Early single dizzy or Twisted type. This name is slightly misleading, as these gearboxes were not only fitted to single distributor motors. These look very similar to the twin dizzy gearboxes, however the main case does not sit upright in the car – they are twisted over slightly (~15deg) to make room for the rotary bottom mount starter motor. As such, the gearbox mount and the sump are not parallel to each other. In detail:

• Late RX-4 Twin dizzy. Later 12A twin dizzy RX-4s have the updated bottom mounted starter motor. Notably, the starter is different to the later single dizzy bottom mount starter motors, which have a smaller body – you can see this by the large (~80mm) hole in the bellhousing. These bellhousings are the only easy way to fit a 5-speed behind a twin dizzy motor – more on this later. The ratios in these gearboxes are the best ratios Mazda EVER offered in a factory rotary gearbox. They are also very strong, being able to withstand over 300 horse-power – beyond this power level, the gear-box case becomes the weak point as it tends to expand and allow the shafts to move relative to each other, which then results is broken shafts and/or stripped gears. Wayne Dyson's 9.7 second 500+hp RX-3 used an internally standard RX-4 gearbox that was substantially braced externally with a "full metal jacket" – a steel sump (rather than alloy), and some hefty steel strapping running around the outside of the case. The most common failure with these gearboxes is jamming in 3rd gear when they are worn. The more power being fed through them, and the more worn they are, the more likely it is that they will jam in gear.
  
• RX-4 Single Dizzy. The actual gearbox is identical to the later Twin Dizzy "twisted" gearbox, but the bellhousing was changed to suit the single dizzy motors' new bolt pattern, and to suit the inexplicably different starter motor. These bellhousing can be identified by their smaller starter motor hole – it is roughly 55mm in diameter.
  
• RX-5. This box is essentially just a 5-speed version of the RX-4 gearbox, and as such, has the same strengths and weaknesses. The ratios, however, reverted to the old top-mount twin dizzy ratios and as such are appalling. Bellhousing is the same as single dizzy RX-4.
  
• 121. Again just another revision of the theme – although obviously fitted with a bellhousing to suit the piston motor. The piston bellhousing can be identified by its lack of a hole for the starter motor. These gearboxes have a shorter gear lever position (approx 100mm further forward compared to the RX-5 gearbox. All previous RX-4 type boxes have the same shifter position length as the RX-5). The 121 box is blessed with the much better later RX-4 ratios. Changing the extension housing to the RX-5's item will create a "close ratio" RX-5 5-speed. Notably, in both 5-speed boxes mentioned so far, 5th gear is removable, and the RX-5's 5th gear is a closer (less over-driven) gear than the 121's. The "ultimate" all production Mazda gearbox would use a 121 gear-set, fitted with the RX-5's 5th gear. Note that later 121s were factory fitted with a 626 style Smooth Case gearbox.
  
• TQX. This is a weird import-only gearbox that you may happen to stumble across. Factory fitted to Jap market RX-5s, they are a 5-speed manual gearbox, which also uses an automatic style torque converter. It is very rare to find one that still has all of the torque converter intact, however, some importers try to sell them as "RX-5 style" gearboxes. They have a very short, very angled gear lever position, and therefore will not easily bolt into anything without the torque converter gear. The gearset however, can be transplanted into a RX-5 or 121 case. Ratios are the same as 121.