FC to Miata Gearset

http://howto.globalvicinity.com/gv_wiki.php?wi=118&co=1&vi=1

Here's a few pics and tips for SA, FB, and FC transmissions. The main focus is the installation of the 94+ Miata MX5 gearset, making a relatively close ratio box. Although the earlier Miata transmissions use the same ratios, a dual cone synchro was introduced in 94. With the current age of these cars, used transmissions are readily available so there's no sense using a pre 94 gearset. Quicker cleaner shifts, more time spent closer to peak HP, and fewer shifts in turns are the only advantages to using the Miata gearset. It's no stronger and no more reliable than the FC gearset.

If you haven't done a few trannies or don't have a factory manual and the correct tools, don't try this. This is not an instruction sheet, but merely an informational page. Anything you do is done at your own risk. Blah blah blah, and the standard "racing only, not for street use" disclaimer. Bottom line, I didn't tell you to do it, or how to use it when it's done.

Here's the rub, Miata shaft on the right is longer and must be precision ground to the correct length.



The grinder cut the spline a bit short (~.100") but it'll work. There's still enough spline for the disk not to overrun either end.


2nd gear synchro...the dogs in the inner brass locate on the hub, the cone has dogs that locate in the gear, and the outer brass functions as normal. When the keys push on the outer brass, it pushes the cone against the inner brass so they work in unison. When released, all three float.

The splined R/5 gears and synchro hub are interchangeable with all versions of this trans that I've seen. Swapping 5th gearsets can give overdrive ratios ranging from .765:1 to .875:1.

If you've rebuilt a few gearboxes, you may have noticed new brass rarely works well on a shiny cone without lapping. As the brass wears, so does the cone. The cone becomes polished and will eventually show a wavy/stepped appearance.




The assembly looks almost identical to the FC. Notice the reverse gears and brass?
With the excessive wear I observed on the reverse brass and cone, it may be advisable to swap these with an earlier style.
To get proper operation, I don't change brass unless it is damaged or approaching it's wear limit. Instead, I resurface the cone. Using fine emery cloth over a piece of hardwood, the cone can be hand finished if wear is minimal.




Proper roll pin installation is shown in Mazda manuals, but even the factory gets it wrong occasionally. The slot should face the direction of travel, if it's perpendicular it can act as a spring. The 1/2 and 5/R forks are installed wrong, the 3/4 fork (center) is correct. The slot faces either end of the rod, but not the side. If you want overkill, 1/8" welding rod can be driven into the pin after installation.



If you want to use the early RWD 626 gears, Mazda says the same mod on that input shaft will accomplish the same task.

Safety Solvent makes as good a lubricant as any, so I usually do it when I'm clean parts. If wear is extensive, the uneven surface will show as alternating lines of scuffed and polished steel. Extensive wear can rarely be repaired and precision grinding is the only way.