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ARP steps the shank of the bolt, leaving ALL the knurl at the smaller diameter. This is not optimum, but should work OK.
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GM steps the knurl, so that the big part of the knurl fits the starter, and the small part of the knurl fits the block. I like the GM bolts better than the ARP.
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The PG260 and many other PMGR "mini-starters" have 10mm holes in the starter housing, but the block is bored and tapped for 3/8-16. Thus the need for stepped starter bolts--big enough to fit snug in the starter housing, but with a 3/8-16 threaded section and smaller knurl to fit the block.