I’ve noticed that the Miata has been a little down on power, and has been getting harder to start these days. I knew that the early generation Miatas (1990 to ~1993) had a design flaw that can caused the timing gear to loosen and start retarding the engine. I decided to dive in and take a look. Here’s what I found.

Initial Look

Here’s a video of me measuring the misalignment between the piston and the timing belt assembly.

Tear Down

I drained the coolant, removed the radiator, and disassembled the timing belt assembly. Here’s what I found after loosening the crank bolt.

Initial Reveal

Here’s a look at the damage after removing the timing gear from the crank.

Damage Analysis

After some cleanup, I took a look at what I have to work with.

You can see that once the crank bolt loosens, key way takes most of the load and begins to deteriorate.

The key itself also starts to get worn down as well. jpg)

There’s about 1mm of gap between the key and the key way on the left side.

This is a comparison between the old vs new key that I got from Treasure Coast Miata. I’m not sure why there’s a difference in length, it must be something that Mazda decided to redisn; Treasure Cost Miata sells all genuine Mazda parts.

Here’s how the gap look with the new key installed. I’ll be filling the gap with some Loctite 660 to repair the issue.

Conclusion

The damage isn’t nearly as significant as I was expecting. I’ll be using the Loctite 660 repair method, and reassembling with a new key, timing key, and crank bolt. I plan on rebuilding the motor this winter, so I might end up swapping in a long nose crank that’s in better condition.

-Dan