The spacer prevents the timing gear from going onto the cam snout far enough to lock up the gear against the thrust plate. If you measure the spacer with a micrometer or a dial caliper, it should be a few thousandths thicker than the thrust plate. I can't say which way the concave goes, but it really shouldn't matter since the spacer is sandwiched tight between the gear and the cam.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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