TECH TALK - Page 8
FITTING MAZDA MX5 SEATS TO A MARK II MGB ROADSTER
Recently I had the opportunity to bid on eBay for a set of Mazda MX5 seats and subsequently won the bid at a very reasonable price.
My interest in the seats had been as a result of an article some time ago in the Chicago Chapter of the MGCC online site, detailing one member’s adventures in fitting a set of Mazda Miata seats to his MGB roadster. My own roadster, a ’68ish Australian built MK II with Overdrive and dubious pedigree, was fitted with an equally dubious pair of seats looking similar to those I remember in my 73 model GT way back in the late seventies. A number of holes had been drilled in the floor pan to accommodate these seats, the original mounting points being of incorrect dimensions and location to cope with the mounting hole pattern of the seat rails. I managed to source a set of very average MGB MK II seats but they were going to cost far in excess of what I was prepared to pay to refurbish.
Enquiries through local (Queensland) second hand dealers failed to bring to light a set of seats; apparently they are very popular items and are snapped up quite quickly. An importer in Adelaide advertises sets for $400 plus freight, however, last enquiries; the price was around $600 and climbing…
The article on the Chicago site has since disappeared; I guess the legal side of following advice of such a nature may have certain ramifications so I offer my experiences as just that. If you intend to follow a similar path as I did then find a mechanical engineer to endorse the modification to your vehicle. I cannot stress the importance of using high tensile bolts to mount anything you are strapping your butt to. The mounting holes for the seats in my MGB are tapped ¼ UNF, any decent hardware or parts supplier will carry a range of high tensile UNF and UNC hardware. Use nyloc nuts – 100’s of aircraft manufacturers cannot be wrong. Using those original mount holes, to me, is critical - someone, somewhere, sometime, designed that part of the car to safely mount the seats to, using any other points is compromising the structure of the vehicle and, ultimately, putting your safety and the safety of your passenger at risk.
The MX5 seats fit easily providing some reasonably accurate engineering is carried out, they are very comfortable seats, the comfort level in the MGB, particularly in hard cornering, increased dramatically. I had read reports that the MX5 seats were much preferable over the standard seats for touring; having done the conversion I can well believe that.
The original article made it sound so easy – just mount the seats straight into your MGB, you need to grind off a locating spigot or two, make up a small adaptor plate for one of the mounting holes and voila! Well, not quite. I achieved the following conversion in about 4 hours, including removing the old seats, fabricating the adaptor plates and fitting the new seats. Oh, and 3 visits to the hardware store for bits I forgot I needed…I also had the assistance of my 12 year old son Cameron, who was rewarded for his efforts with a subsequent 1½ hour drive in the lovely countryside northeast of Oakey as a test for the seats.
The MX5 seats are mounted quite differently to an MGB – any similarity ends with the word ‘seat’. However, the size of the seats and the pitch of the seat rails make the conversion easy. My conversion involves the manufacture of adapter plates that mount onto the Mazda seat rails, the adaptor plates are then bolted directly to the seat pan, using the original bolt holes. The Mazda seats aren’t modified in any way, the MGB remains stock and original seats can be refitted at any time. I used a small bench mounted pedestal drill, a step drill bit (for hole sizes over ½”), plus a vice and rubber mallet for bending. You do not need a fully equipped engineering workshop. Total cost for the steel and hardware to do the job was under $30.
The MX5 seat mounts appear to take 12 - 13mm studs or bolts - doesn’t really matter here but they are angled or pitched down, not flat like the MGB seats. Also, the right hand rail front mount point is offset from the rear mount point and locating spigot. The seat mounts are not handed, i.e. left or right, or mirrored, however, for some reason, the right hand seat fits better in the left hand side of the MGB, and vice versa for the left seat. One of my seats has a map pocket, this ended up being the drivers seat so is relatively inaccessible whilst driving solo. Some MX5 seats apparently have speakers mounted in the headrest – your choice if you want to take advantage of this opportunity. I much prefer listening to the Lukeys up on pipe!
The adapter plates are made from 3mm bright mild steel; extend from the rear MX5 seat mount to 25 mm in front of the forward seat mount. The plate is profiled to match the angles of the seat mounts, drilled to take a 3/8 X 1” UNF high tensile mounting bolt and hardware, with the bolt inserted from the bottom.
Even using nyloc nuts it is worth being able to check to see they haven’t come loose.
