I recently stripped my first spur gear and had to replace it before I could run again. It wasn't stripped due to anything particularly dangerous I just left the truck ticking over for a long time and I think the heat building in the clutch bell transferred to the spur gear and melted some of the teeth from the gear. Won't be repeating that one again.
Also I've really spent too long considering upgrading to a steel or titanium spur gear. After giving it a lot of thought and talking to good number of people I came to the conclusion that this isn't the best place to add in a metal part. Firstly, the plastic spur gears do a grand job. Provided you get the mesh right they will last and do the job they are supposed to do. Secondly, if you rip up a spur gear and trash it thats all well and good ... what happens in the same situation where the energy doesn't get absorbed by a plastic spur gear and instead gets transferred elsewhere by a steel or titanium spur gear? The answer to that is probably a more expensive repair job.
This is of course just my opinion and its also dependant on how and what you use your Savage for. If your bashing a lot best stick with plastic spur gears, if your mainly tarmac racing a metal spur gear might make more sense.
Anyway, here is the deal with fitting a spur gear.
The first thing to do is remove your engine so you can actually get in there and work on the spur gear assembly correctly. This also enables to to set up the engine / clutch bell / spur gear meshing correctly or you'll just be bodging the job and probably getting the mesh all wrong.
There it is ... the lovely spur gear and just in between it and the gear box is the slipper clutch. So, what's the problem I hear you say? Well, firstly remove the old spur gear and I'll show ya! ... yikes ...
See ... that's what you get for letting the truck sit there and idle for most of a tank and occasionally blipping the throttle gets you ... too much heat in the clutch bell and a melted spur gear.
In the image to the left is the new 52 tooth HPI branded spur gear. Best place I've found to buy these is actually Ebay, they come for as little as £2.99 delievered to your door so when I ordered one I bought 2 so that I had a spare. Most model shops want between £4 and £6 for a Savage Spur Gear plus shipping!
They come supplied with a slipper clutch pad which is self-adhesive on one side ready to be stuck to the spur gear itself. There is a 'cut out' on the spur gear that this slipper pad fits snugly into and once its stuck on there its not going to be moving anywhere.
Next thing to do is to remove the brass bush that the spur gear spring braces against to create the bite between the slipper clutch and the slipper pad mounted on the spur gear.
Once you have done that put the brass bush into the new spur gear and remount it all making sure that the slipper clutch plate is facing the correct way and that the slipper pad makes goo contact with it.
Then slip the spur gear onto the shaft, followed by the spring and then finally the spur gear nut.
When remounting this HPI (and everyone I've spoken too about this) says that the nut should be fully tightened so that the spring is fully compressed, then back that but off 1/8 ofa turn to allow the slipper clutch to actually do its job. The spring is obviously key to this mechanism in that the nut is not holding the spur gear in place its maintaining the pressure on the spring which is in tern keeping the spur gear/slipper pad pushed up against the slipper clutch plate.
This is the most important part of the whole job IMO. Getting the mesh right between the spur gear assembly and the engine/clutch is really the whole deal here. If you get this wrong you'll just eat your new spur gear in no time if not straight away.
If you get it wrong with a metal spur gear I dread to think of the potential consequences. It could at least ruin your bashing/racing session or it could eat your spur gear and clutch bell.
Either way it needs to be gotten right first time and there is a really simple test you can do to see if your mesh is set correctly. All you need is a sheet of standard A4 printer paper (nothing too thick) I think the paper I'm using here is 90gsm and nothing too thin either. Cut a strip of it off that is about the same width as the spur gear / clutch bell teeth.
Remount your engine in the Savage (remember to Loktite the mounting nuts) without fully tightening these nuts as the idea is that you'll place the engine by sight and then test the mesh with the paper ...
When mounting the engine I did this making sure that by sight the clutch bell gear and the new spur gear looked aligned with each other. The teeth on the clutch bell were 100% meshing with the spur gear teeth by moving the engine forwards and backwards along its possible mounting positions on the aluminium mounting plate. Check that they are aligned together by looking stright down at the intersection of the spur gear and the clutch bell. Check by spinning the spur gear with your fingers. Once your happy that they are meshing really tight slide the engine away from the spur gear ever slightly (0.5-1mm) (maintaining its front/rear orientation), then ever so slightly tighten the engine mount nuts so that the engine is going to stay where you've placed it but not as tight as you will finally make them once the gears are meshed..
Next thing to do is to actually check the mesh.
Grap the strip of paper you cut our earlier and then as per the picture below, feed it through the mesh between the new spur gear and the clutch bell. The idea behind this is that if the meshing is too loose the paper will come out hardly 'dented' if this too tight the mesh between the spur gear and the clutch bell will simply tear the paper into pieces. In the picture I'm just poking the paper through the mesh and with a screwdriver I'm making sure the paper exits through the chassis so I can easily pick it up rather than it bunching up inside the chassis.
Here are a few shots of the paper that came out the other end!
And another ...
Once I was happy with the meshing I fully tightened the engine mount nuts. After going through this process my spur gear set up has been 100% bang on. Its a simple technique which works well and should see you getting the mesh set up correctly every time.
Go forth and mesh!!!!
