I had to go through my own records to recall a few details about plugs. As you can read and see im my MEL engine build up story viewtopic.php?f=42&t=393
There are two plugs that can be easily skipped by accident.
I assume that you were talking about this plug?
Some machine shops ignore it for a simple reason. Too much time involved to remove and to reinstall.
Please take a look at the second photo for an other plug that might get skipped in the process.
Installation seems tricky but is a lot easier than you might expect. Installation of these plugs should take place right after the block cleaning process and right before you put that biest on a stand.
With the engine standing upright on its' bell housing surface the plug seen in photo 1 can be positioned and driven in a lot easier. The same goes with the rear plug (see photo 2) and with the engine standing on its' timing cover surface.
I hope this is going to reach you by time.
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viewtopic.php?f=2&t=414