I believe it is a technique specific to the stretch bolts. Getting the right initial clamp by tightening to the specified value and then doing the 90deg and 90deg gets the bolts to the right "net" value by taking advantage of the "stretch" feature. I am not all that knowledgable about the metallurgy behind it, but it's what's called for. On another note, several people have suggested to me (and it's what I did) that one should wait a little bit between bolting them up to the torque value and beginning the 90deg turns. Kind of like, tighten to spec, have a beer, tighten 90deg further, have a beer, tighten final 90deg.... Could be witch doctor/old wives tale, but it makes you think about what you're doing for sure.
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'95 RRC LWB, '05 Suburban Z71 (not stock), '94 F350 (only the cab shell & rear bumper is original), '62 International 404 tractor (red & white) & many other project cars in the past!
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