1973 Ossa Explorer - issues with engine transplant

[Triad]

Hello guys, last year I've got a 1973 Explorer with a semi-seized engine. Managed to find another one that should be in working conditions but the owner doesn't know which model it's from.

I tried putting the engine in, it looks the same when it comes to frame brackets but it doesn't fit... it's like it's slightly thicker than the stock one where the engine meets the frame in the swingarm/filter area. Also noticed the right side sprocket cover is different (longer)... any idea?

Is it better to slightly file the engine bracket or the frame?

 

 

Edited Tuesday at 09:06 PM by Triad
added pictures

[konrad]

16 hours ago, Triad said:

Is it better to slightly file the engine bracket or the frame?

Those engine brackets already look pretty insubstantial by modern standards.  I would not want to thin them. 

How much material do you think needs to be removed?  Make some measurements. 

P.S. It may be preferable to heat and bend the frame mounting points.  In general, I try not to make modifications that can't readily be undone. 

Edited Wednesday at 12:35 PM by konrad

[Triad]

Yeah, I plan to measure the old engine "brackets" with a caliper and compare them to the ones of the "new" engine. I didn't pay attention to it before buying, silly me, but from the shape of the right side cover I'd say it comes from the enduro version... who knows what other differences there are. The carb inlet is bigger, can only hope the exhaust flange is the same.

[konrad]

What is "semi-seized" on your old engine? Can you make one good engine out of old and new parts?

[Triad]

Well, the original Explorer engine turns but it takes a lot of force... probably a seized bearing on the bottom? Also not sure the Motoplat works, as it requires 500rpm to make a spark and there's no way to make it turn that fast as is.

It's been outside for years, I think, so rust might be the issue... and the reason for being there might have been a broken Motoplat unit (no spark)? No way to know for sure.

The "new" engine was stored on a shelf with the inlet taped, and the guy told me it was bought from a reputable seller, so IN THEORY it should run. I plan to make the engine brackets slightly thinner as they're massive, while the frame brackets are already thin. 

I'll have to measure them, but as I said I'm now worried other parts might be different and I might find out the hard way.

[woody]

The centre cases are all the same casting so are interchangeable. The reason the ignition cover might look different is because the rear most part sometimes gets cut off or modified, either to stop a build up of mud or they simply get broken by derailed chains

The replacement engine is an enduro engine, the Explorer is a trials engine. The enduro motor will have a different state of tune/porting and different gear ratios. They also use an expansion chamber motocross type exhaust although your Exporer exhaust should still fit into the cylinder. They use a 32mm carb as they're basically a race engine

It's not an uncommon problem to find a different engine won't slide into the rear frame mounts. You might find that the mounting brackets have been squashed inwards slightly with repeated tightening over the years. You can try puting a piece of solid bar against the inside face of one and putting a solid bar through the hole in the other and tapping outwards, then repeat on the other side to try and open them out a bit. Or just take a bit off the engine to enable it to slide in 

[Triad]

Thanks a lot! Yeah, I tried to "open" the mounting brackets a bit by hammering a round piece in but still won't go in freely. I think I'll remove some material from the casing as the frame side brackets aren't exactly ultra-thick...

So I'll need a different intake manifold? Might probably find a silicone one on Amazon, maybe... but my priority is to see if it has spark and runs. If it works I'll gladly put some work into it, but due to my previous bad luck I'm a bit skeptical

[Triad]

Uh, forgot to ask a very important question: with the Explorer I received a Motoplat coil but it's the single pin version. I also bought on amazon a 2 pins "universal" coil... will the engine work with it or it absolutely requires the red 2pins Motoplat unit to work?

[konrad]

I think all those red Motoplat "ignition coils" contain other CDI components too.   Best source of Motoplat information I have found: https://motelek.net/motoplat/

[Triad]

Uh, so the coil integrates parts of the CDI? Damn, in this case won't work.

Can I test the rotor/stator without a working coil?

Today I finally managed to literally kick the engine in place, only to find out my kickstarter lever won't work... different shaft. 😭

Edited Thursday at 07:16 PM by Triad

[woody]

The stator can be checked, you'll need the workshop manual which will give the expected readings, I've no idea, electrics are a mystery to me. In the UK we can get them repaired. The motoplat coil for the Ossa is a combined coil / cdi so you need the two connector type. New replacements are available (at least here in the UK they are)

When I saw the picture of your replacement engine I assumed it was the same year as yours but someone had changed the cylinder head for a later one. It seems the chassis number list I have incorrectly lists the engine as a 1972 but it's actually a 1976 enduro version, so, the kickstart and gear shafts have a different number of splines from your explorer engine. I think that the exhaust fitting on the later enduro motor is push fit with springs, not threaded collar, so your exhaust header isn't a straightforward fit

To be honest you're probably better off fixing your explorer engine, all parts are available. 

[Triad]

I didn't pay enough attention before buying the new engine, the owner didn't know much about it but... even the gear pedal is different?

About the exhaust, I see they have a similar flange... maybe the Explorer one will fit?

If I have to buy all these parts with the risk of the engine non even being completely fine I'm screwed big time.

[woody]

Yes, you'll need another kickstart and gear lever with the correct number of splines, the earlier engine shafts have more splines than post 1973 engines

The exhaust won't be a straight fit, yours has a flange held in the cylinder by the threaded screw in collar. The exhaust on the enduro engine is a push fit held with springs. To get yours to fit you have to cut off the flange so it pushes into the cylinder and weld two hoops on it to locate the springs - as long as it is the right diameter to fit.  But how the enduro motor will run with the trials exhaust I wouldn't know

You also need a carburettor, yours won't fit as it is bolt on and probably also too small for the porting, whereas the enduro cylinder is a stub fitting

[Triad]

The carb was fitted on a rubber manifold, no bolts... but yeah, it's smaller. Didn't check the exact diameter but I'd say it's something like a 24-26mm maybe?

The biggest problem is that I'm missing the kickstarter lever and the coil... I could at least know if/how the engine runs if I had them.

Is there anything that will fit when it comes to kickstart lever? Maybe from a newer bike from another brand?

It's clear I'll have to gamble and buy a Motoplat coil without knowing if the engine runs, but for those old lever sellers tend to ask pretty unreasonable money and they're not easy to find.

Even a borrowed one would be ok, I'd gladly pour more money into the project if I knew for sure the engine runs OK. 

[konrad]

17 hours ago, Triad said:

Can I test the rotor/stator without a working coil?

You can take stator resistance reading as woody suggested.  Although that does not guarantee functionality, you can find some definitely-bad components that way.

A peak-reading voltmeter is better, but again will not guarantee proper function.

https://www.thumpertalk.com/forums/topic/1034259-build-your-own-peak-voltage-adapter/

I have used a drill motor and socket on the flywheel nut (or primary gear nut) to spin the motor for stator testing.  This is best done with the head removed for easiest cranking, but just removing the spark plug may be adequate.