I know this reply is very late coming but I recently went through this same issue about diameter and thought this info may help other people.
First bit of advice is to IGNORE ALL THE SPECS for external diameter (even on this site, or amazingly even the Floplast site) because they are all wrong. A good example as proof is if you buy a 40mm solvent weld Floplast fitting, it comes in a bag that reads 40mm, but the actual fittings in the bag have 43mm moulded into them, and they are DEFO 43mm.
So why ? well the answer is that 40mm pipework (and fittings) are just a reference size. In reality they are available in either PUSH-FIT or SOLVENT WELD versions, and they are NOT the same size, or the same material, and they are not interchangable.
40mm PUSH-FIT pipe is 40-41mm ext dia.
40mm SOLVENT WELD pipe is 43mm ext dia.
So just use/purchase the type you presently have, either PUSH-FIT or SOLVENT WELD, and you will be fine.
Good luck ;-)
Answered by: ScrapheapSteve
Date published: 2021-09-15