This story begins with some sales hype from Vitesse, the company that is modifying a Mazda MX5 (Miata) gearbox to work in the MGA. You can see their sales ad here:
https://www.vitesse-ltd.com/products/mga-5-speed-mazda-gearbox-conversion-kit

The housing is custom modified for the MG cars, but the rest of the parts are indeed Miata gearbox parts. Early production Miata gearbox was a bit smaller. There are some early style units installed in MGA, and the owners report this conversion is somewhat easier. When Vitesse got into the act they use the later and larger Miata gearbox, which is likely a better unit, but does make for a tighter fit.

Vitesse had previously modified a Miata gearbox to work in the MGB, and they casually thought it should work pretty much the same in the MGA. Apparently they had rushed to assemble the prototype kit, and this resulted in a number of rather serious teething problems with the first real world installation in the MGA. What follows is the report from the fellow who had to suffer the consequences of being the (unknowing) guinea pig for the first installation.

"If it Ain't Broke Don't Fix It!" -- A Vitesse 5 speed conversion
by Ken Klay in London, UK -- May 1, 2018

As some of you may remember I asked about the pros and cons of the New Vitesse (Mazda MX-5-Miata) and the Old Type 9 Hi Gear Sierra gearbox conversions for the MGA. As yet I’m unable to comment about the quality of the Vitesse gearbox but can report about the issues I had with the installation on my car.

Firstly the gearbox is substantially larger than the original MG box and from memory is also much larger than a Type 9. It is supposed to be lighter but I'm skeptical about this as it feels as heavy if not heavier than the MG one. The kit looks and is presented well in its delivery box.

It took about a day to strip the engine and gearbox out of my car and to Jack it up to help with access for the conversion. Removing the original gearbox mounts is slightly scary on a complete car as the space within the engine bay/Gearbox tunnel is limited. Using an angle grinder (without a safety guard; access issues) within a confined space has its dangers and even with the use of an air saw I did have a few brown trouser moments.

The replacement Gearbox Chassis mount supplied is quite substantial but has unfortunately been designed to be fitted to a bare chassis and as a consequence the mounting holes that are needed to be drilled into the chassis are in a totally inaccessible location, so I had to relocate the chassis mounting holes. The foot of the gearbox mount comes without captive nuts, and when the gearbox is offered up to the mount it is physically impossible to affix its rearmost fixing bolt and is sodding awkward to affix some of the other fixing bolts. The gearbox foot and leg need to be removed to install the gearbox into the car. I installed the box separate from the Engine, which was a blessing as the Gearbox needed to be jiggled to make it fit through the tight gear tunnel aperture.

I purchased the new rear engine back plate from Vitesse because of the need to relocate my low starter motor to the high starter position on the new Bell housing. Unfortunately the back plate had been made incorrectly. As I understand it was configured for a later 1800 MGB engine (including oil seal). The delay in re-manufacturing a new back plate was not an issue as my flywheel and clutch had been sent off to be lightened and balanced. Just as the New back plate turned up so did a new replacement clutch slave cylinder and new clutch spacing washers. I was told that I would need to replace the ones on my MGB unit with the new ones.

Once all was installed a friend was summoned to help mate up the engine to the gearbox. We struggled for some time trying to get the two to mate, eventually admitting defeat and removing the engine to ascertain what might be the issue. Using the now surplus master clutch cylinder as a template it was obvious that the inner diameter of my original type clutch thrust washer was too small to allow the clutch slave cylinder to pass through. I had to ream out the thrust washer to allow the two to mate. Once again the engine was offered up and after a valiant fight the two were eventually mated together.

At this point it was noted that the engine would not fit on its engine mounts and sat forward by about 1 inch (25 mm). Looking from underneath you could see that the gear box clashed severely with the right hand side of gearbox tunnel (UK driver side, USA passenger side). Tape measures out and a lot of cross checking to try to see where the problem lay, we ruled out a bent chassis, gearbox tunnel etc. The chassis was checked for trueness at the beginning of the cars restoration and was well within speck. The car drives true and In a straight line).

A reluctant decision was made to cut into the tunnel. A 5x7 inch chunk was removed and Bingo the gearbox dropped right into place. The box itself protruded into the cabin by a almost 1/2 inch (12 mm). The top of the gearbox gear stick area also struck the opening on the main gearbox tunnel creating the need to once again butcher the tunnel. It was also noted that it was 100% impossible to affix the speedo cable and make it work without the cable being routed through the tunnel into the cabin. Again another 3x3 inch hole was made to allow access and installation of the speedo cable. Trying to get clearance on the prop shaft proved impossible. Work on the installation was put on hold as travel and work commitments got in the way. Vitesse were informed of the issues and when I returned home.

Christian from Vitesse came down and crawled under my car for a couple of hours to try and work out why I was having to butcher my gearbox tunnel (but also to get an idea as to the serious flaws in the chassis mount design). Some Heath Robinson mods were made to the gearbox chassis mounts which allowed more free play with the gearbox alignment. The top of the gearbox tunnel aperture was also enlarged and this gave us the ability to correctly align the prop shaft and also have the gear stick perfectly centered within its rubber bellows. To Vitesse's credit they gave me a 10% refund of the purchase price but without the VAT (TAX ???) for all the issues I had.

