can be done by others means, even use a soft open flame and damage the old ones,be sure the valve seal is very clean after wards with no stuck on bits of seal left behind, I use a niclely fitting wad punch by hand to clean the external surfaces of the guide, with a bit of wire wool in the end, but beware of leaving debris from that, blow clean with air and wash as well but when replacing you will need (preferably) a purpose made holder/drift to install them to the correct dimension and set squareley, but again, it has been done with deep sockets, box spanners etc, it can work, but then again.......
its always nice to have the correct tools, ie a good set of seal pliers and the holder/drift, I mean if you are trying to a proper job then why bother if you havent got the proper tools?
CLEM
