Hi @PatDIY_Dad,
Thank you for your question on installing a snap tee to an existing stormwater pipe.
While it should be a fairly simple job, it is worth noting that plumbing is a licenced and regulated industry governed by strict codes and standards. For this reason, you must understand your liability in relation to plumbing works. It may be worth contacting a plumber to carry out this job to ensure it follows the relative codes and standards.
With this being said, my understanding is that the advised method of installation is to snap the T fitting over the stormwater pipe and secure it in place with Protek 125ml Red Priming Fluid and Protek 125ml Type N Blue Cement Solvent.
Once secured in place and watertight you would use a hole saw to cut out the entry hole down the vertical part of the t-fitting. The largest hole saw you can fit down the vertical section of the t-fitting is going to be the best size for you because any size reduction will cause the flow of water to back up slightly.
My suggestion would be to pick up one of the t-fittings, and take it to the Tool Shop to compare the opening with the hole saws.
I'd assume the largest hole saw you could fit down the pipe will be around 90mm as you have said. Something like this Kango 92mm Hole Shot Bi-Metal Hole Saw would likely do the trick, but it is definitely worth comparing in-store before buying.
Let me know if you have any other questions.
Jacob
