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685 is the 237hp motor

686 is the 285 hp maxidyne

688 is a non maxidyne e7

690 is the em6/em7 maxidyne engine

On older trucks R is single frame and RD is double frame. Newer truck, guessing around late '80s, RD is all R models, single and double.

Suffixes of ST, S, SX will tell you what suspension is on the rear, which will tell you single or double frame.

ST is 38k Camelback, and will be a single frame tandem tractor.

S is 44k Camelback, and the truck will be a double frame.

SX is 55-58k or even heavier Camelback, and will likely be an even heavier double frame than a truck with suffix S. 

Example. My 1980 dm686sx has the 285 maxidyne endt676 engine, two 3/8th inch frame rails, and 55k rear camelback suspension. 20k front axle too.

Edited by JoeH
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have to disagree with a few things, I have seen lots of early RD's with single frames, but the frames are thicker and deeper than a R model. And have seen lots of 38k suspension with factory double frames. I think a lot of the differences are the area they were ordered for, for everyone ordered one way you can find a different one the other way.     terry:MackLogo:

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