I fully agree with Tankme here.
The DKLMRC 550 gearboxes are as sturdy as they come, and fit any 540/550 motor.
The only problem could be the width available in the chassis tub. The 550 motors are f.i. Too long for the M1Abrams. But fitting 540 crawler motors is no problem al all, provided You got a gear puller to get the pinions off the big 550’s, because they are of a hard to get teeth count.

I use those gearboxes in my Leopard. They’re very silent after some proper running-in, and make the Leo ridiculously strong off road. The only thing stopping it is lack of grip. Added advantage: Because the motors do not have to work as hard as little 400-sized ones, they sip amps. Despite the heft of a 6kg Leo, I haven’t emptied the 4000mah packs even once. Mostly after 45 to 50 minutes I call it quits.

My Leo has about 12,8 to 13cm of space in the hull and the DKLM gearboxes fit fine, with a little wiggle-room left.. So, Your hull/chassis is wide enough for the big ones. If You can afford the $90.- price-tag, and are interested in pure grunt (and, maybe even start using 3s/11.1v LiPo) they are the way to go imho. (And with that grunt, it becomes a pretty good tow-truck for all sorts of accessories)





But, for the sake of completeness, let me elaborate on motors and relative power in our tank-toys:

It all depends on the types of 400-motors used, whether smaller motors will do.

The low rpm, short 400-type motors (white endbell) that come stock with HL tanks have high winds, and consequently will run slower than the black endbell variations. It also means they use less amps, but are not very powerfull and heat-up when under heavier loads, which increases amp-draw until they get overwhelmed and start to burn their collectors, or worse, blow-up the RX18 controller..

The bigger/longer RS390, also called “480” motors (to confuse people even more) have more torque, and run higher rpm’s, and as such are -if one wants to keep the stock gearboxes- a better choice when the tank starts hitting that 4,5 to 6kg. The price is, however, that they will gobble more amps, and potentially could overwhelm the stock RX18 esc/controller.

That doesn’t mean they cannot be used, but heat-management and amp-draw becomes more of a consideration.

In all of my years of fiddling with tanks ( plùs 35 years of RC-Cars) , I’ve built quite the selection of little motors, and I tested all of them in several weight classes of tanks. In the end I prefer the somewhat exotic Igarashi 5-pole/5-slot 2738‑51 400-sized motors ( Igarashi 5-pole 12v ) for slow, WW2 tanks, and the larger and longer, high rpm, 3-slot 480/490 motors for more modern tanks. These 480/490 motors run from 13000rpm up to 25000rpm, and have a lot more power to haul heavier tanks, but -as stated, at a cost in amps, but they completely fall away against the hefty 550 sized motors in the DKLM gearboxes. All the power You could ask for, and -if Your controller ( IBu, Elmod etc.) supports 11.1v LiPo- are scary fast, allthewhile staying cool and sipping amps, completely incomparable to the teeny tiny 380/400-size motors that come stock with most tanks and that get overwhemed quite quickly.

Super-simplified It really comes down to this:
-Little 380/400 motor, more heat when under load, smaller rotor, less metal to dissipate heat, more heat=more resistance=more amps=more heat=again more amps etc.
-Longer 390 /480/ 490 motor, more metal through longer rotor, more metal to dissipate heat etc.
-Big high-turn 540/550 motor, more metal to dissipate heat, less amps when under heavier loads (depending on wind, but the DKLM 550’s seem to be 25 to 27-turn 550’s, looking at the claimed rpm)

And I’m not even mentioning the difference in torque, efficiëncy and wattage between a wheezing 400- and a big, brutish 550-motor.

The simplest upgrade is 13000rpm 390 long-cans, also known as “480” motors in electric flight, say, a Multiplex Permax 480, or for a bit more speed, these :480 pro.
For Your tracked truck, I firstly would check if there’s about a cm left between the endbell of the one -stock- motor, and the gearbox on the other side. If not, a 390 is a no-go, and You should look into cooling solutions, fans, clamp-on coolers etc. to keep the tiny 380’s cool (rs370 is actually an even smaller motor with a very thin shaft, 2mm,where an RS380 -as used in HL tanks, has a 2.3mm shaft) often confused/mixed-up by shops and advertizers.
It’s a Mabuchi-specific size denomination, while in f.i. the electro flight world the sizes are slmply 400, 480 to 490, basically the length of the can.

Planetary is even better. Bit more complex to realize, though. In that case, the bigger length of 550’s is no problem whatsoever. For cheap 55- to 80 turn 540 motors, take a look at the Absima offerings (If You’re in Europe)

Hah! Then -when using the DKLMRC gearboxes- The Sky Is The Limit. 2x180 amp will comfortably pull even fast 12-turn 550’s. Then You’ll have a tracked truck that potentially could hit 25 to 30km/h.. mad! But I lìke the idea.. :P

by the by. Here’s my thread with a bit more detailed review of the DKLMrc gearboxes, and pictuurezz.. :)

DKLMRC gearboxes in Heng Long Leopard (realized too late You already checked those posts out. Well, mebbe handy if someone else stumbles on this gearbox discussion.. )