decrease in radiator water quantity after every 5 days.

you have a leak and from the sounds of it, it may be a blown head gasket from overheating.
you need a leak down test to verify the integrity of the motor. the miss fits in with the air bubbles that you describe.

Roy

If the recovery tank is also decreasing, then it might not be too serious. There is a leak somewhere and a pressure test would allow you to check for leakages.

If the recovery tank is not decreasing but overflowing instead, you either have a bad radiator cap or as Roy mentioned, a bad head gasket.

thanks for the reply. may be its head gasket because i didn't find the decrease in quantity of water in recovery tank.

so if i am going to change the head gasket then it is compulsory to change valves, pistons and piston rings ?

Nope, not necessary to bother with the engine block components unless you have wear of the cylinder liners or compression is poor. The valves need not be replaced unless they are bad but valve lapping and replacement of the valve seals would be recommended if the heads are removed.

Check the radiator cap and if condition is bad, replace it and retest.

sir,
how i will check the engine compression.

and i already changed the radiator cap but no effect.

You need a compression gauge plugged into the plug hole after removing spark plugs and crank the engine to get the reading.

thanks guys for the support... :-)