Hornby 040 chassis

[Rana Temporia]

I have been fixing and modifying numerous versions of the Hornby 040 chassis for years but I recently came across one with different gears. This has a black worm and a smaller black gear on the drive axle but the gears don't mesh properly. The motor has some kind of black 'gunk' under it and is wired to the pick-ups. 

 

I recently ordered some spare 040 motors from Hornby (delivered very fast) and they are the type I would have expected. 

 

Is the new chassis I got a more up-to date version and if so why don't the gears mesh properly. Has it been modified by someone?

[Going Spare]

Which locomotive did the 'new' chassis come from?  Was it purchased new?

How do the motors obtained from Hornby differ from that in the loco chassis?

From what I can remember, black worm/wormwheel has been standard for some time. 

[96RAF]

The early motors had the brush connections underneath so they pressed directly onto the rail pickups.

 

Later ones had the motor the other way up with the brushes on top and were wired to the pickups making them easier to convert to DCC.

 

Most recent locos seem to have had a gearing change as they run much less like a Scalextric car than before, although this does not reflect in any service sheet info or spares listings that I can find.

 

The margue has been the subject of many forum discussions over the years as folk try to improve what is a reasonable little family of locos in attempt to tame their speedy nature.

[Rana Temporia]

I don't know which loco it was from as it came in a job lot off an internet auction site. It has the outside rods so probably the Holden tank or Caly Pug. Looking at it the spoked inserts in the wheels look to be finer than the others as well. There were two of the type I'm used to in the box of assorted chassis plus this one. It all looks original but I can't get the gears to mesh properly which is the main problem I'm having. The motor is fine and my intention was to use it in one of the BMR diesel shunter body kits with the side rods removed.