CZ 550 thread size

[guntech]

Maybe a metric machinist can tell me what the "sp3" means in a thread description of M26x2sp3 thread.

26 mm with a 2mm pitch but what is the sp3? Does it refer to the rounded thread point?
I think it is also 60 degrees ...?

 

[PEI ROB]

Metric threads are 60` by ISO 965 standards as well have the same basic profile and pitch diameter calculations . M26 is special purpose, neither coarse not fine. There is a 3 tolerance class in metric, external only and its tighter than UN class 3. Internal only goes to a tolerance of 4, lower being more exact. ISO Class 3 is half the tolerance of ISO Class 6 which is the common class about equal to UN 2. sp3 makes no sense to me technically speaking, far from an expert on this though.

Oh yes, if MJ is used instead of M it means the same profile as when UNJ is used instead of UN. Both are controlled root radius to allow a larger minor diameter for external threads.

 

[guntech]

Thanks, that's enough info for me to thread this barrel...