I'm trying to identify the major differences between a 1974 DT250 and the earlier DT1/2/3 models.  I've searched the forum, but haven't been very successful.

Are the engine cases and frame the same or different?

 

There are several distinct differences.

power 74’ less hp
weight 74’ heavier
front fender 74’ high
74’ reed valve 
74’ some are CDI ignition 

the parts for the most part are not interchangeable 

a

 

Yes, many differences.

If you want to look at specific shared parts, then clicking on a part on Partzilla or CMSNL will show which models used it.

Different motor & different frame.
Essentially nothing the same except for piston kits that you could find listed in aftermarket sites as good for DT2&3 plus DT250 & so on.
Motor from "74 on is longer & mounts are different--& so it goes on.
"74 '250'was still points, all the way to "78 i think" then went CDI for the "79 last year & borrowing it off the DT400 which had used the same one for a couple of years.
Actually the points mag & cdi mag off all those models including DT400 will swap over onto DT1-2-3.
Sounds like you have a project, i like projects--what is yours?.

RT325, super helpful response.

I might be able to get a good deal on a local '74 DT250, but I was hoping it would be more similar to the DT1/2/3 models since I'm building up a Trackmaster with a DT2 engine.

 

"74 is a good bike & good strong motor.
Lower frame too, but does it put you out of class? being "74.

 
Hmm, seems like somebody else said the '74 frame was taller?

I'm not racing.  My first bike was an early 80's DT175 so thought I might add a two-stroke to the 30+ bikes in our garage while I'm building up the DT2 Trackmaster for my girlfriend.

You could be right but i always thought the "74 looked like it was dragging its bum on the ground compared to earlier 250 & 360 models.
photos.app.goo.gl/U1zPEfBepKNkW8Vc6

I agree RT325...  I always thought the engine looked lower in the 74 on frame?   If it was, I guess someone could say the "opening" from engine cradle to backbone in the frame was taller?

I think the 1974 Service manual in Tech Library has fairly detailed frame dimensions near the back?   Not sure where to find them for older models.

Probably the frame grew taller & lower taken from a point halfway up, so the exhaust could go over the motor & through the frame. Something like that.