[Healeys] Gearbox jumping out of 4th gear coasting downhill or decelerating

Bob Spidell bspidell at comcast.net
Tue Nov 7 11:56:51 MST 2017

Thanks, Andy.

WRT the indents in the shift rails, I'm of the opinion that the shift 
rails and forks should not be depended on to hold the gearbox in gear.  
When my BJ8 started popping out of 4th, I tried holding it in gear with 
the shift lever; the force required to do so was substantial, and it 
occurred to me that fighting a gearbox that wanted to go out of gear 
could result in a broken shift fork (which would have created a real 
inconvenience when 100 miles from any town and well out of cell phone 
range).  I plan on ordering DWR's improved 3/4 hub, which they claim 
helps alleviate the jumping out of gear problem, along with new 
synchros, shifting forks and layshaft.

Curiously, when I pulled my input and main shafts apart, I don't recall 
the caged bearing set falling out.  I had noticed that the input shaft 
wobbled a bit more than I thought it should; if we neglected to put 
those bearings in at the last rebuild it's a wonder the box stayed in 
gear at all!  Also, the tip of my input shaft had more wear than 
expected, which could well have been from it wobbling a bit more due to 
the missing bearings.


On 11/7/2017 10:44 AM, A H List wrote:
> Magnus K does a neat YT vid of rebuilding the centrechange box
> https://www.youtube.com/watch?v=9rz6b8ZWXfc
> There is a bit of play in the needle roller between the 1st and main
> shafts when the tip is not supported in the crankshaft pilot bearing.
> Earlier boxes have very long individual rollers so have less play
> whereas the later box has the caged bearing which is only half the
> length and so appears to have more play.
> One issue is that the factory did a rather average job of aligning the
> indent grooves in the shift rails with where they are supposed to be
> and the indent ball sits only partially in the groove so is very easy
> to pop back out of gear again. The recommended mod consists of
> grinding the groove slightly deeper and further forward (or backwards)
> so the ball sits correctly in it when the gear is engaged.
> You can check if your shift rails are ok by assembling everything and
> engaging each gear with a dti gauge on the corresponding indent ball
> (or a screwdriver). When the gear is engaged, the ball should drop to
> the lowest position and begin to rise up slightly if the gear is
> forced further into engagement. That indicates the ball is correctly
> in the indent. If the ball continues to fall as the gear is forced
> further into engagement, you have the same problem as I had in that
> the indent is simply in the wrong position. I tried several shift
> rails until I found the closest fitting ones then ground them with a
> dremel to finish.
> Beware of junk quality synchro rings...
> Andy.
> On 11/6/17, Bob Spidell <bspidell at comcast.net> wrote:
>> I've finally removed my geabox and done some preliminary inspection.
>> Shifting forks are worn down some--not surprising in a 200K mile box--but
>> all gears look to be in pristine condition.  One thing I noticed is that the
>> first motion shaft seems to wobble a bit, unlike the output shaft with turns
>> freely but doesn't wobble much if at all (both large bearings appear to be
>> in perfect condition).  Should the rear of the first motion shaft fit snugly
>> into the front of the output (main) shaft, or is a bit of play to be
>> expected?   I've thought perhaps the increased compression in the new engine
>> might cause some 'buckling' in the first/output shafts--more than the
>> sliding hub can restrain--so could this be the cause of jumping out of 4th
>> gear?
>> Also, DWR sells an uprated 3-4 sliding hub, which it claims eliminates the
>> 'jumping out of gear problem,' so this issue is apparently not uncommon.
>> Bob
>> --------------------------------
>> Bob Spidell - San Jose, CA

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