Abstract: A T-joint fitting comprising two parts for clamping to an existing length of pipe encloses a piston, which is axially displaceable by firing of a cartridge. The piston cuts a hole in the wall of the pipe and is provided with a transverse passage, which will serve for conducting fluid from the pipe to a second passage in the fitting.The length of the piston is selected so its front, cutting edge rests against the envelope wall of the pipe, when its back face is about level with the open end of the first passage. The cartridge is a disc-shaped shell, having about the same diameter as the portion of the housing enclosing the first passage, this portion having external threads. There is an eccentrically located socket for a firing cap at the face of the shell to be turned outwards in use.A cup-shaped nut engaging these external threads has in its inner bottom surface a recess for reception of the cap containing socket, the recess being further adapted to make the firing cap accessible to a firing pin.