Identifying 100 motor

[johneymack]

Hello, I am a new member here, but have spent many hours over the years as a guest, looking and enjoying all the BS stuff here.  As a kid I owned a Trail 100, but traded it for a car and have been regretting it for well over 40 years.  I finally bought a true basket case TMX 100 last month, thus my plunge into membership.  QUESTION if anyone can please help.  This old bike is pretty rough and some missing parts.  I am concerned The motor is still assembled and looks better than the rest of the bike.  It may not be the original motor, possibly a 90 substituted in the past.   I just cannot find a method of telling the 90 apart from the 100, without dis-assembly of the motor and measuring the bore.  I know there is differences in the cylinder bolt size and spacing, but without a 90 available for comparison, I am lost.
Is there any data or external measurements to help me identify the motor?  the Motor number is  20M017786.  There are no similarities in the VIN number and the motor number.  Any help is appreciated.  Thanks John

[mqtsteve]

Hey johneymack, welcome to the group.  The "20" in the serial number indicates the model as 100 sport.  Check out the serial number id guide to determine the month/year it was made.  Another way to tell, the 90's have separate cylinder nuts.  The 100's have long studs holding both cylinder and head.  Here's a link to the guide.  Good luck, Steve  

http://bridgestonemotorcycleparts.com/index.php?topic=449.0