Ossa Explorer Questions

[Tony T]

Hi all,
I'm just starting my first trials bike (and first Ossa) resto so I've got a few questions to start me off.
FYI it's about a '74 Explorer (not MAR) and I'm not really stressed about using OEM parts as long as they're reasonably period correct.

1. I've been trawling the internet for the last week or so and found a couple of suppliers with potential. Mainly in the U.K. Is there any one that stands above the rest?
2. Is there a way of working out whether I have a '73, '74 or '75 model? And does it even matter when ordering parts?
3. Are the engine bearings and seals a special item or can I get them at my garden-variety bearing supplier?
4. Are Gonelli or other generic guards available in Australia?
5. What's the deal with the rear sprocket? Are they difficult/expensive to replace?

That's about it for now, but give me a couple of hours and I'll be in need of more help......

Thanks in advance for any words of wisdom.

[David Lahey]

Hi all,
I'm just starting my first trials bike (and first Ossa) resto so I've got a few questions to start me off.
FYI it's about a '74 Explorer (not MAR) and I'm not really stressed about using OEM parts as long as they're reasonably period correct.

1. I've been trawling the internet for the last week or so and found a couple of suppliers with potential. Mainly in the U.K. Is there any one that stands above the rest? In Motion (UK) are very good as far as customer service goes, but are not OSSA specialists. SM Products (UK) are good too but also are not OSSA specialists. Keith Lynas in San Diego USA is a gold mine of OSSA knowledge and is also a parts supplier.
2. Is there a way of working out whether I have a '73, '74 or '75 model? And does it even matter when ordering parts? Look at the rims, fork bottoms and the carby mounting to work out which year. The first OSSA MAR (1972?) had a flange mount IRZ carby, then followed a MK1 (flange mount) AMAL, followed in 1975 by a spigot mount carby and I can't remember which type of carby that was, probably AMAL MK2). Early OSSA trials bikes had shouldered Akront both ends and by 1975 they were shoulderless Akront both ends. Fork bottoms started out cut-off square at the bottom, by 1975 the axle clamps were waisted down. Also only from memory, the Explorer came out first in 1974. The very early MAR came with a swingarm that is shorter than later MARs and Explorers and some people reckon that MAR triple clamps are different to MAR triple clamps but I doubt it. The motors did not change mechanically in that period (1973 to 1975) apart from the previously mentioned carby atachment method.
3. Are the engine bearings and seals a special item or can I get them at my garden-variety bearing supplier? Don't know - am yet to rebuild mine. The primary chain is same as one model of BMW car engine cam chain.
4. Are Gonelli or other generic guards available in Australia? Yes Don Newell in Brisbane has them but maybe not the full range of colours. There is now a longer rear Gonelli that works a treat on OSSA and Bultaco trials bikes
5. What's the deal with the rear sprocket? Are they difficult/expensive to replace? SM Products makes a kit that has a spacer with an extra bearing and a flat sprocket. This helps with one of the weaknesses of the OSSA. The standard rear axle bends easily, and the outrigger bearing design in the SM Products design reduces the bending moment the axle is subjected to.
That's about it for now, but give me a couple of hours and I'll be in need of more help......

Thanks in advance for any words of wisdom.

[Tony T]

That's terrific, David. Thanks for going to the trouble to post all that info. It helps a lot.

I owe you a beer at CD10!

[Takhli44]

What a great community we have in trials.

[Tony T]

Absolutely!

One more question:
Does the Explorer need a battery to give me spark?

[TriCub]

Tony T.

David forgot that 72 MAR's had a fiberglass bash plate, but as you have an Explorer that is not an issue.

All the bearings and seals are commonly available.
The only tricky thing when ordering parts is the bigend assemble, the 72 models and I think the 73 had a smaller diameter and width crank which take a different rod kit to the latter. The early crank is about 100mm in diameter an 50mm wide with the late version at 105mm diameter and 60mm wide.

The ignition doesn't need a battery. Pretty sure the ignition is the same Motosplat that is in the MAR. They have a lighting coil output separate to the ignition.

[David Lahey]

Another thing about pulling the cases apart Tony, the primary drive crankshaft sprocket is mounted on a cush drive device that is supposedly tricky to remove from its taper. I don't know how successful it will be, but I bought a special puller from Keith Lynas for the job but haven't tried it yet. There may be better ways to get the cush drive off and mentioning this might prompt people to suggest them.
Yet another thing. Be very careful when you split the cases because there will probably be multiple shims on the ends of the gearbox shafts and it will save you a lot of time when reassembling if you can avoid getting them mixed up.

[David Lahey]

David forgot that 72 MAR's had a fiberglass bash plate, but as you have an Explorer that is not an issue

Tricub is a very good source of info about OSSA motors, having very capably rebuilt multiple OSSA MAR motors. He has also been mercilessly teasing me since 1977.

[Tony T]

He has also been mercilessly teasing me since 1977.

Now that's a recommendation!

Thanks fellas. I've gotta say, I'm really enjoying playing with this bike. Hopefully, it will be just as much fun to ride.

[David Lahey]

Mainly for Tony's benefit, here is what the OSSA primary drive cush puller looks like. I'm thinking that the bigger pieces fit behind the cush drive hub and the smaller diameter piece fits over the end of the crankshaft. Ah the joys of working on OSSAs