NA Miata Starter/Engine Mount Bracket
While changing the right engine mount today I took another look at the differences between NA and NB starters and motor mounts. NA cars have an additional bracket that connects the starter with the motor mount; NA and NB starters are different as a result and there are differences in the engine side bracket that the motor mount attaches to.
First, the starter differences.
Here is a 97 starter:
The two studs on the front of the starter are circled in yellow. For future reference, they have M5x1.0 thread.
This is a 00 starter:
It has no studs on the front. There are two bolts but they are spaced wider than the studs on an NA starter, making NA starter brackets unusable with NB starters.
Starter/Engine Mount Bracket
The bracket on NA cars, when assembled, looks like this:
There are actually two separate brackets connected to form somewhat of an L shape. The photo shows view from the front of the car.
Right Engine Mount/Engine Bracket
On both NA and NB Miatas, the engine mount itself is sandwiched in a pair of brackets which provide bearing surfaces against the engine and the front subframe. The engine side bracket is different between NA and NB Miatas to accommodate the additional attachment point for the starter bracket.
The NA bracket looks to be beefier, while the NB bracket looks thinner. The reason for this is probably the fact that the extra hole on the NA bracket is threaded, thus needing a certain amount of material thickness. Another difference is the edges on the NA bracket are flat whereas the edges on the NB bracket are curved; the curved edges are probably increasing the strength of the NB bracket.
With the starter secured to the transmission housing, install the starter side of the L shaped bracket using M5x1.0 nuts. I strongly recommend using vibration-resistant fasteners - either lock washers, lock nuts or serrated flange nuts. The result:
Then install the engine mount and its enclosing brackets. To retain the subframe side bracket I place a couple of loops of electrical tape around the whole assembly.
Finally install the rest of the L shaped bracket. Final product:
An NB engine mount bracket will not work with an NA starter bracket because the edge of the NB engine mount bracket is curved. The starter bracket can be omitted from the car, at which point an NA engine mount bracket will bolt up into an NA chassis, but see below for concerns.
An NA engine mount bracket will work in an NB chassis, with the extra hole for the starter bracket remaining unused. However, the NA engine mount bracket is heavier than an NB engine mount bracket.
Multiple sources indicate that NB starters are lighter than NA starters:
- 1990-97 5-speed (and 1990-93 auto): 9.80 lbs
- 1994-97 automatic: 7.05 lbs
- 1999-05: 6.00 lbs
Perhaps Mazda determined that the lighter NB starters do not need additional support that the NA starter bracket provided.
According to these reports, NA and NB starters differ in one of the mounting holes. Multiple posters say that attaching NB starters with only two bolts is fine, although they do not specify whether they track their cars.
As the starter mounts to the transmission case rather than the engine, using an NB transmission with an NB plate between the engine and the transmission in an NA chassis should make an NB starter bolt up to the combination with all 3 factory bolts, though I have not tested this.
Finally, here is a side by side photo of NA and NB starters from miata.net: