Whoa....so many choices for one "generic" motor......
The RAD 400 motor will in all likelyhood, have a bigger can size. Most 400 motors are larger than their 300 counterparts.
Graupner does offer 6 and 7.2 volt versions of that 300 can motor. Though they also offer other sizes like 400 (I.E. like the RAD 400), 280, 300, 480, etc....They all offer different RPM's and most are different shaft sizes.
Confused yet? Lets try to simplify things. For our RC purposes, if we use a Graupner motor for surface RC's, the 300 6V can is the motor of choice. Some surface RC's will fit the bigger 400 can, but those are still in the micro/mini range of 14th or 16th scale. The "regular" 10th scale RC's have a ton more motor choices available to them. I state SURFACE becuase the flyers will typically use the other "odd" sized cans.
The Graupner motors offer a good balance of speed to money ratio
. They are typicaly found localy for UNDER or at the $20 mark. Yes, that's here on our rock in the middle of the pacific
They are typicaly not taken apart and re-built, as they are not designed for that. But at that price, you understand why the closed end motors are termed as "throw away".
This site lists Graupner spec's....
http://www.greatmodeldeals.com/motors.htm
Looks like the Graupner might be slightly shorter and narrower? Shaft size looked similar.
One thing to consider is mounting holes. While most micro RC's will accept most 280/300 sized motors, alot of them will need modifacation to cram a 400 motor in there. Not sure what type of mounting options you have in your gun? But most times even the screw size will differ.
I bring up that last issue since i'm wondering if that RK-370sd motor you have is of the "300" varity? Not sure, but after briefly reading through both sites, your motor might be bigger?
This place detailed the mabuchi code....
http://www.rcgroups.com/forums/attac...hmentid=162984
One of your best bets will be to hit a local hobby shop here. Most are friendly and knowledgable. Not sure where you are at? But some palce like hobitat in Kaimuki and sandy flemmings in Pearl City are both good options. The guys there (or guy in flemming's case...lol
) are knowledgable, or are usualy able to find out. I know that 10th ave. (hobitat), stocks or used to stock a number of the Graupner 300 and 400 motors. They offer parts for surface and air RC's. They are also other modelest source for parts and support. Most shops will be able to get the Graupner motors, as they are generaly easy to obtain. There are other motors in that same class and size they will be able to offer. They will also have other pinion sizes if you want to maximize your torque
Your best bet will also be to take your motor down with you.
Good luck and aloha's......