My rotary MG Midget's History


Well over 10 years ago, Tom (the previous owner, see his cars here) got this car on advice from his mom who had an MGB. Tom had to do a brake job and other maintanence to get it back to driveable condition. The old boinger (4 banger, er uh, inline four cylinder) was old and slow. It had torque, but wasn't fun to drive.

So what did Tom do? He researched rotary engines and decided to see if he could install one in this car! Ken Durkee (of Rapid Rotary/Mazdas and More fame) helped with the engine swap. They chose to use a '71-'73 RX-2 twin-dizzy 12A and top mounted starter tranny, for obvious reasons (very little room in a spridget platform). The engine was streetported and a single distributor front cover replaced the twin-dizzy cover so an RX-7 electronic distributor could be used. The exhaust system was custom made with a short primary header (a little too short if you ask me, but not much else would fit), and a single presilencer-like glasspack from... oh I don't remember the name, they went out of business uh, Rotary Performance or something, and a Monza dual tip at the end. (It is a bit louder than my REPU)

This car easily exceded its unofficial design limit of 95MPH! Anything over 100MPH was just scary! But it was still fun beating Cameros and the like.

But the fun didn't last forever. Appearantly somebody dropped some washers down the carb which destroyed a rotor and rotor housing and must have done other unseen damage because shortly after Tom rebuilt the engine, it seized on him due to an oil starved front stationary bearing. The oil pump's little rotors were also scored pretty badly. Not enough oil cooling? Oil pump capacity too small for the shimmed rear pressure regulator? It was one of those old 12mm pumps which should have been upgrade to at least a 15mm or better yet a 17mm. Did the Mazda OEM oil filter bypass too much? Was the mix -n- matched rotor set Tom used from two different engines possibly out of balance? He never mentioned anything about vibrations.

Tom moved on to other projects so the MG sat for a few years until I decided to buy it in '96. After I got it, I started to mess around with the interior. Then shortly after that I had the opportunity to get an REPU which needed an engine (Tom had an old style 4 port 13B in his garage). I got the rotary truck and worked to install the 13B in it. The MG just sat there.

It wasn't untill the summer of 2000 that I finally decided to do something to make up for all the time I spent on other projects. The year before, I was taking carefull measurements of the MG here and there in order to prove a theory I had come up with. I wondered what a 13B would be like in the MG. Well, for one thing, no 13B engine ever came stock with a top mounted starter tranny behind it. Only the twin dizzy '71 to '73 10A and 12A did (I think the 13G used a top mounted starter too, but it was a racing engine). I started scratching my head wondering if there was a way to swap out the RX-2 tranny for a '74 or later bottom mounted starter tranny. This would open up a whole new world of later engines (can you say 13B?).

I studied the info on the MG12a Project page and saw that this guy came up with other solutions than what Tom and Ken came up with because he had gone with an RX-7 12A engine and 5 speed tranny. He kept the oil-to-water cooler and shortened the shifter linkage so it would fit better in the MG hole (he still had to trim some metal aft of the original hole, but it was still better than not shortenning the linkage at all). In Tom's project, he fitted a piece of sheet metal to reshape the tranny hump and the shifter came up in a great position because the output shaft housing of the top mount starter tranny is 40mm shorter than any '74 and later tranny, and far better than an unshortened FB RX-7 shifter.

Anyway, I had my old REPU 4 speed sitting in my garage so what I had decided to do was make a compromise between the RX-2 tranny and the RX-7 tranny. If anybody has ever seen a top mounted starter tranny side by side with a bottom mounted starter tranny, you will notice some obvious differences. In order to move the starter to the lower driver's side, Mazda engineers rotated the whole gearbox by 15 to 20 degrees to make room for the starter. The output shaft housing was altered and lengthened by 40mm. They also enlarged the bell housing and moved the clutch release fork up to where the starter used to be; they basicaly did a location swap with the clutch fork and the starter. This allowed the heavy (at the time) starters to be mounted lower in the car thus lowering the center of gravity a bit. They also moved the shifter rearward a bit which was ok in their increasingly larger cars. Infact the shifter in the later (FB) RX-7s was moved even further back for a more sporty feel and/or easier to reach position, hence the need to shorten the linkage if using that sort of tranny in an MG.

Satisfied with my measurements, I removed the RX-2 tranny and compared it to the REPU tranny. They shared a common set of rear mounting bracket holes, but the REPU tranny also had a second set of holes further forward. The distance is only 10mm, but it was a start. The rear most set were used in the original truck installation. I would be using the front most set in the MG installation. This was going to move the tranny rearward by 10mm. I also drilled two new holes in the tranny mount bracket 10mm forward of the original holes. This also moved the tranny further rearward by another 10mm. This added up to 20mm, basicaly allowing me to mount an engine which is 20mm longer than what was previously in the car (going from 12A to 13B) without even touching the front cover motor mount bracket thingy (crossmember, as most people refer to it). That was all that I needed to install the tranny! The next step was to remove aprox 60mm of material from the tranny hump so the gearstick could fit in the new position. Remember that the 40mm of extra tail shaft length + 20mm of rearward relocation had to be eaten somewhere. I haven't gotten around to finishing the carpeting yet, but I test fitted everything and it looks pretty good. That extra 60mm of rearward relocation doesn't seem like it's going to cause any probs as far as comfortable shifter locations go (but I've got to drive it first to be sure).

I didn't want to have to build a new header to fit the 13B, so I simply measured the bolt spacing on a 13B and slotted the holes on the old header (not just horizontally, but diagonally due to the RX-2 12A's different stud spacing). I also had to remove some metal on the inside of the flange to give the 13B exhaust ports a smooth flow into the header. 10mm worth of filing can take quite a while if done by hand. I know it's not the best idea to remove metal from these areas, but there is still plenty of thickness left and it sealed fine (didn't leak at all, and my exhaust gasket is really old too). I suppose that if I ever install a high powered 13B, I should rebuild the header with a new 13B flange and new pipes with good bends for better flow.

I borrowed an '86 starter (because it is small enough to fit) and proceded to remove the engine from my truck. I knew I couldn't drive the MG since the driveshaft wasn't shortened yet and I had no slave cylinder, but I continued anyway. I got the engine out of my truck that night and actually got it installed in the wee hours of the next morning. I finally decided to quit when it was getting light outside and I noticed myself falling asleep under the car on the creeper! The next day I installed the header, radiator, and some of the wiring. The day after that I finished a makeshift temp sensor and wired an electric fan. I finally got it running on the fourth day (if memory serves). It was cool! My younger brother was totally excited because it had been 10 years since he last heard it running. I was happy because I was able to prove my theory correct, and I was able to prove wrong all the people out there that said I couldn't put a 13B in my MG! I put the engine back in the truck about a week later (it took all of 5 minutes to seat the engine on the tranny and bolt it up, I couldn't believe it!), but what a cool week!

A couple months later I had the driveshaft shortened by 60mm and have since installed it. I am also finishing up the center console and I may even put my truck engine back into the MG for a while before I rebuild it. This time I probably will be able to drive it if the clutch master still works after 10 years of sitting. We'll see!

I guess I am lucky since most of the boinger-to-rotary work was already done. All I did was modify existing hardware. But this project has opened up other opportunities to install rotary engines where smaller ones (12A and boingers) used to be. I better understand the thought processes involved in an engine swap now and I hope to develop this skill even further.

 

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