Steve's Triumph Spitfire 1600
The ongoing saaaaaaga
Transmission Selection, Modifications & Shifter Relocation:
One of the best reasons for undertaking this swap is the fantastic transmission that Mazda offers.
There are many people that spend thousands of dollars to fit a T-whatever tranny, which is great, except that they are stuck keeping the absolute worst part of these cars, the lump in front of them.
When making my decision on this combination I had a general idea of what was available, and combining that with what I already knew about transmissions, differentials, gear ratios, and how to actually mount them, I was very confident that I had the best answer for me. Since then I've learned more of the details and am happy to share them here.
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There are really two big considerations for which transmission to select, Shifter Location, and Gear Ratios.
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Fortunately most of the gear ratios for the Miata transmissions are pretty good and minor tweeking can be done by selecting a differential to compliment the tranny and get that final drive ratio right where you want it.
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​​Shifter Location is super-important; A poorly placed shifter would really take away from even the best execution. My view on this is that like many performance cars out there I think it's important to have the shifter as close to abeam the steering wheel as possible. Not only is this the most comfortable and natural place but it also allows easiest and fastest transition when moving one's hand back and forth.
Gear Ratios:
For many years now I've been running my factory Spitfire J-Type Overdrive with a GT6 3.27:1 Differential. I've been super happy with it, but with the added power of the Mazda engine I knew a change was in order. I have since acquired two Nissan R200 differentials, a 3.63 from a G35 and recently my real target differential, a 3.54:1 LSD from a Q45. It will go in sometime in the future (with an updated suspension), but for now (and while writing this 3 years later) I ran the 3.27 diff and have not blown it up, but I think my time is coming. I loved the gearing of the 3.27 but admittedly I think the 3.54 will be best and I think the following numbers will show that.
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From the following chart you can see how the ratios compare between the factory Spitfire and various Miata set-ups. Final Ratios are Highlighed in green.
Thanks to Flyin' Miata for the gear ratios, what an excellent site. https://www.flyinmiata.com/tech/gearing.php
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Choice of Transmission :
Looking at that I went further to get a final ratio when mixing Transmissions and Differentials
In reality that doesn't help much so I used a website (listed below) to calculate RPM vs. Speed. As mentioned, I'm running on 185 55 15 tires that have the exact same diameter (23") as the factory 155SR13 tires (modern description is 155 82 13) so that is convenient.
Below are the RPM expectations when running at 100kph (62mph) and 120kph (74.6mph).
To me, even in hindsight, these number tell a great story. I suspect that the gearing for the Miata was intended to hit a sweet spot on the highway of somewhere between about 3000 and 3300 RPM at 60-65 mph. Given the light weight of the Spitfire I've found that even with the 3.27 gears operating at 2800 RPM on the highway the engine does not labour, but I suspect the new RPM will be an excellent all 'round ratio.
Shifter Position:
Early in the process I quickly realized that compromises would need to be made, but with further research I quickly knew where I would not compromise.
Although it is very tempting (and maybe cool) to jump into a later 5-speed or even the 6-speed transmission they really weren't was was best for me.
I believe that the early 5-Speed is really the only Miata transmission that is practical to modify as required. The later versions including the 6-speed have the remote shifter assembly incorporated into the Extension Assembly and would, if even possible, require much more surgery (maybe one day if I can get a cheap one I’ll give it a go).
There is some conflicting information, I think mostly due to Domestic Markets, but from what I could see these are examples fo the two other Transmissions.
5-Speed 1998-2005 6-Speed 1998-2005
Trimming the Case:
My Choice:
​I was very quick to rule out the 6-speed, mostly because of shifter position, but almost equally because I didn't see the benefit to extra gears with such a close ratio, and a higher top gear ratio. The later 5-speed would have been nice with the speedo pulse generator, but fortunately that piece can be easily installed into the older 5-speed.
As mentioned earlier, moving the shifter forward 4" is pretty straight forward and has been done a number of times, again here is the best write-up I've seen.
Our objective was 6" and it worked out fabulously giving us a better-than-stock shifter position.
