MASCOT January-February 2008

Ask Aunty - Roadster gearbox sticking

Dear Aunty. My Roadster gearbox has started sticking in first gear, a problem which has developed over the last year to a point where it gets well and truly jammed, and no amount of heavy handed persuasion will shift it. For a start, I removed the top cover cap and found the selector fork and springs in good shape. Then I took off the top cover assembly, when the cause of the jamming became clear.

As first gear is engaged it slides rearwards over the second gear syncro assembly. But in this case it seems to slide too far before it contacts its stop, which is the later, Hunter type. This allows the spring loaded 2nd speed syncro balls to become partly exposed, which prevents any forward movement to disengage the gear. There seems to be a lot of play along the mainshaft, so my first thoughts are that an additional spacer fitted behind the first gear stop might prevent the balls from becoming exposed and causing the jam. What do you think?

Best wishes, Sidney M Xavier

Dear Sidney.
In addition to the 6 detent balls on the 2nd gear synchro unit, the Hunter type box also has a clever 7th ball and pin operated by a partially relieved spline in the 1st speed gear. This mechanism locks the synchro unit to a groove in the mainshaft when 1st gear is engaged. (Fig 1 & 2).

If engaging 1st gear exposes the detent balls, it will also be exposing the locking ball, and I suspect this is allowing the sleeve to become partially unlocked, such that the detent balls, under their spring pressure, will force the sleeve slightly forward, thus holding the ball out against the unrelieved end of the spline, so that the 1st gear cannot slide forward out of engagement and is therefore `jammed'. (Fig 3).

Putting a spacer behind the stop would have a similar effect, as it would push the 2nd speed synchro sleeve forward and force the locking pin part-way out of its groove, thus jamming its ball against the unrelieved end of the spline and locking the whole synchro unit together.

I can offer no ideas as to why the 1st gear is going so far back, but to prevent this from happening I suggest you machine 2mm from the front face of the stop and place a 2mm thick spacer behind it. This will move the stop face forward without affecting alignment of the locking pin with its groove. It will also prevent the detent and locking balls from being exposed. (Fig 4).

I am confident this will cure the problem, but please let me know how you get on.

Best wishes, Aunty.

ASK ASCO. (March-April 2008 issue)

Dear Aunty. Many thanks for your help with the drama of my 4AD gearbox jamming in bottom gear. Your advice in the January 2008 Mascot was spot on! But rather than take the risk of modifying my only Hunter type stop, I found a spare threaded type stop from an earlier gearbox and turned the threaded portion off that, plus an additional 2mm as suggested, then made a 2mm thick spacer to fit behind it - and hey presto! - I can now engage, and, more to the point, disengage, all gears with ease.

Who needs the AA helpline when we have this super service in ASCO from our very own engineer - Aunty?

Sidney M Xavier.

Dear Sidney - Glad to have been of service. But had you not realised that, in addition to `Alcoholics Anonymous', "AA" also stands for "ASK ASCO"!!