I have what I think is an 1972 DT100 LT2 on engine. When I look up parts It always come up with 1974 and up. What years did they make the DT100?
Is there difference in parts before 1974. Where is a good place to order parts. I am new to this group. Thanks

All enduros changed to the DT identifier in 1974 except the GT80. JT-60cc, GT-80cc, LT-100cc, AT-125cc, CT-175cc, DT-250cc and RT-360cc. You can check and see exactly what year you have in the tech library under VIN numbers. If yours checks out to be a 1972 it would be an LT2, a 1973 would be an LT3 even though your VIN probably starts with LT1. I will usually go to Partzilla and get part #s and then punch that into ebay if it is unavailable or more than I want to spend. One must be careful as some of the diagrams will cover more than one bike.
Cheers

Greetings from Tennessee. 
 

Yes, the letter system is a remnant of Yamaha's 1950's and 60's model designations.

"Y" prefix used to mean a conventional "gas tank between the knees" frame.
"M" prefix meant "step-through frame with gas tank under seat".

Then came the letter code for the general displacement of the engine.

Then there was "S" for street and "T" for trail.

Then a model version number, 1, 2, etc. Back then, the same models were often sold for several years with little to no changes. Maybe a new color option occasionally. There wasn't a new model line every year.

For example, a "YDS2" was the second version of a 250cc street bike with a conventional gas tank between the knees.

By 1968, the step through frame models were all but gone so prefix was dropped.

So then in 1968 a 250cc trail model was a "DT"... motorcycles started to have new models every year so Yamaha first went with 1a, 1b, etc... and then "2" coincided with "1972", 3 coincided with 1973.

The first model year of the 100cc enduro was 1972 so it was an LT2. 1973 was an LT3 but Yamaha used the LT2 prefix in the VIN for the LT3... you need to use the VIN table to see if it's an LT2 or LT3 by the rest of the VIN number. (This happened with other Yamaha models too... for example an RT3 model will have an RT1 VIN prefix... you have to look up the suffix number in the VIN charts to see it's an RT3 by the VIN)

In 1974, most of the Enduro line got a complete re-design... Yamaha used the proven race designs and applied them to the Enduros.

Not much will be the same between the LT2/LT3 and the 1974-on DT100

They also decided in 1974 to capitalize on the "DT" popularity and as was said they renamed pretty much all the Enduros with a "DT" prefix... from 74-on the engine size was no longer a letter code... now it was spelled out DT100, DT125, etc.

Thanks for the info. My serial # is LT2-036011 and from the tech info I read it sa a 1973 year model even though it has LT2 in serial #. Am I close?

Yup...  LT3 started with -035101 so you have an LT3.