I just found a thread with someone saying the valve seats have been reinforced for the ST170.
http://www.lpgforum.co.uk/viewtopic....t=st170#p83943
Cylinder head
The compression ratio has been increased from 10.0:1 to 10.2:1. This has been mainly achieved through
the use of a revised cylinder head gasket together with a larger cylinder diameter. The bore has been
increased from 84.4 mm to 84.8 mm.
The knock sensor, the fuel distribution pipe, the mass air flow sensor (MAF sensor), the barometric air
pressure sensor (BAP sensor) and the injectors have all been modified.
The lift of the cams on the intake and exhaust camshaft has been increased. The diameter of the intake
valves has been increased from 33 mm to
33.5 mm, and the valve springs have been uprated (purple). The valve seats have been reinforced
Still like to get more confirmation about this... There must be some place where you can look up the engine parts and see which material was used right?