As I cracked on with the installation of the engine ancillaries another issue was found. The Aperture for the starter motor was found to be a tad too small. The solution was to file down the fixing plate on my brand new Hi Torque starter motor (my original and the new starter both fit an original back plate). I worked till late to rebuild the car as I was going to be taking it on a week long driving holiday with my local MG club to one of England’s National parks in a just few days time. This trip had been organized months earlier, and I was going to have no shake down time on the car.

With all but the engine manifold and carbs to install my phone pinged with a email from Vitesse. “We don't think that the clutch setup is correct” was the essence of the email. My friend had turned up to help me bleed the brakes and clutch, so we set about testing thing. Yup NO CLUTCH. Engine out again and my MGA driving holiday to the national park now in ruins. Vitesse's solution was to replace my 5000 mile old MGA clutch (balanced to my flywheel) with a non balanced MGB clutch but this needed specialist bolts to be made. I asked for a Meeting with Vitesse to talk about the long list of issues I had on what was supposed to be a quite simple DROP IN conversion with very limited modifications to an MGA.

After I returned from my MG (sorry, Renault) “Scenic” driving holiday in the national park I was informed that the clutch and bolts were ready. Because of work commitments, my friends availability and an up and coming 3 week holiday to France in the MGA (all ferries and hotels booked weeks ago), I raced the 2 hours up to Vitesse to collect the Clutch and parts and also to discuss the issues. They showed me a twin cam that they had converted ( via a removal of the gearbox tunnel and floors boards so as not to disturb the engine setup) but had not encounter any issues with the gearbox clashing with the tunnel, but had needed to bring the speedo cable into the cabin. They talked about the conversions made by MG specialist and what they had learned from them whilst transplanting 1800 engines and their gearboxes into MGAs.

It become apparent that many assumptions had been made about the commonality of the “B” series engines which had led to the back plate and clutch issues. As they say in French “Assumption is the Mother of all F'ups“. I and just about everyone I discussed this with felt that that Vitesse should refund me the full cost of the gearbox. I felt that given the amount of time it took to remove and reinstate the new clutch and engine for a second time, the loss of a quite specific MG driving holiday, these issues along with the sever modifications and massive inconvenience that the lack of R&D (understanding of the differences within the “B” series engines) had caused me, and the fact that I had unwittingly become the R&D Guinea pig for the 1500/1600\1622 MGA conversion, this request wasn’t unreasonable. Vitesse were not forthcoming with any refund.

Vitesse are now re-designing the gearbox mounting bracket and will as I understand be supplying all new kits with a MGB clutch as a way of standardizing the clutch setup. They are ware of the fact that the speedo cable stands a snowballs chance in hell of being fitted without it entering into the cockpit of the “A”. I hope to have the car on the road in a week or so, so will let you all know whether the conversion was worth the aggravation.

I'm 100% sure that the Mk 3 Mazda box will be a joy. Having driven several Sierras in the day (home of the T9) I was never impressed, so the new Mazda box seemed a better choice.

I suppose my issues sit in two camps. The R&D re gearbox mounting design and also the Clutch, 100% a Vitesse issue, but also the fit of the Gearbox within my tunnel. If I'm being told the truth re the other dozen or so conversions - why did my cars gearbox tunnel prove to be such an issue, as I stated my car seemed to be within tolerance, the tunnel seems perfect and followed the lines of the chassis. I replaced nothing but the floorboard mounts and a touch of metal around them so had not messed around with the major datum points. A couple of others in my local club have "A's" so when i get the chance I'll crawl over them to do some cross checking. -- Ken
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And a week later Ken wrote:
"For all those waiting for a report on how the 5 speed Mazda box feels and drives, I'm having to censor my language a tad but ... F,ing brilliant! Did I say F,ing brilliant? Smileys with beer, yes F'ing brilliant. So the Vitesse/Mazda birth was difficult, problematic and painful (what a shame), but the baby it has produced is beautiful. The car always was a joy to drive even with its original box but the addition of an all synchro box and a 5th gear really adds to the pleasure of driving her. I'm probably on 400,000 plus miles on manual gearboxes (most cars in the UK are manual). The Mazda box, as expected is smooth and clean with a very nice action (short and very close in terms of original MGA gearstick movement). The ratios seem close to the original.

I probably only put 40 miles on her, but it consisted of some town driving, some UK country lanes (single track) where the ability to swift change was a joy, some motorway driving where the 5th gear dropped the revs by 800 at 50 mph-ish. This effected the overall engine noise and it was clear that the car was not struggling or being strained. I need to do a tad more work on the cars set up but I know a 5th gear conversion was the right thing to do. I have a big trip to France coming up where I'll be able to give a greater appreciation of the gearbox.