There has been a lot of discussion with some fire departments about using BAT thread instead of AMA thread or BCT thread so we wanted to inform our customers if they should hear the chatter on this topic. Below is a short explanation of the thread terminology we use.
ODM = outside diameter of the male thread
TPI = threads per inch
AMA = Alberta Mutual Aid thread (AB)
BCT = British Columbia Thread (BC)
BAT = British Columbia and Alberta thread (BC & AB)
AMA is specified as 2.990 ODM x 8TPI
BCT is specified as 3.000 ODM x 8TPI
Now the male and female threads obviously cannot be cut with the same size as they will not fit together. So the allowable safe difference either way which is 0.03 and normally 0.02 is used in the industry. So 0.02 on the male and 0.02 on the female so a total of 0.04 tolerance on a set of couplings. Keeping in mind we are allowed 0.06 difference from the male to the female.
So if we take the AMA thread and remove 0.02 from the male it is 2.97” and the female will be 3.01” at its very tightest.
So if we take the BCT thread and remove 0.02 from the male it is 2.98” and the female will be 3.02” at its very tightest.
So when we supply a BAT thread we use 2.97” on the male and 3.02” on the female. This is a difference of 0.05” which is well within the tolerances required. This will insure that if a department has a BCT thread or an AMA thread the BAT will safely fit their existing hydrants and hardware that are cut as BCT or AMA.
Now we have hydrant manufacturers, hose manufacturers, truck manufacturers, valve manufacturers and adapter manufactures all cutting threads and it has been very rare for us to have heard of any incidences of couplings coming apart. Threads do wear down just like everything else in the fire field so it is wise to check any threads you feel are loose or damaged and take the most used connections and rotate them out on a regular basis to assure a good fit. We do not even see departments changing their gaskets let alone their fittings. So maintenance is very important issue with the fire threads on all your appliances. And with a good set of calipers you can compare the difference from the male to female to see if the tolerances are within the guide of 0.06.
Remember the best way to be safe on the fire ground is training, maintenance, and common sense.