G12 with a mystery motor

[Papulski2]

Hello, first post here. I bought this bike about 4 years ago. It's my first Matchless and it was someone else's project. Typical story...guy starts a rebuild and then stops. Complete disassembly,frame stripped,boxes of parts. He said the engine was rebuilt. It was together with gearbox mounted in a crate wrapped in plastic. Took my time and got frame back together, mounted engine then took a break for a couple of years....easy to get sidetracked!

Spent the last 2 months really determined to finish this. After a lot of research I discovered how mismatched this bike is. Checked the frame with the clubs Dating Officer and it's a '61 G12CSR even though it doesn't have siamese pipes. Wrong seat,wrong tank ('62-'63 with very large badges),even the headlamp bucket is off a Norton.

The biggest mismatch though is the engine. It's a Matchless 650 as I count 8 fins but there is no serial # anywhere. Engine is very clean, just no numbers. It uses a magneto and alternator. The alternator only has 2 wires...brown with green stripe and green with brown stripe. Possibly an RM21? The gearbox # is M33414 and # on mag is K2F 42379B. Also on the left side there is a drive which I assume is for a tachometer even though no tach on bike. The carb is Amal stamped 389/95 with 9 64 on other side.

The electrical is another disaster. PO said there was no battery as everything ran off magneto. I've learned he didn't know much about bikes, he bought it this way and drove it for a few years. If no battery is it possible alt can run lights only when running? My plan is to rewire everything but haven't found a schematic with a 2 wire alternator. I'll have to look harder in the archives.

Lastly the engine is stuck! Won't budge. I pulled plugs and poured some Marvel Mystery Oil down cylinders. Let it soak about a week and tried to turn using rear wheel but no luck. Any advise on this would be great.

Sorry for the long post. I love working on this bike, just wish I was more knowledgeable. I'll try to post some pics

Thank you!
Steve

[MalcW]

Any engine number should be on the LH crankcase, just in front of the LH cylinder. Any sign of one having been there once? Just a thought, but if you're trying to turn the engine over via the rear wheel, are you sure the problem isn't elsewhere in the drive train? Can you select neutral? is the gearbox jammed? It would be worth trying to turn the engine directly via the crankshaft.

Malc

[Papulski2]

Does not appear to ever have a number. No sign of being ground off. I just wish I knew the year in case I need to replace parts.

When in neutral the rear tire turns freely. What would be the best way to turn the engine by the crankshaft? Pull primary case and remove sprocket?

First time working on an engine so any help greatly appreciated.

[Mick D]

Hi

Put the bike into gear and tie the clutch lever in, then if you can rotate the rear wheel the gearbox is not seized.

To rotate the engine, remove the primary chain case cover and primary chain, then you can get a spanner onto the drive side of the crank shaft and try to free the engine.

Regards Mick

[mdt-son]

Hi

I've dealth with a couple motors which were stuck. Turning the crankshaft at the PTO end will not help if the bores are really rusty.
You need to take off the cylinder heads, and take the engine out of the frame. Then, on your workbench, use a piece of hardwood and give each piston a good whack (or ten).
Pistons will be junk anyway, so no need to spare them. Just make sure you don't hit the cylinder spigots (make that piece of hardwood long enough).

If pistons still resist, you may try heating the cylinders using a propane torch.

Good luck!

-Knut

[Papulski2]

IMAG2194(1).jpg

[Papulski2]

thumbnail_IMAG2198.jpg

[Duncan]

Hi Steve

That's a clean looking engine I cant believe that the bores would be that rusted to stop the engine turning, I also cant believe that the crank is broken bit a quick check is to confirm that both pistons are the same height through the spark plug holes. It is possible for the Magneto to seize if it is original if the shellac melts, additionally Alternators can fuse together if they have been rubbing, it looks like uou might need to do some light dismantling and testing.

It is also worth removing the rocker boxes to check that the valves are free (slacken them off) and that the engine was not mis-timed and they have hit the pistons.

[Mick D]

Hi

If I understand you correctly that engine has never run in its current state, it has been 're-built' but not fitted.

I'd still try rotating the engine from the drive side crank shaft rotor nut, just gently in both directions, is there any sign of movement at all?

I'd suspect incorrect valve timing, misplaced push rod(s), misaligned main bearings, incorrectly sized piston(s) or broken/picking up piston ring(s).

Its difficult to imagine someone re-building an engine to that standard externally and not protecting the internals in the process so I'd be pretty sure there's an error in the assembly. It would also be strange for an un run engine to be immovable in both directions of rotation but a broken ring or wrong piston/rebore could cause this and forcing things will only result in more expense.

Regards Mick

[robcurrie]

Have you removed any pieces of cloth that might have been put in the ports for protection?

Rob C