Interesting thread. Shows the potential incompetence shown by places you would expect to be fully competent and equipped.
As a new owner to the 172, but a competent home mechanic, I thought i'd share my views of a belt service including Dephaser pulley.
Out of 10 (difficulty), i'd rate it a 6 or a 7 *IF* you have the right tools.
Out of 10 (fiddly-ness) it gets an 8 cos of the god-damn aux belt tensioner. When i turned it back with a 3/8ths drive ratchet, it broke the fitting in half (too lazy to go get the right size square lol) and I had to figure out another way to loop the belt over.
So... The biggest issue to the home mechanic is the floating pulleys. No woodruff keys, no timing marks which seemed stupidity of the highest order to me. Retrospectively though, its not that bad at all. In fact it makes it easier, provided you have to correct locking tool.
I have the right tool to go in the cams on the flywheel side, and the locking pin for the crank. With these you can do the job perfectly well.....you dont need the pulley lock- just remember to crack the pulley bolts off AFTER you lock the crank but BEFORE you release the old cambelt. I found the exhaust pulley bolt very tight and needed a breaker bar to crack it off.....with the pulley free that would bend the cam-lock tool. (Ok, ok, I confess, it DID bend it and I had to revise my approach as described lol).
With the cams, and crank locked in exactly the right place and the free spinning pulleys with loose nuts (oooer mrs), looping the belt over the crank first then idler-cam-cam-idler then tensioner is ridiculously easy. Simply tension the belt before you tighten up the cam pulleys.
I took the headlight and front bumper, and slam panel off to do mine. I'd definitely do that in the future if I do any more....WAY easier for access for the time it takes.
After doing my own I could quite happily do a few more I think, and easily for under £450 including all the parts. Pity the dephaser pulley is so expensive really!