Flexible exhaust pipes are available in different thread sizes.
10mm size (above) for 50,56 singles and 100 twin (thread size is M10x1.0)
12 mm size for 65, 72, 80, 91, 100 engines. A flex pipe exhaust without pressure tap is part number SAI65111A shown below.
A 12mm flexible exhaust pipe with a pressure tap is Saito part number 182TD-1112 (Horizon Hobbies part SAI182TD1112)
The Saito-125 exhaust size is 13mm. The flex exhaust is SAI125A111
14mm size (see left and below) for 120, 150, 180 singles and 270,300 twins.
The Saito-220 has a 15mm thread. SAI220A143 is the flex pipe part.