I have recently become all to aware that RH4 head are commonly cracked by the intake guide(s).
Out of nine 850 heads in my garage at present, 3 are small port RH10 and none show evidence of cracking when processed by a dye penatrant test.
Out of the other six which are big port RH4.....4 have cracks next to the guide going out toward the carburator end of the port. The heads are quite thin at this spot, being just below the spring pocket.
Anyone else aware of such things?