The relief valve seat in automatics has a 3/8 UNC thread in it. You can get these out easy with a head stud, a nut + washer and some spacers.
However most manual seats do not have a thread in, last time I had to remove one it needed a thread cut in it first. I checked my other 4 manual blocks and they were all like this.
If the seat is dodgy, a 9/16" steel ball will seal it better than the plunger does.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
