A size .12 motor is a small block and an .21 is a big block. they both have different mounting specs. you may have installed a OS .18 or maybe a 21TM which is just a bored out .18 as they the same mounting specs of a .12. An OS .21 will not fit the stock motor mounts. lets see some pics of your GT2.

here are some specs on the mounting hole for small blocks compared to big blocks.

OS .12 TG
Distance between mounting holes same side: .43" (11mm)
Distance between mounting holes opposite side: 1.24" (31.4mm)


OS .18 CVR
Width between mounting holes same side: 0.4" (11mm)
Width between mounting holes opposite side: 1.2" (31.4mm)

OS .21VG
Distance Between Mounting Holes Same Side: 0.83" (21mm)
Distance Between Mounting Holes Opposite Side: 1.4" (37mm)

OS .21TM This one has the same mounting holes as a small block but it has the same power output as a an OS .18TZ at a claimed 1.8PS each but the TZ is almost half the price.
Distance between mounting hole centers same side: 0.43" (11mm)
Distance between mounting hole centers opposite side: 1.2" (31.4mm)

My last RC10GT build had an OS .18TZ in it so I wanted to go a different route. the Losi Mach 427 engine is in a whole other league compared to the .21 TM, .18TZ the power difference is unreal you really need to experience it to understand it.

I have another GT I am working on too and I will be dropping an .21 in it. I have a Ofna Hyper .21 I may drop in it but it won't have the same power the Mach 427. I also have a OS .21 Vspec and though about putting it in one of these trucks just for kicks and to see how it performs compared to the Mach.