A hole is drilled aft of the front mount to clear the locating spigot. I used 3 X 50cm lengths 50mm wide and 1 X 50 cm length 25mm wide. I also used pieces of hardwood as spacers, to the width of the relevant adapter plate and 5/8” or about 16mm thick. These act in compression so are simply pinned to each adapter plate using 2 self tapping screws.
Even with all this extra packing you end up sitting an extra inch lower because of the lower profile of the MX5 seats. Your MGB may have hardwood spacers fitted under the original seat rails. This conversion does away with the need for these spacers. Keep them as souvenirs or for retro fitting original seats later. You don’t throw anything away, right? If you have any doubt as to the structural integrity of 3mm steel, just look at what was originally mounted to the seat pan…
I first manufactured identical adapter plate sets for both seats then found minor differences meant I needed to approach each seat installation separately. I found the RH MX5 seat fitted better into the LH side of the MG. This means the recliner lever is towards the centre of the vehicle, however, it does not interfere with the seatbelt operation and is still readily accessible and useable. The seat rack adjuster lever ends up on the outer near the door sill but is hidden under the seat, is readily and easily operated and does not interfere with entry or egress from the vehicle. For this seat I used one adapter plate 25mm wide for the mount and spigots that are in line, plus one adapter plate 50mm wide for the offset mounts. Mount the adapter plates to the seats then transfer the pattern of the MGB seat mounting holes, starting with the forward holes. These are drilled in the forward extension of the adapter plate. Measure back for the rear mount holes, these end up in the area of the adapter plate supported by the timber packing piece, so you will need a longer ¼ UNF high tensile bolt to cater for the extra thickness. Check, measure, check, measure, drill.
I fitted the adapter plate assemblies to the seat pan, using ¼” UNF high tensile bolts with spring washers under the heads. I have this thing about ensuring attaching hardware has some form of mechanical locking – I have spent over 20 years maintaining and flying in Army helicopters as an Airframe/Engine Engineer and have a healthy regard for ensuring all parts remain securely in their allotted location. Make sure the 3/8” bolts for attaching the seats are in place, the head of the bolts are easily accessible with a spanner after installation but very difficult to install if left out. Drop the seat over the mounting bolts (a variation on this would be to have studs welded to the adapter plates), fit washers and nyloc nuts and the seat is fitted. Lovely.
Buoyed by my success with the now LH seat, fitted nicely and neatly into the passengers seat well, I manufactured identical adapter plates for the drivers side. Well, the seat fitted, but wouldn’t easily slide rearwards, fouling with the curved intercostals at the rear outboard of the driver’s seat well. Being 6’2” and often wearing Army boots I need as much rearward seat travel as I can possibly get. A good think, strong coffee, a bit of head scratching and measuring and I found I had room to move the seat inwards laterally 25mm. This was achieved by replacing the 25mm wide adapter plate with a new 50 mm wide plate, shifting the mounting holes 25mm laterally, giving the clearance required to allow the seat to travel fully forward and rearward and still have adequate clearance on the handbrake lever. After all this I found I had to have the seat a couple of clicks forward to comfortably reach the pedals! The rear squib of the MX5 seats seems to be thinner; however, the lumbar support cushion on mine can be adjusted with packing for us in mid life crisis sporting ongoing back problems! The problems with the RH seat may be a quirk of my vehicle; my advice is to check your own carefully before drilling any holes…
The original article made comment about the tonneau not needing any modification. Certainly, the head rests are in the same location but are physically larger and significantly taller. My tonneau was modified by a local canvas place for $25. Champions!
Finishing consisted of priming and painting matt black all metal components. The timber spacer I painted with clear epoxy. Araldite works okay if you are quick, but a modelling epoxy like Great Planes 20 minute epoxy will be absorbed by the timber a bit better. So, I also build model planes…and helicopters!
This seat conversion is quite achievable – provides a relatively low cost, comfortable and stylish alternative to genuine MGB seats and does not alter the structural qualities of the vehicle. So far I have seen the seats in tan leather and black cloth – seat covers are readily available for the MX5 in a range of colours and finishes which should be enough to complement any colour scheme.
I have deliberately not included any plans, drawings or photographs of the conversion process with this article. However, if you are contemplating fitting MX5 seats into an MGB, having a bit of a head scratch and want to see how I did it, drop me a line firstname.lastname@example.org and I will be happy to oblige.