Black water pipe - blue water pipe

[Cpd]

Good afternoon hubbers, I need to brake into my main water line (private gravity fed supply) and install TEE fitting.
The existing supply is old black pipe and the fitting that I have exposed says 3/4 - 20

i need to tee of onto blue 25mm pipe 

 

Does the 3/4 -20 on the fitting indicate that the black pipe is 20mm ? 
and if so is the only requirement a 20mm - 25mm adaptor ? 
 

I am hoping someone will reassure  me before I go out and buy the wrong fitting ! 
 

thanks 

[ProDave]

I would read that as being a 3/4" one end to 20mm the other end adaptor.

 

I would leave that alone.  Cut into the pipe on the right hand side where you can be more certain it is 20mm and fit a tee there.

[TonyT]

http://www.philmac.co.uk/products/

have a squint at the website, make sure you cut the pipe square, use the pipe insert and watch out for cross threading when starting the fitting off.

 

[Cpd]

2 hours ago, ProDave said:

Cut into the pipe on the right hand side where you can be more certain it is 20mm and fit a tee there.

I put the callipers on the pipe and it was about 27mm compared to the 26mm I measured on the new blue pipe...... so at 1mm thicker I took the plunge and cut the pipe and then we had a right old time squeezing the 25mm fittings onto 27mm pipe..... got there in the end and no leaks. Next job.