can separate it from it's hangers somehow rather than have to cut it as the exhaust is in quite good condition. Looks like I will have to raise the rear of the car quite a bit higher for access to this which may stretch the capacity of my largest set of axle stands.

[Not_Anumber]

Despite one sunny day which typically coincide with when i was at work, the weather hasnt been dry enough to lie on the ground under the donor car outside but my retired mechanic pal came round yesterday and we did a first trial fit of the engine and gearbox in the GT's engine bay. The first mistake was to have tried it with the exhaust manifols still on but that was easy enough to seperate. The next clearance issue was the alternator. This should be fine to go back on when the engine is actually in place but proved an obstacle to the trial fittiung excercise. Typically it's longest bolt proved a swine to remove, just what you really dont want whilst the engine and gearbox are supported by an engine crane. When that finally succumed I realised the alternator's front bearing was a bit rough and there seemed no point in using that and hoping for the best so now have a new one on order. Thats pretty much the first money Ive spent on it so far aside from the cambelt and clutch kits which I havent yet fitted and a replacement rear crankshaft oil seal which fro £ 10 would have been rude not to change whilst the flywheel was off.

At one point I had the gear lever line up perfectly with the hole in the transmission tunnel. The next thing will be to support the gearbox in that position, making sure the output shaft is centred and begin to mark out the positions of the new engine bearer plates I'll be most liklely attaching to the rear part of the front crossmember. My helper hasnt turned up today so its just a day tidying up, cleaning and painting the block and some ancillaries.

[Not_Anumber]

The gerbox mounts have been worked out and this weekend will hopefully see them in place.
Theres no prescriptive approach for this, people have taken different approaches and I am erring on the side of caution by using two different methods together. As some of you may know the method of natively supporting the MX5 gearbox is ususual to our eyes as it is braced to the rear axle assembly on one side by a lattice beam with no other mountings (other than being bolted to the engine of course). There are two fairly mighty locating points for this on one side of the gearbox that would be overkill were this just for a conventional gearbox mounting. In addition, a few inches further back, it has two more conventional threads in the centre of the rear underside, An angled hanger bracket connected this on the lattice beam for a 2nd support. It was decided to tackle this one first. Refitting said bracket at 180 degrees provided a mounting point on the other side of the car's centre line. A plate was welded on the bottom of this to which an original MGB rubber engine mount was bolted (2 bolt side) as it is just the right height for the job. I am now going to make up a thick plate of metal which the mount will bolt down onto (single bolt side). That plate will be welded and braced to the B's fixed crossmember( the non removeable one thats behind the actual gearbox crossmember). Despite the additioon of more metalwork the gearbox can readily be accessed and the mount removed when needed as there are now 3 ways of unbolting it.

The next thing will be the forward gearbox mount. Ive sand blasted the MGB removeable gearbox crossmember and cut off the diagonal plates for the original rubber gearbox mounts. As the B usefully has 2 sets of threaded holes for the crossmember I have moved it forward which will better align with the mounting positions on the side of the gearbox. I will need to trim a bit off the leading lip of the crossmember for clearance but will then reinforce to ensure no strength will be lost. It remains to now find a method of introducing a mount into that space. Depending on the height measured here this could incorporate another repurposed MGB rubber engine mount or an MGB gearbox rubber mount.

By using 2 sperate mounting points either side of the centre line the gearbox will be prevented from twisting on it's mounts under power.