In 1984 i restored a TWN ‘Cornet’ at the time the motor was in good running condition so it was left alone now almost 35 years later having gone full circle i am about to restore another ,But this time the ‘Cornet’ is all in pieces!The problem is there are no workshop manuals available! all i have is a illustrated spares book that covers both the kick start and electric start models and with all the parts i have got there are parts from the TWN’Contessa’ scooter!(The ‘Contessa used the same engine) so the situation is a bit like doing 3 jigsaw puzzles at the same time! But at my age keeping the brain active is a good thing,So pictures below will show how i am getting on.

parts of the gearbox,It is a simple gearbox(no selector drum) the arm on the right with the curved yoke fits between the middle 2 gears(top shaft) and pivots to 5 positions for the gear changes.each gear held in position by the ratchet on the arm plus the pointed pin and spring.
The 2 shafts in position,the arm’s yoke between the 2nd and 3rd gear and the ratchet pin holding the arm in the neutral position.The washer on the top shaft flings most of the oil away from the bearing as there is no seal on that bearing.
Remember all the parts were available as you see them!! can you spot any differance? (apart from the oil flinger on one shaft)When i first put the shafts back in the gearbox housing, i put the layshaft lower right with the main shaft top left! i then spent a least a hour trying to get the outer cover on, stripping down  all parts to find the problem. 24 hours later i solve it!!! the layshaft lower right is from the ‘Contessa’ scooter and the main shaft top left is from a ‘Cornet’ the differance is in the picture below.
The 1st gear left is ‘Contessa scooter and the 1st gear right is from the ‘Cornet’ the gear left is about 2mm larger(different ratio)which was the reason the outer cover would not fit!!
The outer cover with the gearchange shaft and the return to neutral shaft(shorter one) The gearchange shaft in the forefront is from the ‘Contessa’ scooter,shorter and with finer splines.

The crankshaft’s below will need all new bearings the 2 bearings either side of the crank known as M25 & M20 angular contact bearings made by SKF are like hens teeth!! none available in the UK i manage to track down a specialist bearing dealer in Germany who still had some in stock! but they come with a very expensive price tag!!

My 2 crankshafts with the usual damaged pistons . The crank left has the original M25 & M20 bearings unavailable in the UK,The crank right has substitute roller bearings which will cause problems because they cannot stop lateral movement.
Old bearings ! To remove the the journals from the shaft, they are scored with a anglegrinder disc and then hit with a chisel and hammer in the cut and they split(most times?)Then they can be removed.
Crankshaft already for it’s new bearings The M25 & M20 bearings came from a German specialist bearing dealer total of 4 bearings for the crankshaft.
Total cost of bearings for the engine rebuild plus seals and gaskets £260!!!!! plus the £220 for 2 new pistons! Beware restoring a German lightweight is not cheap!!
The M25 bearing in the electric start side crankcase,the ball journal fits on the crankshaft so the crankshaft is a sliding fit.

Two new pistons were needed ,which were found in Germany. Now fixing 2 pistons into barrels on a split single is one of the hardest things to accomplish without braking a ring but pictures below show the safe way to do it. Also one of the gudgeon pins has a blank end! so where does it fit.I got in touch with the Triumph(German) Owners club and they  kindly sent me a diagram showing it’s correct position.I dont think it has any effect on engine performance! it just helps with pin removal.

Place the pistons in the barrels one at a time you can see all the rings easerly then join up with the conrod ends.
Pistons showing the blank end gudgeon pin Which way does it fit?? Answer below.

TWN Pin blanc end