Can you provide more details please?
like - reason, parts changed and total cost?

basically, the input and output shaft of fifth gear had worn out so changed it.

the cost of fifth gear input and output shaft is around 1000 rupees.

but you need a few other oil seals and engine/gearbox packing too - to repack the gearbox.

and since you don't open engine/gearbox frequently, it is better that once you open it, replace all faulty parts like a few bearings etc.

in my case i changed both gear shaft bearings, a few oil seals, main crankshaft bearings, connecting rod bearings, etc.. and i changed piston also and got the block rebored. so for me the cost was huge, but i changed a whole lot of parts.

generally mechanics won't open gearbox just to replace 2 tiny parts. and labout charge to open complete gearbox is also huge.

so it is best to change it when you are opening the gearbox for somethings else too.

Thanks for the info Princesirohi.
So you changed almost everything in the engine!
I have some basic things to fix like ball race, front forks oil change, head light stay etc and above all my engine is burning oil !
So planning to get these fixed, before touching the gears.

Re: Noise when in fifth gear

My 180 UG3's 5th gear is making a grinding noise too. Only the 5th gear. I have replaced chain/sprocket set, this time with P220F's one, removed the chain cover and the grinding noise can still be heard. It is 100% certain that the noise is coming from inside of the crankcase; probably the transmission unit, precisely.

In my case, even with stock Piston block and stuff and odometer at 69k+ Kms, fortunately my bike is still able to go 111 km/h at 4th gear (~9300 RPM) without much time, this signifies the compression seems okay if not the best for now but when I up shift to 5th gear, the RPM decreases to somewhere between 7000 and 7600 and then the grinding noise starts and the speed reduces to about 102 km/h and doesn't go above 106 km/h.

LOL I can't fathom that Weird Bajaj had created a bike where piston block survives but one of the gear set (probably the 5th drive and driven) goes kaput. Now I don't want to go back to that stupid and unreliable UG3's part numbers and this is why I am considering to get all the gears replaced with that of the new 180 UG4+ or 220F ones.