Ossa 250MAR sprocket replacement

[Black111r]

Hi All

Having just completed my first ever trials event at Conondale may I say thank you to everyone that offered words of encouragement and advice. I had a great day, fell off a couple of times, got muddy and even managed to clear a few sections.

One thing that I found was that my Ossa seemed somewhat over-geared with only first gear being usable in most of the tighter sections (even then it was somewhat 'fast' compared to some of the other bikes i watched).

Following advice I have now purchased a 10t front sprocket to replace the 11t existing one and will be fitting it in the hope of lowering the gearing.

Can any one confirm that the nut holding the sprocket in place is reverse threaded (that is clockwise to loosen)?

Thought this might be worth checking before I start shearing off bits and pieces!


Thanks in advance


Chris White

[David Lahey]

Hi Chris
I'm the bloke at Conondale in the bright yellow Yaamaha T-shirt who was probably one of many who told you your gearing was too high.
I've looked up what I did to my MAR to make it nice to ride and it is a 10T front and 52T rear. Your rear looked standard to me so I would say you might be wanting a bigger rear to get it slow enough.
I also did a drawing of what I had to do to the sprocket nut to allow the use of the 10T front without the chain riding on the nut. The inside 6 mm of the width of the nut was machined down in diameter to provide the chain clearance.
That thread is right hand (ie normal) and replacement nuts are available from (vintage) OSSA parts suppliers. The thread is M25x1.0 and the hex is 36mm AF.
David

[Black111r]

Hi David

Many thanks for your reply. Yes I rememeber you and trust that you enjoyed Conondale as much as I did, Great day!

I have the new sprocket and nut that I ordered from In-motion (Bultaco UK). The sprocket already has a step machined in it so one less things to worry about. I have checked the rear sprocket, it is a 42T.

With regards to the thread i am somewhat confused.

In-motion are not sure of the 'handing' of the thread, Keith Lynas at Ossa Planet informs me that it is a left hand thread and I have the Clymer workshop manual for Ossa 1971-1973 which states that the nut is loosen by turning clockwise and have found these pictures and info on Mat Nybergs site which is mentions the left hand thread.

Quote........from Ossa shop manual

3. Fit the OSSA countershaft sprocket spanner to the countershaft sprocket. Fit the OSSA countershaft nut tool to the countershaft nut. Rotate the tool clockwise (CW) to loosen and remove the countershaft nut.

Ossa Disasemble.jpg (37.78 KiB) Viewed 7181 times

Ossa reassmbly.jpg (37.58 KiB) Viewed 7181 times

So before I reach for the spanners worth another check!


Thanks


Chris

[Black111r]

Hi David

Can you remember where you got your 52T rear sprocket from.

Been researching all morning but found nothing firm.


Kind regards

Chris

[David Lahey]

Yes good to check all right! I judged the thread to be RH by checking my notes (no mention of LH thread) and looking at the thread rather than undoing it.
I would say that I have probably been thread-dyslexic when I looked at it and that the OSSA manual pictures and Keith Lynas are much more likely to be correct than me. I will also undo that nut on an OSSA motor later today and report back

I will also post up a photo showing the way the bigger rear sprocket was achieved
David

[David Lahey]

I just loosened one of those nuts and yes mine was left-handed ie clockwise to loosen

Photos of rear sprocket. The original sprocket was machined to form a spigot and and the new sprocket was machined to fit onto the spigot with a matching recess. The 6mm bolts hold the parts together.

Another alternative to this is to weld the joint instead of bolting.

Another alternative is the Sammy Miller Products rear sprocket kit that includes a hub extension and another bearing and allows flat sprockets to be used.

David

[Black111r]

Hi David

Many thanks for you prompt response.

I still have memories of shearing the nut off the mageneto bolt of an old bike when i was aged about 15Yrs.

Think twice, spanner once or something like that!

I will look into the various larger rear sprocket tricks you mention, Unless you have a spare one no longer need?

Chris

[chippy]

Hi all,
Chris I was the other guy with the Ossa and that was the first time I had ridden that bike in a competition and by the end of the day I was getting used to it. I'm glad you had a good time and I wish I would have more time to chat with you.
My original rear sprocket was a 46 tooth, but I was running a 12 tooth front and with limited time, I didnt get a chance to source a smaller front sprocket prior to the event.
I am about to start restoring a 350 MAR and may need more parts.
Email me and we can talk more re parts etc....
Regards
Andrew

[David Lahey]

My original rear sprocket was a 46 tooth, but I was running a 12 tooth front and with limited time, I didnt get a chance to source a smaller front sprocket prior to the event.
Regards
Andrew

Riding OSSA MAR + 12/46 gearing + unfamiliar bike + riding Open Twinshock class at Conondale + placing well = LEGEND

[Black111r]

Hi Andrew


My personal email address is [email protected], easier to make contact via regular email

I am currently living in the Ashgrove, Western Suburbs of Brisbane area.


Looks like someone in the states has a 46T dished sprocket that I can get unless someone has one to spare?

I will try that with a 10T front before any major mods.



Rgds

Chris