I have a (tiled) block cavity wall with a 100mm dia. hole in it, for a fan. I
want a concentric 150mm hole (for a bigger fan duct). It seems to me a
well-fitting cylinder (?mandrel) with an axial pilot hole would help, to guide
a diamond core dril, especially for cutting the porcelain tiles neatly. Is
such a thing commercially available? The two problems about making one are
that it would be hard work to make it fit tightly and it would be an expensive
chunk of wood. Also my carpentry is usually crooked.


--
Roger Hayter