why is it that only on the honda engines you have to have the rocker arms hardend when doing mods.

 

It's not just honda's. But it's almost all honda's that need it.

It depends a regrind cam will need it due to they weld a build up to the lobes. The build up is a harder metal than the rocker arms, hense permoting ware to the rocker arms. You can get drop in cams wich don't require the hardening, but they are usually about 3 times the price. Another reason is the heavier valve springs. The heavier valve springs give a more possitive push on the cam wich will permote ware as well.