Looking at the judging standars, it says that the powerhouse 1 mounting bolt was 7/16" until Dec 1927, then P2 used 1/2" -20 by 2 7/8"until September 1928, then P3 used used the same bolt as P2 until they swaped to a normal generator in July '29, so check the timing gear cover and see what size bolt it is. Hope this helps