What are the muffler thread sizes on the 60 and 120 SP. I am looking for a more compact muffler piping possibly a flex pipe like Satio's.

No problem. The thread sizes are as follows:
58CD, 91CD, 90SP & 120SP = M11x0.75
60SP = M10x0.75

There are numerous alternative pipes you can use, please ensure that it is suitable for a 4-stroke engine. The Saito's flexi pipe should be fine.

Best regards
Laurenne

hello i just purchased the 91 cd for my GP Stuka,you said that saito has a flex pipe that i could use?. i would like to have an incowl muffler.
but i went to their site and the thread size you wrote down wont match any of the saito flex pipes sizes available
thank you for your help

P.S a part number would be very helpfull and i would be very greatfull