# Changing Crown Gears in a 4 Gear Specialty Chassis



## bondoman2k (Jan 13, 2003)

OK gang. Got a problem. I would ask Jim (tjetsgrig) this question, he's my 'go to' guy lately, but he's in Mexico so I can't..lol. I know some guys on here have done this, so I'm asking here. 
I have several original Aurora "Specialty Chassis" (otherwise known as 4 Gear). I know guys here have done this, but it is giving me some problems doing it. I am trying to put in a stronger 15t crown gear from an original MagnaTraction car. The gears I have are brand new (American Line replacements). After shaving some plastic from the top plate so it will fit, I still have fit probs everywhere. I already had to shave the 'sleeve' on the gear that fits against the chassis itself to just about non existant, but now it seems the chassis is binding at the bottom of the gear! Do I need to grind the inside of the chassis? I have to be careful on this since we are not 'allowed' to modify the chassis much, other than for clearance of things. I did bevel off the sharp outer edge of the gear and it helped, but not enough. I can tell there is a huge bind in it. What else do I need to shave or trim? I'm stumped. I really got to get this done, I have 4 cars to build in a week or two, and this is one of the main mods I need to do. BTW...these are all drag cars, and two of these will be using poly mags with a 6 ohm or lower armature (the ones I am using are actually 5.7 to 5.9 matched poles, balanced, polished, and trued), and the other two chassis will use ceramic mags with the same type of armatures.
ANYBODY got a clue as to what I need to do to make these gears fit?  I'm in a serious time crunch here so any help would be MUCH appreciated 
HELP!!!!:freak::freak:
Ron (Bondo) :dude:


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## Gary#8 (Dec 14, 2004)

Ron, Mount the gear on an old axle and put in a dremel or drill then sand or file the back side away from teeth to round back side of gear to look like the 19 tooth you took out. i have done this and the chassis will not have to be touched. May take you a couple tries (15 tooth crown) to get a good one. Gary


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## Bill Hall (Jan 6, 2007)

I dont know that there's any exact path to get that particular fit. Everyone of them has the potential to be a hair different. In the past I have just bevelled the outside edge of the pinion window when viewed from the bottom, because I have no roolz restrictions.

As Gary pointed out the first thing is to roll off the back edge of the crown using the chuckem' up technique. I prefer a sharp hobby blade for this, but the result is the same whatever you use.

This allows clearance in the gearplate's pinion window, BUT doesnt guarantee accurate lateral placement of the crown. Adjustment of the back shoulder on the crown is pretty much a given, although I have had those that do require some shoulder after all. Therefore I prefit it and work the bind out incrementally. So, I've removed too much in the past and had to shim back the other way....duh. :freak:

After you get the hang of it, you'll be able to feel the difference between those which are so bound that the shoulder has to come all the way off and those that merely need some of the shoulder scuffed off. If the shoulder is to be completely removed, just flick it off with the hobby knife, and quickly polish the wound with 600 and 1200 progressively.

I dont worry too much about the shoulder removal. As the pinions got bigger, the engineers moved the shoulder to the inside like the Tuff Ones/AFX style, rather than the outside shouldered T-jet style. The outside shoulder of the Tuff Ones AFX style crown is merely a spacer. The actual impinge fit to the axle got moved the toothside of the gear.

Regardless of your pile of parts, custom gear fits can take a few minutes to poke around. Ya gotta take them apart and set them up a few times to get the mesh just right. Sometimes I think you'd be time ahead to just roll the top edge, whack the shoulder and shim it back into the pinion.


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