The Evolution of The M Series AirChime Train Horns

M Series Train Horns Evolution

In 1950 was the first introduction of the M series train horns. The series replaced H and N horns. The configuration they were produced in was 'round base'. This means that the bells' base (bolted to the manifold), is thoroughly circular. This 'round base' configuration was retained bu Burnette, Holden, and AirChime through the horns' production runs. In July 1952, Nathan started changing the horn castings to 'scallop-base' configuration. This was done because tests revealed that keeping the base round requires an unnecessary extra metal. Because of this, the horn's weight was considerably reduced along with manufacturing cost. This difference was the most apparent in any later and early-productions comparisons. (There were slight differences of the castings made by Burnette, Holden, and AirChime. These will not be extensively covered here though.)

The line of Nathan M horns has experienced plenty of changes and modifications and these will be discussed in detail here. The dates given in the following section were obtained by examining earliest M train horns. They still had a stamp of their manufacture date until about 1952. Shown on the back of the horn's heads and sometimes stamped on the manifold is a 3-character code. Specifically there were 1 letter and 2 digits. The letter (starts from A to L) is a representation of the months, so for a example a B would be February while an L is December. The 2 digits means the last 2 digits of the year the horn was manufactured. Example, an M3 that has a stamp of 'H50' was manufactured August 1950. Earlier Ms contain other letters but it is unsure what they mean.

The oldest Nathan M horns (1950), used some A5 horn leftovers. A few earlier M train horns then would have H/N-style back caps, predating M horn spanner wrench usage. Several of these older horns utilized bells with “Nathan AirChime” bell-mouth cast too. Newer bells had the word cast on raised blocks on their sides. There are 2 M5s existing with no marks to be found on the bells. There was the bell number though and atop bell 1 was “Nathan AirChime”. Production dates for them are unknown.

Taking a spanner wrench is the starting design of M horn back caps. Nathan designed the earliest train horns though with back cap outside edges spanner holes, thus each back cap required a different-sized spanner! Fortunately, AirChime corrected this early (Nathan quickly followed). The new design already allowed the use of one spanner for each back cap. The latest manufactured “non-standard spanner caps” was in May 1950, before the introduction of improved AirChime cap. “Pat Pend” was cast into earliest AirChime caps. Later 1950 caps had a part number cast instead of “Pat Pend”.

At that time, the Nathan name was also cast on the train horn bell sides rather than the mouth. Until late 1950, AirChime and Nathan words were cast on raised blocks separately. Later, “Nathan AirChime” got cast on a single raised block. Until bell styles changed to scallop in 1952, this was the final major modification.