The Miata’s Manual Gearbox is Refreshed

The PLAN was to spend free time during this past winter working on the Miata. What happened? Where did the winter go? Of course, I ask that based on the CALENDAR, not on the actual WEATHER. (As I sit here composing this missive at 7:24 p.m. EDT on April 15, it is 38 degrees F outside, and the rain and wind make it feel like 31F. Clearly, it does NOT feel like spring!)

The to-do list for the ’93 Miata, drawn up last November, included: rear brake service, transmission service, new lights, new tires, and an engine compartment detail. I haven’t gotten very far. The first item to be tackled, the brakes, wasn’t started until March, and still needs bleeding and parking brake adjustment before it’s crossed off the list.

Since e-brake adjustment requires removal of the center console, I combined that with servicing the shifter. Here was a case where online forums provided information not to be found in a service manual.

My Miata service book, published not by Mazda itself but by an independent publisher, is quite good. However, it says nothing of servicing the shifter “turret”. The turret is an oil-filled box at the rear of the transmission, in which the shift rod connects to the external shift linkage. It does not share its oil with the rest of the gearbox.

The turret service info was found at www.miata.net, and I’m grateful to the contributors who provided both a step-by-step tutorial, along with a list of suggested replacement parts. I was also greatly helped by the YouTube video posted by Miata Mike, self-proclaimed King of the Miatas.

Under the leather faux boot is the real rubber shift boot. Old (L) and new (R) side-by-side.

Once the shift knob was unscrewed and the center console lifted out of the way (the leather boot attached to the console is but a decorative item), it was obvious that repair work was overdue. (This is what happens when you drive the same car for 21 years, and the small deteriorations are not noticed.) The large rubber shift boot was shredded, and the flexible rubber cap, bonded to a metal plate which forms the top of the turret, was equally damaged. Removal of the cap allowed the shift rod itself to be extricated. The plastic bushings at the bottom of the rod were worn but not broken. Most of the turret’s gear oil was gone.

Bottom of shift rod, with old plastic bushings still attached

One of the major forum findings was just that: “You’ll find the turret to be empty or almost empty. Service it by refilling it with oil”. The mystery remains: where did the oil GO? Using a turkey baster which has been appropriated to the garage, the scant remaining oil was sucked out, and fresh 75W-90 gear oil was added until it almost reached the top of the turret.

Peering into the now-empty turret

It was time to rebuild the shift knob. The aftermarket replicates all the needed plastic and rubber parts; however, scanning the various online listings convinced me that spending a bit more and getting OEM components was the wiser move. A Mazda dealer in Vienna VA, Priority Mazda, runs an eBay store and had the best combination of price/availability/shipping cost/delivery time. I placed the order and had all my parts, in Mazda bags, at my house in 3 days.

Old (bottom) and new (top) service parts lined up

The new pieces went together quite easily. With the turret full, everything at the center console was reinstalled. While I was there, I drained the gearbox oil, and again using a recommendation from the forum, refilled it with Valvoline “Manual Transmission Fluid”, GL4, NOT GL5. After visiting 3 auto parts stores looking for this stuff, I had to order the Valvoline online also. What did we do before the World Wide Web?

Valvoline MTF, hard to find in stores

 

The trick to getting 75W-90 gear oil to flow in 50 degree weather

 

Rebuilt shift rod reattached to now-filled turret

 

The Miata is still up on 4 jackstands; just as well, because it ain’t goin’ out in this weather just yet. Once it warms up, I’m excited to take that first test drive and try out the shift action. With fresh tranny oil, refilled turret, and new rubber booties, I have great expectations. But I better put a hustle in my bustle. The NJ Region AACA annual car show is Sunday May 6, EXACTLY 3 weeks from today, and my now-25-year-old Miata will be making its AACA debut there. It’s at the Mennen Arena in Morristown. If you’re in the area, I expect you’ll come by.

All photographs copyright © 2018 Richard A. Reina. Photos may not be copied or reproduced without express written permission.

The Isetta Saga has many more chapters to go before reaching its inevitable conclusion. With the help of some colleagues, I’m working on a big surprise, and hope to have it available for your viewing pleasure soon.

FUN FACT:

Road & Track magazine, in its July 1989 edition, ran its first full road test of the new 1990 Mazda Miata. A sidebar article crowned it one of the “World’s Best Cars”. Here’s what they said about its manual transmission:

“…. performance is further enhanced by a close-ratio 5-speed that rates nothing less than a 10 for its smooth, positive operation. With the feel of a Formula car, this tranny is fun just to run through the gears.”

 

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