RC40's aren't made in 1 7/8" you might be measuring the flange...
Mine is stamped '1 7/8" ' on the side just by the rear silencer. I measused the 3-into-1 manifold pipes today and they are 1.5" going into a 1 7/8" straight through pipe.
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Is a 1 7/8" classed as big bore and will this set up be OK for my 998?
No, it's perfect for a small bore.
That is good to know. I just need the time to be able to swap it over now.