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Spitfire & GT6 Forum

Mazda MX5 Gearbox Swap for MkIV Spitfire

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MKIVspanner Rob Tomlinson
Farnham, Surrey, UK   GBR
Hey gang.

I have been looking to upgrade the Spit to a five speed gearbox to help with the noise (and engine life) at motorway speeds. After doing some research I have come across the Ford Type 9 and conversion, which proves to cost an arm and a leg, even if you can source a box on a well known auction site. I stumbled across a Mazda MX5 gearbox coversion kit for MGBs and I was wondering if anyone out there had any experience or knowledge about doing this type of conversion on a Spitfire, primarily as they are relatively easy to source and will not break the bank. Any and all suggestions are welcome!

Thanks in advance,

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clshore Carter Shore
Beverly Hills, FL, USA   USA
The issue with fitting a Miata gearbox is the integral bellhousing.
Notice that the 'pocket' for the starter nose is located opposite to the Spitfire position, and higher up.
Do you plan to use the Spitfire flywheel, or a Miata flywheel?
This is important because the distance of the starter pinion from the crankshaft centerline is determined by flywheel diameter.
Some type of custom adaptor plate will be required in any case.
This gives you the opportunity to perhaps fit the geared starter, which requires minimal bellhousing clearance behind the flywheel,
since the pinion gear is moved into mesh with the ring gear from the front by a solenoid, and does not project behind.
Note that there were several varieties of Miata transmission that were fitted over the years, which may not
all use the exact same bellhousing pattern and dimensions.
The custom adaptor may perhaps be fabricated from flat aluminum plate, which requires only the bolt patterns of the Spitfire block
and the Miata transmission, with their centers concentrically aligned. Thickness suitable for the length of the trans input shaft.
The two patterns may be available as CAD files, or you may have to capture them yourself by careful precision measurement, or
by 3D laser scans of the two parts, which can be merged to create the CAD file for fabricating the adaptor plate.

Perhaps you can orchestrate a group buy or CrowdFunding operation, as there will be quite a lot of interest in such a conversion
from Spitfire and/or GT6 owners.

HTH

MKIVspanner Rob Tomlinson
Farnham, Surrey, UK   GBR
Wow! Thank you very much. I was planning on exploring if a standard flywheel and pressure plate could work with an a different clutch (as I assume the spline patterns on the MX5 box is different from standard). The logic behind this is that if an adaptor plate is being used I might be able to retain the original starter and blank over the MX5 starter location (with some firewall fettling too).

I'm tempted to get a box for cheap and see how this works out.

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clshore Carter Shore
Beverly Hills, FL, USA   USA
The cheapest clutch solution is to have a custom friction disk made, having center spline that fits the Miata input shaft.
The adaptor ring thickness determines where the splines on the Miata input shaft line up with the friction disk center,
that position can be accomplished on the custom friction disk.
The Miata mainshaft tip must also penetrate the correct distance into the end of the crankshaft to engage the spigot bushing.
Hopefully that will not require any machine work to shorten the tip of the shaft .
A custom spigot bushing can be easily made from bearing bronze to adapt the shaft diameter to the bore diameter inside the crankshaft,
although perhaps a standard needle bearing can be found that fits.

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
In reply to # 1595491 by clshore The cheapest clutch solution is to have a custom friction disk made, having center spline that fits the Miata input shaft.
The adaptor ring thickness determines where the splines on the Miata input shaft line up with the friction disk center,
that position can be accomplished on the custom friction disk.
The Miata mainshaft tip must also penetrate the correct distance into the end of the crankshaft to engage the spigot bushing.
Hopefully that will not require any machine work to shorten the tip of the shaft .
A custom spigot bushing can be easily made from bearing bronze to adapt the shaft diameter to the bore diameter inside the crankshaft,
although perhaps a standard needle bearing can be found that fits.

I would try to make it so a future owner can order a clutch kit.

RobTAR Robert I
Seattle, WA, USA   USA
I posted a thread a while back about a GT6 that has a Mazda 6 speed tranny in it. Could be a NB,NC,or ND tranny. I don't think anyone ever got any details from the guy.


https://www.triumphexp.com/forum/spitfire-and-gt6-forum.8/interesting-gt6-build.1573154.1573658/#msg-1573658

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1595543 by Doug in Vegas
In reply to # 1595491 by clshore The cheapest clutch solution is to have a custom friction disk made, having center spline that fits the Miata input shaft.
The adaptor ring thickness determines where the splines on the Miata input shaft line up with the friction disk center,
that position can be accomplished on the custom friction disk.
The Miata mainshaft tip must also penetrate the correct distance into the end of the crankshaft to engage the spigot bushing.
Hopefully that will not require any machine work to shorten the tip of the shaft .
A custom spigot bushing can be easily made from bearing bronze to adapt the shaft diameter to the bore diameter inside the crankshaft,
although perhaps a standard needle bearing can be found that fits.

I would try to make it so a future owner can order a clutch kit.

Look at a friction disk.
Tell me what part will be a challenge, especially when the splined center can be had from a stock Miata, and the disk can be had from a stock Spitfire.
Might be as simple as drilling new holes, and riveting the Miata center in place of the Spitfire center.
Else a steel adaptor ring, childs play on a lathe.

You think it will be cheaper and easier to fabricate a custom flywheel, or custom machine the input shaft?

Stock Friction disk: $100
Custom Friction disk: $200

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