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My partner in crime had some serious doubts about our ability to implement my plan to fine enough tolerances to avoid some resistance and/or binding of the Control Lever. My turn to say "I told you so" :). Maybe we lucked out or just maybe patience and accuracy paid off. Or more likely it just wasn't that big a deal.... but it worked out great.
Quick Recap:
A slight recap if you read the General Fitment section....
Looking at the Miata Transmission it is very obvious that the Cast-In Bracketry that holds the PPF (Power Plant Frame) would need to be cut/ground off. The casting on mine and I'm sure all of them, as I could see from the inside, did not have any channels or such that went into the bracket portion. Cutting this off was a pain my brother endured with the wrong tools, sorry Klaus; best to get aluminum cutting and grinding wheels.
The Plan:
The Control Lever was the easiest part of the equation and by simply measuring the delta of before and after locations of the housing we knew exactly how much the rod needed to be cut. It was left till the end and I'll discuss it later.
Hopefully we could use the aft securing holes of the Change Control Case lined up with some new holes through the casing of the Extension Assembly. We would come up with some way to put nuts on the inside of the casing without having them back out and FOD the tranny. (FOD= Foreign Object Damage)
Because of the potential of damage from even a little bit of force pushing up on the Change Control Case we would also have to secure the back of it.
I had picked up an extra 5-speed for a hundred bucks just in case we buggered this one so we tromped on in confidence.
Extension Assembly:
Taking off the Extension Assembly wasn't too bad by just following the Miata Factory Shop Manual instructions; I think it’s easiest with the tranny in neutral, but I forget now.
The Extension Assembly has no gasket and was a touch stuck on, but not bad.
Then we followed the instructions to disassembling the rest of the Extension Assembly and drifted out the roll-pin being careful that there was room on the other side of it; then the Change Control Case came apart nicely.
Once apart, although out of order, just to make sure we didn’t damage it, we removed the Oil Passage right away and put it in a safe place (it has a small rubber end to it that can fall off too).
We also removed the two locating pins from the Extension Housing with a touch of PB Blaster and by simply clamping on some Vice-Grips, twisting, and pulling 'em out.
Mounting the Change Control Case Further Forward:
Because the Extension Assembly tapers wider as it goes forward, the Change Control Case required a little bit of massaging and creativity to fit. By sliding the still unaltered Control Lever in through the housing we were able to see how the Change Control Case would need to be trimmed and shimmed for perfect alignment allowing smooth movement on final assembly.
We needed to trim part of a small ridge on the Extension Assembly and taper part of the Change Control Case as well as use a few washers (ground to a wedge) to get a nice solid flat fit.
When drilling the pilot holes further forward to put the bolts through we noticed how thick the Extension Assembly housing was and at that point decided to just tap those holes for some bolts we had on hand and use tread sealant and some lock-washers. Worst case scenario we'd have to enlarge the holes to go back to my original plan. We were pleased at how nicely it went together.
We also installed the forward left bolt with some Loctite and ground it down to have something nice and solid to brace the Case on. Taking our time with multiple fittings we got it to sit just right.
After a number of fittings with the Control Lever in place and the bolts torqued down to about 10 foot-pounds we achieved unimpeded movement of the Control Lever and the Change Control Case was secure.
Control Lever:
We took our measurements and put in a new hole for the Roll Pin and then cut it to length. Obviously clamping down the rod and using a nail-set makes the job that much easier. We also did a minor counter-sink to help in aligning the roll pin.
One of the biggest blunders I made in this build was making that roll pin hole ever-so-slightly too small. When I went to drive in the pin it went in just enough to not come out again. There is no access to the back side to drive it out so my next effort was to cut it off with a rotary tool and then drill it out even though I figured I'd just end up breaking the bit. I did. The only way to get it apart now would be to cut either the Control Lever or the box. Fortunately the pin did go in enough to do its job so we just left it. Unhappy with my mess-up we carried on. (Just occurred to me this second that I could just drill a hole in the opposite side of the Change Control Case, just large enough to fit a drift, drift out the Roll Pin and then tap the hole to accept a simple bolt as a plug. humph)
Rod Cover:
The Rod Cover allows transmission lubricant to pass through it and has seals melted on both ends. We felt it important to be very careful with those seals and ensure that after cutting out about 6 inches from the middle the weld would be complete and not seep. The Manual mentions that there is a dedicated forward and aft to that Rod Cover so we tried to replace it as it came from the factory.
