Triumph Repair/1978 Spitfire pops out of 1st gear
Hi, my Spitfire 1500 (with overdrive) keeps popping out of 1st gear. It pops out usually when the car is in motion but sometimes occurs from a standing start. When it won't stay in 1st from a standing start and I have to pull away in 2nd. When I am able to put in 1st it doesn't crunch at all. What is the likely cause and is it 'fixable' (I have taken on DIY car projects before)or is it better to replace the gearbox?
There are several things that can cause it to jump out of gear. Look at the simpelest first and that is to remove the shift lever boot and confirm that the torque over of the engine when starting out does not foul the shift lever while in 1st gear. A broken motor mount or transmission mount can casuse that.
Remove the transmission tunnel and hold the top single shift rail that extends from the shift lever forward to the gear box. And move the shift lever to confirm a positive transfer of movement. If the shift lever moves right / left and forward /aft a lot without moving the shift rail then you may just have a problem at the bottom of the shift lever. But if there is a good transfer of motion of lever to shift rail then remove the top cover plate from the gearbox.
Take a long screw driver or such and check how much forward and aft free play there is between the gears. There should be very little if any. Now check the forward and aft play of the 1st/2nd gear in it's brass shift fork. This is the large rear one and it is like a cup and fits over the gear teeth. The forward one (3rd/4th) is a sliding hub with the front fork fitted into a grove.
The rear sliding hub (1st/2nd)is a gear and the fork fits over it. If the forks get worn then they can't move their hub far enough to lock in place. There is only a short piece of spline that actually engages so if it can't engage all the way it will jump out under load. When this happens the splines are only engaged on the edge and after time it wares away this edge and is then beveled and will jump out most of the time.
1st gear has another problem and that is that the engine is running and the mainshaft of the trans is not. So you push the clutch in to disengage the clutch and the first motion shaft going into the gearbox. The problem is that the first motion shaft, clutch disk and every gear in the gear box (except reverse) is spinning and probably at cold start engine speed (about 1500 RPM) If you don't hold the clutch pedal down for 15 to 30 seconds before engaging 1st gear the syncro ring (a cone shaped clutch that gets the hub and gear turning at the same speed.) In the case of 1st gear it is at a stand still. It needs to hault low gear as the hub is stationary.
So it boils down to check the shift lever first and then for excess free play between the shift lever and the shift rail. Then remove the top cover and check for end play in the gears and end play between the rear fork and the gear /hub it moves.
It is far more costly to replace the gearbox then is is to pull it out (which you will have to do anyway if the problem is inside) and correct the problem. If you have not done gearbox work before you will need a good service manual. (Haynes or Bentley) Then if you have any questions as you proceed contact me and I will try to help.