konrad

I have an opinion, but you're not going to like it. ONE OF THE BEST FEATURES OF ELECTRICS IS THAT THEY ARE QUIET!!!!!!!!!!!!!!!!!!! See, all that hollering is annoying. Yes, they are even quieter.

I'm not saying your coil is good, but oil won't cause it to short out. Lots of large transformers are cooled by being immersed in oil. Everything is in parallel with the source coil.

Which bearing did you decide was the possible issue?

The clicking sound is not normal. But it seems the repetition rate is too slow to be something in the gearbox. I'd look for something external like Zippy's problem first. Inspecting the drain oil is a good idea, but knowing the true history of the bike is more valuable. Did the prior owner sell because there was debris in the drain oil? Is there a chipped tooth somewhere? Out of curiosity, I looked for gearbox parts. It appears KTM is good about making GG parts available. You can buy individual gears and even a complete transmission assembly (MT40000CT-CSV-1) for $840 USD. https://trialssuperstore.com/products/gasgas-transmission-complete-19

I could not tell much from listening to your video at 1x, 0.75x, and 0.5x speed. I think a better diagnostic would be to video the bike running on a stand in 4th (probably would have to load the motor by using the back brake) and slowly letting it rev. Note that both 1st gear and 4th gear use three gear-pairs to transmit power though the box. So the whine should be similar for 1st and 4th. There is a large ratio change between 4th and 5th, and this will make a clunk and not be smooth. The farther apart the ratios are, the more pronounced this will be.

At first, I was going to say the "assembly" number is the service limit. But viewing that table in the context of the other displacements, it makes no sense at all. It's a horrible table!

That's a very nice manual! Anyone learning to do maintenance on a trials bike (regardless of brand) would be well-advised to study it. Unfortunately, there is almost no content about the gearbox (see page 49).

It should NOT sound like it's going to blow up! But 4th gear is "complicated" and actually requires 3 gear pairs to create the ratio. Here is a link to a YouTube video that demonstrates this, as well as the original GG patent in Spanish: https://www.electricmotiontech.com/home/em-factor-e/clutch-gearbox#h.2ajazdf2rs5

Can you feel anything with a sharp-tipped scribe? A crack could explain the problem you are having. Another thing to consider is the history of the crankshaft. After repeated pressings, the pin/crankwheel interference fit can become too loose. If you have a dial indicator and a means of supporting the crankshaft, measuring the runout could tell if the crank wheels have shifted position,

Is that a crack?

I think this relates to your prior post about huge side-clearance on connecting rod. I have read about the washers on the piston pin, but have never seen a motor that used the technique. What struck me as odd, is that the big-end bearing itself seems to have a lot of clearance to the crank wheels and could walk from one side to the other. So, even though that motor may use washers on the piston pin for rod centering, it may still be that the crank wheels are too far apart. But this is all speculation on my part. I don't work on "antique" motors.

Ciao! I think you should start a new thread describing technical details of your installation. Many of us would find it interesting. I would be curious to know: Had the old controller failed? How often does the controller's fan run? How did the performance improve? How long did the project take? Total cost? What seat is that? Did you upgrade the battery also? I see a clutch lever. How does that work?

After a seizure, the piston probably must be replaced. If the seizure was very light, it's sometimes possible to sand a damaged piston smooth again. But if there has been any aluminum transferred to the cylinder, sanding is not advisable. Furthermore, if any rearranged aluminum has impinged on the ring lands, the piston must be replaced. If the cylinder damage is just aluminum transfer, it's possible to remove the aluminum with hydrochloric (aka muriatic) acid. But, if any peeling of the plating has occurred, the cylinder must be re-plated. Sometimes imperfections in the cylinder plating are tolerated in areas above the ring travel or below the piston travel. Note that Nikasil plating is very hard, and a diamond hone must be used. A standard cylinder hone for iron won't work. It's probably not possible to determine the extent of seizure damage from photographs. If in doubt, a local specialist should be consulted. But finding OSSA parts is not easy either. ☹️

This link shows the many Lifan engines produced: https://gzbawo.en.made-in-china.com/product-group/lqKanTtOOzcr/LIFAN-engine-1.html Search in your own country, but this is an example from the USA: https://www.amazon.com/CUBELLIN-200CC-250CC-Motor-Engine/dp/B0D81ZTJDH

You are probably not going to find much information about that enduro bike (?) on a trials forum. It appears to currently have a Lifan motor (which is a Chinese copy of the Honda XR100). Lifan built them in a variety of sizes up to about 200cc. You can measure the bore and stroke to determine its displacement. Have you considered that it will cost more to repair that bike than it will be worth?