Alfa Romeo Repair: Spider 5 speed, input shaft, output shaft


Question
I purchased a used spider 5 speed transmission knowing it needed work. The input shaft does not rotate, but the output shaft does rotate. The shifter mechanism goes in all the gears. I have not opened up the case to examine it yet. The previous owner said it jumped out of gear and the car was driven into the shop. Any speculations?

Answer
Joseph,

It's hard to give you a concise answer without looking inside. I will say that, based on your statement about it jumping out of gear, the pinch bolts on one or more of the shift forks, may be broken or slipping.
They are notorious for this.  It's a small 6mm bolt that locks the shift fork to the shift rod.  Sometimes they work loose and allow the rods to move inside the fork.  There is a notch milled on the rods to locate the bolt and give it a flat surface to pinch on and if they work loose, it allows the rod to move about 20mm or so, without the fork moving.  Gears don't engage completely.  They will usually stay engaged while accelerating, because of the helical cut gears, but as soon as you back off the throttle, it jumps out.  Worn shift forks can cause it to jump out also.
If the bolt is gone completely, or backed out so that it doesn't catch the notch at all, you can actually engage two gears at once.  This happened to us in the pits at a race, while moving through the gears without the engine running.  When you move the lever from 2 to 3, since it's a different shift shaft, second stays engaged but it goes into third also.    Without looking at a parts breakdown, I can't say that could cause the input shaft to lockup and the tailshaft to still turn, but it's possible.
It's deceiving because you can still move the lever like normal, but since the fork isn't fixed to the rod, you aren't actually doing anything.  If you can move the lever through all the gears, with the input shaft locked up, then I'll wager this is your problem.
We have had the bolts work loose on the race car a couple times.  If you either safety wire them or use loctite on them (or both), they will stay put.
These gearboxes are fairly easy to work on, if you have a manual and any experience at all in transmissions.  Be careful about the little detent springs and balls that locate those shift shafts.  When you pull out the shafts, they can go flying!  If you look at a diagram, you will see what I mean.  If you want to write back, when you get it apart, I'll try to walk you through the checks.
Sorry I can't give you any other ideas without looking at it.

Dave