That is common around 1988 1989 as I recall. It is a core shift or casting
defect. It happened right after they moved the O-ring groove
from the rotor housing to the end housings. Not enough meat around there.
I think Mazda changed the patterns later to fix this problem.
Bruce Turrentine says even some new end housings have this
problem right from the get go.
Paul Lamar

Blake Mantel wrote:

Paul (or anyone else)  do you know what the dimensions are on the O-ring groove?

I have a friend who wants me to make a cutter path for making a new groove on a part.

Thanks
Blake

I think you will have to get one and measure it with a height gage. 
My measurements would be a rough guess at best.

Paul Lamar
 
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