[northNH]

You should understand what I wrote above, have a fair conceptual and/or pictorial image of the cutaway drawing of the M40/41 gearbox (e.g. Haynes, FSM), and understand clearly what you’re going to do...

1) Park car with nothing in front of it, parking brake on.

2) Raise engine speed to mid-throttle (~3K RPMs).

3) DO NOT DEPRESS THE CLUTCH PEDAL AT ALL ‘TIL FINISHED!

4) For LHD cars (RHD cars, you’ll figure it out...): With your left hand grabbing the shift lever just below the knob (motor @ 3K RPM) gently and firmly push the shift lever towards 3rd gear AS FAR AS IT WILL GO.
With sufficient engine speed the lever will stop and will not engage 3rd gear.


5) With the gear lever’s forward progress firmly held by your left hand and stopped by further movement forward by the rapidly spinning internal shafts/synchros, with your right hand give one good smack to the top of the shift lever (i.e. shift knob) towards the front of the car.

6) You’re done. Drive it.

7) You may need to smack it harder a second time if you didn’t have the balls to smack it hard enough the first time.

What that smack does while the 3rd/4th gear brass selector fork is internally fixed and unable to move further REARWARD toward 3rd gear is to slightly bend the tweaked selector fork FORWARD, allowing it now to push the shaft coupling far enough forward to firmly lock the input shaft to the mainshaft for 4th gear operation.

Remember, at least 3K, and don’t touch the clutch pedal.
And with all due respect to Volvo Joe from central VT, RIP, the cat’s out of the bag...