I have a Systema magnum motor in an MP5 with a Systema Mecha box with a systema 130 spring,no issues other than the motor does get alittle warm and thats it.But that might be from the Lipo pushing as well.I don't think its anything to do with the fact of it having an Alum. end bell,more like alot of carbon deposits from the brush to comm. contact in an enclosed space (clean it). I'v been around the R/C type motors for about 25 years now and it being because Alum. enbell would have done the same with those. But a build up of carbon can cause all sorts of issues. R/C cars were using thoes types of magnets 20 years ago (Astroflight 05 motors) were the first and were banned from the race tracks pretty quick because of the unfair power difference.Anyways on that note my Systema motor and gear box set is the first one they came out with and not the newer ones,so I can't comment on the new ones.
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