While cutting the Rod Cover with a normal plumbing pipe cutter it occurred to me that a half inch domestic copper water pipe might just fit inside it to keep it perfectly aligned during welding, and of course the weld would not bind to the copper. Man, the pipe fit perfect and we ended up with a beautiful piece. I ground down any protruding material from welding so it wouldn’t interfere with fitment.
Before
After
Speedometer Delete Modification:
As mentioned earlier, using the Stock Speedo Cable was not going to be an option for us; the cable comes out of the tranny perpendicular to the casing and would not fit around the frame.
We may have been able to get more creative modifying the frame but, seeing as I was going with aftermarket gauges that would have the speedo using an independent pulse generator I pulled the speedo gear off the output shaft.
We also modified the Speedometer Cable Fitting by cutting bits off it and then tapping the hole to use a bolt as a plug.
Really only one of these two things needed to be done, but we did things at different times and missed that. Should've left the Output Shaft Gear in place.
Since this was done I read on Flyin’ Miata / Solo Miata that the newer VSS or Speed Sensors (1999-2005) were electric pulse generators. What really surprised me is that apparently they fit where the earlier Speedo Cable Fitting goes. I’ll look into trying that and report back here if there is a way to use that device rather than the stand-alone I have planned for. I may still struggle with the frame in the way of that sensor, so not getting my hopes up.
After installing a new Oil Seal at the Output Shaft, the Extension Assembly went back together as expected and the Hylomar and thread sealant worked great.
Bracing The Aft Part of The Change Control Case:
As mentioned earlier, with only the two bolts holding the Change Control Case in position I did have some concern about possible damage from harsh handling of the Stick Shift, perhaps during an AutoCross. After all, those two bolts were only bolted into threaded holes in the Extension Assembly Casing.
There were two vacant threaded holes in the top of the Extension Assembly aft of the Change Control Case that were pretty easy to incorporate. However, with limited areas to attach to the Change Control Case I decided to use 2 of the 4 bolts that hold the Blind Cover on.
Out of cardboard I cut a few templates and then moved to steel. By bolting the pieces on the transmission I was sure to weld them in with exactly the correct geometry so as not to change the alignment of the Change Lever at all.
I put a slight bend in the vertical piece, or gusset, for two reasons. Firstly the Extension Assembly bolt on the right side would have gotten in the way of it, requiring a shorter, less effective gusset and the bend would also help prevent any rotation of the Change Control Case as a straight gusset may have bent slightly with hard movement. Generally I’m not that Ham-Fisted, but may as well build it the best I can.
Aside; all of the bolts and extra washers in the image were merely for the fabrication process; later they were replaced with the correct fitting pieces.
Reassembly:
The notion of adding a gasket for easier removal down the road occurred to me but that would add thickness and hassles...
Mazda knows what they are doing so we just used Hylomar and some thread sealant on the bolts. In fact I've had such good luck with Hylomar, even on that British engine, that I just used it as a thread sealant.
Transmission Fluid:
Later when it came time to fill the tranny I looked to the Miata forums and found a lot of guys recommending Ford XT-M5-QS Transmission Fluid.
51 Page Thread here.... http://forum.miata.net/vb/showthread.php?t=270393
This was no doubt over-kill at over $80 for the 3 litres required, but stupid me bought it.... $80 of fluid in a $100 tranny! Sheesh.
Of course following the workshop manual includes adding fluid to the Change Control Case as well.
Results:
After a season of decent use including a bunch of AutoCrossing all is still well; absolutely no signs of any issues. :)
Misc:
While we had the transmission separated from the engine we inspected the clutch components. The only issues we identified were 4 small cracks on the clutch friction surface of the flywheel; they were radiating outward from the inner edge. The young fellow I bought the donor from was a little enthusiastic and really beat the crap out of the car. We sanded down the surface and the cracks weren't really bad so we just pressed on. Fortunately the friction plate still had lots of meat on it. Cleaned everything, sanded the friction plates and pressure plate, lubed the throw-out bearing and shaft, installed the modified fork and put 'er back together.
