Fourth Gear Skip - GB Gearbox

[DaveH]

Hello all,

Thought I'd pass along my latest adventure with the '52 Square Four. I was riding along peaceably in fourth gear when I felt a skip - a momentary interruption in power, but with no misfire or change in engine sound. Very brief, and sudden - not a slipping clutch. Normally this would have been a mystery, but as it happens I had the exact same problem a few years ago on an AJS 18S, which also uses the Burman GB gearbox. That was on an 1100 mile tour, and it rapidly worsened until there was no drive in fourth gear. I made the final 300 miles in third, inconvenient but could have been worse.

It turned out this is a known problem with some GB boxes, at least to the AJS/Matchless world. From 'AJS Motor Cycles' by F. W. Neill:

Top gear disengages under load.jpg (44.66 KiB) Viewed 2235 times

I'm not sure how "uncommon" this fault is, given that both my GB gearboxes had it. The good news is that adding the shim eliminated the problem on the Square Four, just as it had on the AJS. (The bad news is that you need to remove gearbox internals, clutch and most of the primary drive to add it).

Hope this is of help to someone at some point.

Dave

[john.whiting]

Jumping out and into gear is very destructive for gearboxes.Best to immediately change to a lower gear and run home for repairs.I think the machining error refered to was pre war.......anyway,endfloat and wear in the box has the same effect.Some boxes have a cover or plate where the endfloat of internals can be checked,but not Burmans.Another cause is backing off and rounding of clutching teeth on gears.......Norton boxes had undercut engaging surfaces ,and never jump out.

[johnwalsh]

Very interesting and concisely put, thanks.

[marcus whatling]

Had this happen a few years ago on a 54 sq4 riding along in forth gear then suddenly no gear, like neutral . Pulled over , Nothing visibly wrong so carried on home with 3 gears . When I stripped the gearbox there was no circlip to retain the main drive bearing .Also not much left of the gearbox case as 4th gear had ground it away .One casing and circlip later I have 4 gears again .

[paul.jameson]

I had the problem on my first Healey. It boils down to lack of engagement of the dogs into the final drive gear. Either you shim this inward, as per previous posts or you do what I did which was to eliminate play in the gearchange mechanism and thus push the dogs further home. The worst point for me was the selector shaft bush in the outer case but it is easy to check. Press the gear indicator inwards and if it moves significantly you have a problem. There is a washer in the service kit for a Sq 4 SU carb which is ideal as a quick fix to remove this play. Oh, and for goodness sake, use a thin paper gasket on the joints at the very most. Using the thick modern gaskets is a sure receipe for trouble.

[Eero.Korhonen]

This also caused by too much clearance in the fourth gear sleeve, it is
the last one before chain sprocket.
Br, Eero