Ossa Mar - Jumps Out Of 1St After Losing Oil.

[biffco]

I purchased my bike last year from a company that deals in 2nd hand trials bike , and have done about 6 trials in it so far.

Following returning from a trial I put the bike in the garage. When I next saw the bike I noticed a small pool of oil. I left a tank under it to catch the drops and drain the rest. In total I caught about 200ml of oil plus what ever went on the floor. So there was probably only 250ml Max in it after the trial.

On investigation I found a crack in the sump from the oil plug hole to the mating face of the crank case.

As a temporary measure I have covered the crack with 'plastic metal' to allow me to fill it up.

Once I fires the bike up it initially ran ok, and went through all the gears. After about 5 mins of slowly looping round the garden in 2nd, I dropped it in to 1st. It would stay in 1st for about 5 seconds and then slip in to neutral. It seems to be a 'false' neutral as when selecting 1st again it doesn't clunk into gear, But again, as you pull away it it will go back into neutral after moving about 1 or 2 bike lengths.

From experience, what damage do people think I have done, and what do I need to do to fixed it ??

[goudrons]

Have you checked the detent assembly?

 

The detent locates each gear in place with a pin under tension of a spring in dimbles which are machined in the end of the shift drum.

 

It's behind a slot headed bolt with a plastic washer on the right side of the crankcase, just below the output shaft/gearbox sprocket.

 

See pages 16 & 17 here.

http://ossaengineering.com/wp-content/uploads/2014/02/OSSA-5-speed-manual.pdf

Edited May 20, 2016 by goudrons

[biffco]

I've not done anything yet, looking for suggestions, before I start.

Is your suggestion something that can be done with the engine still in the bike ?

[goudrons]

Yes, it's on the outside, just below the sprocket/output shaft.

Large slot headed bolt with a plastic washer.

 

looks like this

http://www.ebay.co.uk/itm/GEAR-SHIFT-DETENT-BOLT-1973-OSSA-MAR-MICK-ANDREWS-REPLICA-250-TRIALS-72-73-74-/171581306804?hash=item27f30af7b4:g:GxMAAOSw0vBUixiv

 

You can see it here, it's the only screw/bolt on this pic, so it can't be confused!

http://www.ebay.co.uk/itm/1974-76-OSSA-MAR-250-CRANK-CASES-ENGINE-CASE-MOTOCROSS-TRAIL-TRIAL-ENDURO-/272054238399

 

As the shift drum rotates (when you change gear), the pin/spring flick into different dimples machined into the end of the shift drum and holds it in position.

Without it they tend to slip out of gear as nothing is stopping the drum from moving and you loose that positive "snick" when you change gear.

Edited May 20, 2016 by goudrons

[dadof2]

Sump plug crack was well known Ossa Mar problem. Too exposed and gets hit on rocks. Best solution was to get a special pug made out of an allen bolt head by threading it, leaving just a trace of a "head" then fitting it with Loctite or PTFE tape.

 

If the detent aligning solution does not work the engagement dogs or their mating holes on the gears are probably worn. If they are and you can't get new gears you can sort them with a dremel and diamond file.

[biffco]

I tried measuring the plug, it appears to be tapered ? From about 14mm to 14.5mm and somewhere between 1.5mm pitch and 18tpi ?

So I guess either M14 fine or 9/16 UNF ?

The plug does look modified already,(badly) which allows it to protrude lower than it should.

[dadof2]

I can't remember the thread but when the special pug was installed it was beneath the surface of the alloy, not protruding at all. The alloy used to be countersunk by about 1 to 2 mm.

[woody]

If your bashplate has taken a beating and pushed up in the middle it can touch the engine which means impact shocks transfer to the crankcase and can cause cracks  -  it's not always the sump plug that causes the problem.  When the bashplates were new there was enough clearance that the sump plug was well recessed and nowhere near protruding.  Bashplates can be pressed back into their correct shape.

[biffco]

When I purchased the bike I spaced the bash plate away from the engine, is I will look at bending it back to clear the engine without spacers.

[twinshock giles]

Look carefully at the frame top tube from the head stock back, it should be straight, I had one bike for breaking that had had the bashplate spaced down previously that had a dog leg in the top tube....

That said its far more likely to be a bent bash plate and a weak or sticking detent spring - the wear can be difficult to spot though, I resorted to fault finding by substitution.

The Ossa box (and the whole engine really) is tough in my experiance, I very much doubt you would have done any damage to it running it low on oil for one trial unless that trial included a lot of road work.

[biffco]

The last trial was at the Christmas trial for the GVCMCC at Cowcombe.

Thank you for your replies.

I'll remove the detent spring this week and report back

[biffco]

I removed the detent plunger and found the end worn. I re machined the end to a point and put back together. The first couple of times I tried pulling away in 1st it again initially stayed in gear, then slipped in to the false neutral but after that it stayed in gear every time I tried. I'll take it for a ride in the woods soon to give it a proper test.

Fingers crossed. ?