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Caption this...

Self balancing

And that was putting it on!!

Is that an ATI Damper crank gear? I assume that isn't wear, but you removing it from the crank?

I had acquired an aftermarket harmonic balancer, and then discovered that it had a manufacturing fault. The fault is described here:

https://www.miataturbo.net/engine-performance-56/incorrect-tooth-clocking-ati-dampers-96402/page2/

TL:DR - the teeth that drive the timing belt are out of alignment with the crank key way in a batch of the product which affects timing in relation to both cams.

I contacted the manufacturer, and they offered to replace it without charge, on the condition that I destroy the faulty part of the balancer and provide photo proof.

The new balancer is on its way.

One of the rare occasions you can ‘properly fix’ with a big fulsome hammer.

ManiacLachy wrote:Is that an ATI Damper crank gear? I assume that isn't wear, but you removing it from the crank?

I think you posted while I was typing (see above).

One thing that I read while investigating this was something about the ATI balancer requiring measurement of the balancer and crank, and then hone of the balancer bore to fit. This might explain some of the difficulty had by some when fitting?

Ah got it.

I didn't have too much trouble getting mine on (then off, then on again). Just watch out for the woodruff key, mine snagged on the initial install and caused some minor damage. You do need something to hold the crank while you install, in gear and hand break on isn't enough. If you don't have something let me know and you can borrow my FM crank tool.