Rebuilding GB7 Gearbox

[daveleek]

Well its nowhere near as bad as Gui's problem , I took my Mk2 for its first test ride and after 10km I lost drive and no clutch , I felt something go , and coasted to the hard shoulder , the clutch cable was slack and I could move the kick start with no connection to the engine , I removed the clutch cover and the plates were all loose then I realised it was something to do with the clutch pushrod , put it back together and called the ANWB [AA] I was taken home on the back of a heavy goods recovery vehicle , my 10km ride was 4 hours . I removed the gearbox outer cover to discover the clutch operating arm plunger was broken and fusion welded into the mainshaft end it took a lot of hammering to remove it , when I built the box the plunger was a little bit tight and I removed a little bit with emery in the lathe , I realise now I should have paid attention to the mainshaft bore and used a flex-hone on it , I wondered if this is a common occurence or the roughness of the mainshaft bore that caused it , I will now carry a spare plunger with all my other odds and ends just in case when I can get the parts

[Roger Gwynn]

I have never come across this happening before, there is usually enough lubricant finding its way down the little hole, but a bit of oil on the push rod is always a good thing.

[daveleek]

When I recieved my new GB7 mainshaft I never thought of taking all measurements of it , I believe the broken plunger was caused by the push rod bore size being undersize at 8.56mm and the plunger a tight sliding fit , the original mainshafts push rod bore is 9.10mm , the length from the bearing face to the end of thread on the new shaft is 38.19mm and the original shaft measures 36.82mm , the clutch pushrod is the correct length and I did lubricate the whole assembly , when the clutch lever was pulled in , the plunger was forced against the mainshaft and friction welded itself together.
I am now thinking I will have to strip the box and remove 1.37mm from the kickstart end of the mainshaft , I will enlarge the pushrod bore at the plunger end too , If I am the first one to have this happen in 70 odd years it must be the mainshaft , or am I just Unlucky Dave

[Roger Gwynn]

Dave you must have had about .054" extra end float on the mainshaft unless all the extra length is thread. As the plungers are 5/16 dia and your hole is 8.56mm there should be enough room. You would have to be holding the clutch in for a long time to friction weld the plunger to the mainshaft as it has to get very hot, like lots hot.

[daveleek]

Roger , yes the extra length is thread , I was exagerating a bit with the friction welding , the plunger had picked up and was difficult to remove , I had to use a hammer , luckily I have a suitable Flex-hone to clean up the bore when I recieve a new plunger

[daveleek]

After my gearbox rebuild last year and the clutch operating arm plunger breakage, when repaired the gear changes were very good,but I drained the oil and found traces of bronze dust,when I returned from the AGM this year I again drained the oil to find even more,when I got to the Dutch rally the gearchanges were very bad and on my return trip home got progressively worse,once home I immediately drained the oil and removed the outer cover, to find the bronze dust was everywhere in every nook and cranny,after removing the g/box and opening it up,the innermost drive gear bush had been winding its way out into the second gear,half of the bush had gone.I replaced the bushes and drive gear bearing,which were sourced in Europe,a bit more expensive than the UK but no import duties and handling fees to be paid, so only worked out a little bit more expensive.The cause of this I suspect was either the new brake drum/chainwheel which was oval and caused the chain to be loose then tight and doing 2000 miles or me Flex honing
the bushes and sealing the pores of the sintered bronze,would that be a possibility,or could there be another cause

New bush on the right,there is a mystery [what looks like] hacksaw cut on the L/Hand bush

[nevhunter]

Maybe you are the Man with the midas touch? Sintered bearings are not really heavy duty. Your primary chain loads that bearing . The rear chain loads the big Ball race. Nev

[daveleek]

Nev,All that glitters is not gold,thats a shame,it would pay for the ball bearings.I cant be the first to have this happen to,any ideas?