Maximum lift?

Can somebody give me a ball park figure for the maximum valve lift a stock J head will handle before you need to cut down the guides? I seem to remember reading way back that it was around .510", but I'm old, so my brain doesn't work as good as it used to. :sad1: How much clearance is required between the top of the guide/valve stem seal to be safe?

Another question. Does anyone know the proper name, or a part number, for the "spider" on roller cam engines? I've tried searching Mancini's site but can't find it. I found the tappet alignment yokes & the lifters, but no spider.