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As the engine is "503" with no letter, it is an original run engine and as such, is the least desirable of them all due to problems Mazda had with these engines failing. The problem is with the irons and these were redesigned in the B series and greater engines. FYI - I have C339 which makes me happy. From the "other" forum:
I do not know the total numbers of "runs" that were
made but it goes like this:
the first run went from 001 to 999
then the next run was A 001 to A 999
there may have been more than 1000 per run but this is
what I have seen.
next came:
B 001 to B 999
then last was:
C 001 to C 999 (actually D was the last series)
the differences between the run numbers are strength.
the first 2000 motors were the first run and are the
ones that mazda had the problems with, and had
changed lots out on warranty to the newer motors
at great expense.
I was told that the first run motors have a short life
and its not a matter if they will fail, rather when,
especially under higher boost levels (anything over 12psi)
the strengthened housings of te 2nd run (B toC)
make the motor much more reliable and hold higher boost.
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