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

David Porter frogeye at porterscustom.com
Tue Nov 7 13:51:58 MST 2017

....as a general finding in my experience it is excessive front to rear 
thrust in the main shaft. dp

On 11/7/2017 11:56 AM, Bob Spidell wrote:
> 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.
> Bob
> 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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Dave Porter Porter Custom Bicycles 2909 Arno St. NE Albuquerque, NM 
87107 505-352-1378 Go HERE: my world www.porterbikes.com/

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