I found that as soon as the spur gear started wearing, it would jump and break the stabalizer mount on the diff. Never had a problem with the engine moving. My biggest concern about the spur gear is the cost to get it shipped. I'm in Canada, so I've gotta have a huge order in to Tower to make it worth getting a $7 spur gear sent to me. Anyway, it has kind of a cool " turbo " whine to it now!