To strengthen flue pipe connections use short metal to metal self tappers, available in our Useful Items collection.
As with all vitreous pipe the bottom end narrows to a lesser diameter to enable insertion into other pieces of pipe (male is always on top). Key reason is that any condensation that trickles down inside pipe cannot escape. Straight lengths can be cut using a grinder and 115mm cutting disc (available from this online store).
Top tip: No fire cement or glue of any kind is used when connecting vitreous pipe to vitreous pipe - it is just push-fit. When to use fire cement on flue joints? Use the SEARCH function and type "sealing flue joints".