The OEM gaskets were 0.8mm thick (0.032").
The modern aftermarket gaskets (like Vesrah's or Cometic's) are 1.0mm thick (0.040").
Hondaman,
Last question (you seem to be a guru round here) I'm putting a 650 cam in my 550. Being very careful to not repeat some horror stories I've found on here. What should minim valve clearance be between the valve and the piston?
Well, I'm not the 'racing guru' around here, but I've worked on (a lot of) these bikes for a long, long time.
The number for piston-valve clearance I've used as a minimum for the SOHC4 engines has always been 1.0mm (0.040") as measured using the clay-on-piston method, and the old head gasket I took off (so I don't wreck the new one).
On those rare occasions when Honda Racing (in the old days, when we still had to step around the still-molten lava pits to get to work...) would leak any of these tidbits to us thru the Honda reps of old, we'd stick to their numbers. The old 'bike gurus' back then were always speaking with H-D or Triumph/BSA wisdom, which was a whole 'nuther world (like 0.006" piston clearances!) and would make Honda engines unable to even start after their 'hop-up' was built with those numbers. Imagine the battle when I'd take a set of 750 or 500 cylinders to the machine shop and tell them I wanted 0.0004" piston clearance, which they would immediately tell me was 10x wrong, after which we'd argue...etc. That's why there is often confusion on the web about how to build these bikes nowadays.
Cams in the 750 were sorta tricky, but cams in the smaller Fours are more so, because taller lobes in those tiny cavities can make things too tight, needing 'secret' mods. Others here will chime in, but watch carefully for mechanical interferences in the 500/550 head when installing taller cam lobes, as they can bang into the rocker arm webs and head cavities pretty easily!
