Roll some toilet roll so you can place on sealant around the bath soak with bleach leave for 10-15 mins then take off and rinse with cold water .Works ok
Pipe is not 7/8 Pipe thread is 7/8 BSP 30.2 m/m diameter
I turned fittings 50 yrs ago and there is also 1/1/8 BSP thats the size of thread on most gas meters.Have a look at.
http://en.wikipedia.org/wiki/British_standard_pipe_thread