plunger gearbox odd behaviour

[RDfella]

One for our plunger experts:
The gearbox on my vee twin is ex plunger. Overhauled it before fitting to engine - ground the layshaft, made new bushes, fitted new bearings, new layshaft gear, final drive sprocket etc etc. Made a roller plunger for the camplate detent. It's been selecting / working fine ever since assembly and starting / running the engine. Done a few tentative road miles since the starter was fitted - all fine.
Except yesterday evening. After two or three miles it suddenly jumped out of gear. Thereafter difficulty selecting a gear - seemed to be neutrals everywhere. In fact, at one stage I wondered if the chain had come off. Fortunately I was only about a mile and a half from home, so headed straight back - with difficulty. Managed to keep it in third mainly, by pressure on the gearlever. Then got delayed by roadwork traffic lights - difficult to find any gear when green light appeared. Thereafter only half a mile home, but by then the gearlever was solid - wouldn't move.
Having got home, put bike on stand and left it. This morning thought I'd check the oil level, assuming it had run low. Drained the gearbox - just over one pint. Tried the gears (engine stopped) and everything selects fine. Gear selector plunger tube hasn't moved and is tight.
What is going on? Could it be because I've recently changed from straight 40 engine oil to EP80/90? It has, however, already done more miles on the EP than I did last night. If for some reason a bush was tightening, maybe I should change to R40? After all, if anything seizes on R40 it will seize on any oil.

[Greybeard]

You may remember I had a strange gearbox problem with some of the symptoms you describe after I had the box overhauled. It turned out to be caused by having a gasket between the main box and the intermediate section. When I tried without the gasket, just Hylomar, the box was good and has continued to be OK. . It amazed me that such a tiny offset could cause the gearbox to basically stop working.

I found this:

After completing the reassembly of my gearbox and engine I was gutted to find that gears were not selecting properly. Oh boy I was not happy to think that I'd have to take the engine out again! I spoke to Richard Clamp who'd renovated the box. He said that he had fitted a gasket on the inner case because there wasn't one on the box as I'd taken it to him. I had a feeling that I'd read something on this forum about plunger gearbox gaskets so I searched the forum and from various posts I could identify that I probably had the same symptoms. With Richards agreement I stripped off the gearbox covers and quickly refitted them with no gaskets. Gear selection immediately felt positive. I cleaned off the old sealant, applied Blue Hylomar to both sides, waited for it to dry a bit, reassembled the covers and took the bike for a run. The gearbox is now GREAT!

[muskrat]

G'day RD.
Ditto what GB says.
Check the claw moves and isn't worn on the teeth.
Cheers

[RDfella]

More mysterious than that, Musky. To recap - it's worked fine for ages previously. This morning it's fine again - everything free and selects the gears fine. I believe it was probably seizing last night - by the time I'd reached the workshop where I keep the bike, the gearlever was solid in the case, as if it were welded there. Didn't think to check the 'box for temperature. Convinced myself last night that the box must be low on oil, but it was actually slightly over full. Won't risk gear oil again - toss up between straight 40 engine oil and R40. On the other hand, if it was a bush tightening up, how would that make it jump out of gear and be almost impossible to select a gear (before the gearlever seized)? Jumping out of gear is more a detent issue. Or can the selector mechanism seize up, which would explain the seized gearlever and the possibility gears were only partly being selected?

[chaterlea25]

Hi RD
When you made the roller detent plunger did you drill a small hole near the top of the plunger ?
If the hole was omitted then the plunger could have hydraulically locked jamming the gear change ??

John

[RDfella]

Good thinking John. Been a while since I made / fitted it, so I don't remember. Seem to recall I ran a flat up the side but will drop it out to check (haven't refilled the box yet so saves draining it again).
 
addendum.
took the plunger out this afternoon. Yep, no hole, nor flat on the side. Must have been my other A10 where I did that, as that box has been no bother. So drilled a hole under the roller and re-assembled. What a pain that is, though, as there's a frame tube almost directly under the plunger assy meaning a lot of twisting and fiddling to get it out. Putting it back is even more fun. Anyway, job done & box refilled with straight 40. Roadtest hopefully tomorrow. Wonder why the problem has only just surfaced, having previously worked fine? Was changing to 90 gear oil the tipping point?
Anyway, thanks for the tip, John.  Just shows how useful forums like this are and how more than one mind can shine a light on issues not thought of.

[chaterlea25]

Hi RD
Sometimes I hate to suggest a possible "fix" in case I am barking up the wrong tree and cause the owner a lot of unnecessary trouble.
But happy to have been been able to help on this occasion

John

[Jules]

if that is the issue RD, then maybe the thinner oil allowed enough "bypass" whereas the 90 grade trans oil didn't??

[RDfella]

Sometimes I hate to suggest a possible "fix" in case I am barking up the wrong tree

Know the feeling, John. My concern when trying to assist is judging how much the questioner knows - are we dealing with a beginner or someone who probably knows more than I do? For example, for a non-starter 'have you tried swapping the plug leads' might be something I'd suggest for the former but not the latter. For the experienced owner I'd contemplate more unusual possibilities.
As to my issue, I reckon Jule's explanation is probably the answer to 'why now and not before'. I can think of no other explanation. And without John's tip I'd probably have stripped off the outer case to find a reason for the gearlever jamming solid.

Anyway, took it for a short ride yesterday and gears are back to normal. Nice sweet change too - those roller plungers really make a difference.