Query Approved

Yes possible; go through several threads here or contact some of the experts like SriramEfunds, dual_disc, NitinGirish, JD666 etc

You don't need to change the gearbox.Existing gearbox can be converted to 5 speed with 5th gear wheel,shifter etc.

Santosh,
we tried the very same thing on one of my freind's 4S. but it dint worked well. we managed to have 5Speed GB, block, piston but failed to have Head of 5S.
its too slow. its not providing that quickness which RX is famous for. we tried borrowing head of 5S and compaired it with Original 5S. modified 5S wasnt comparable with Original one.

FYI, i am lisitng down the products which you'll have to arrange to modify it to 5S.

head
block(bore)
piston
carb
lightweight connecting rod
5S gearbox
sprockets.

last thing we failed to have in our experiment.

if you want street fighter, 4s is much better than 5Sin innitial 3 gears.

For all of ur kindest information who wants go modding around, the compresson ratio of 4S & 5S is different and that is not the only difference. The ports are different too. There's no much difference between the heads other than the compression they give. A 4S head can be shaved to give better compression levels of that of a stock 5S & there are a bunch of guys who can do it perfect. Also there is no difference in piston used. The 4S & 5S pistons are the same 58mm ones and 4's & 5"s doesn't have different sizes & so does the final gear ratios of both the models. You should have also checked for Jetting issues or atleast should have tried swapping the carb of 5s with the test mule that you had. Finally light weight connecting rod?? All the 135 Crank shafts are the same what you get in India irrespective of 4s or 5s. just ensure that 2YA is embossed in the rod, that would do the job. Btw. expecting the 5S performance without their sprockets would never give you smiling results.

P.S. 5's are never too gud in the city as you think, they are the most sluggish ones that can severely test your patience but on high ways it's a different ballgame altogether. Reverse the result & there you end up with a well behaving 4S.