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Hi

Shimano SIS Compressionless Derailleur Cable Housing SIS-SP51 has an outer diameter of 5mm.
The usual shift housing diameter is 4mm.
Is the 5mm housing compatible with Shimano Alfine and Nexus shifters and cassette joints?
Also, with Shimano RDs and shifters?
How about housing stops?

Thanks

Kontact 

4mm housing uses thick walled cable stops, and 5mm housing uses the thin wall stops that brake housing uses. The outer diameter of both housing end stops are the same (5.5mm, I think?) so they will fit in the shifter, derailleur and frame stops. So there shouldn't be a compatibility issue.

Some non-Shimano derailleurs used to use smaller stops, but that's only on vintage stuff.

KCT1986

Shimano specs. for cable stops is 6.05mm. If your frame follows that recommendation, then you should be fine. Ferrules for 5mm housing can vary a little in OD, based on material.

If it doesn't work, then step-down ferrules are available.