Smith 30 series regulator doesn't fit acetylene tank.

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Hey guys.  I'm not sure if this is the right forum for asking this type of question, but I figured I'd give it a shot.  I've just purchased a set of Smith 30 series gas regulators.  It was the twin pack for Oxygen and acetylene.  When it arrived, the oxygen regulator fit the oxygen tank fine, but the acetylene regulator would not fit.  I'm assuming it is because I have the small acetylene tank, i guess it is a "B" tank, but I'm not sure if this is the reason for it not fitting.  If anyone with more experience on the fitment of these can help me, it would be appreciated.  I broke the problem down to one of these:1) Either the acetylene tank is too small for the regulator and has different threads"or"2) The new acetylene regulator's threads are damagedIf the problem is # 1, do they sell a fitting to make it work?
Reply:The answer is most likely #1. The threads on a "B" tank are not the same as those on larger tanks. You can buy an adapter to go from one to the other. A welding supply can hook you up, tractor supply may have the fittings also.The small tank you have is a "B" size 40cf tank and is a CGA 520 and a full sized tank is a CGA 510. Be careful a 10 cf acetylene "MC" tank has a different size thread, it should be a CGA 200 valve. I do have an MC to B adapter for my air/acet torch and a B to CGA510 to run my big reg on my B bottle in an emergency. Your LWS or Tractor Supply may have these.If you look on the tank valve and on the reg fitting it will have the CGA numbers marked on it clearly.Another option besides the adapter is to just get the correct CGA nut and nipple that fits your bottle and remove the existing one. Most regs are probably threaded 1/4" NPT. Again the welding supply will get you the right fittings to do this.A "B" tank is quite small, and if you plan to do a lot of cuting, you will have issues with withdrawl rate on that tank. You'd be better with a larger tank if you plan to do a lot of cutting, or use a rosebud..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:left hand threads on acetylene