Hi,

i am assembling the gear box and having problems with the first and 4th gear: the one gear in the main shaft is a spin gear and has no slide gears close to engage and transmit movement to the countershaft. as a consequence when i get first in, no rotation is transmitted to the sprocket.

any advice please?  

So how are you checking It? Partially assembled or all the way back together and no first gear.

all back together including first gear and closing both engine covers. sprocket is not getting any traction

 So the engine is on the bench not in the bike?  You might try spinning the crank and the sprocket to get it engaged. I take it you are getting the other gears but checking before you put the bike back together.  

thats correct, on the bench and before clsoing checking all gears work, which is not the case with the first gear