How To Unseize A Locked Engine?

I would be oiling every bolt on it, waste a couple of tins of spray 3 in 1 all over everything, or fill the bath with a thin oil and submerge the whole bike for a week.
Your going to have to go in to the engine so start making it as easy as possible to get there.
Make the whole bike a oily mess. Look at the engine mounting bolts especially.
IMHO

 

Surely a full strip down is required, gears will be rusty as will bearings, trying to get it running without is asking for trouble in the long run.

 

39 years sitting.

 

Try this http://www.baconsdozen.co.uk/Seizedengines.htm

 

Diesel is what I would try. Heard it works well but have you tried the key first
Looks for somewhere to hide.

 

Lubricate with diesel for about 1 week then turn over slowly with a spanner on the crank. Check ports as you go - It's a job not to be rushed. Good luck hope the internals are ok

 

Key as in to unlock a lock Jez not to actually start it

 

Remove the cyl head with engine in frame and put some WD 40 in the bore that is stuck and with 2nd gear selected rock bike back n forth, you can cut down old feeler gauges to poke any piston to bore gaps to encourage fluid penetration

 

Try using hot molasses, mix with distilled water, molasses eat/ dissolves iron based rust/corrosion. It's used by quite a few of the local vintage car and motor cycle restorers here in Australia.
Heat the bore/motor up a few times , then add some oil based lubricant before trying to turn it over, I got a Yamaha 850 triple to free up using this method, the owner took it back once it was able to be kicked over. He got it running again some 18 months later. Not sure how much he did in way of stripping down.

 

Al this talk of how to free the pistons in the bore is fine but what about the rest of the engine? Full strip down, only way to be sure it’s all ok.

 

One step at a time, no point doing anything else till you get the pistons free.
Then as you said a full strip down and check bearings etc

 

you have seepage, good molasses will work on rust/corrosion in the bottom end too. put some in gearbox if that proves to be seized too

 

I would start by being suspicious of why an otherwise serviceable bike was parked up and neglected. The ‘patina of age’ look is fine up to a point but really makes no functional sense for wheels, suspension, engine (less covers) and drive train.
Viewed as a fun investment, a proper nuts and bolts restoration makes more sense. Having soaked the internals I would be having the heads off to see if the pistons can be tapped with a lump of wood to inspect the bores for signs of broken rings etc. The storage conditions matter too if there have been cycles of condensation allowing major corrosion (a re-bore job) or merely fresh honing.
The main thing is patience and pleasure in a job well done.

 

By the way, if you pm me I have an ultrasonic cleaner that does a fantastic job on carbs. I assume you are not too far from Drumfries?