Why not on a head gasket.
 

Like ^ he said. The stop leak will work (sometimes) for the water pump, radiator, thermostat gasket, heater core, etc.

That is because the system get under pressure pretty quickly, and that 10-12lbs of pressure will force the sealant to fill the tiny leaks by plugging the hole.

But a leak at the head gasket into a combustion chamber does the opposite. The super high pressure made when the cylinder fires will force gasses INTO the coolant, causing it to build up pressure way over 13lbs (cap) and push coolant into the bottle.

Mine was very tiny for a month or so, and would suck coolant back into the engine during cool down. Eventually it stopped sucking it back in, as the leak / hole got bigger.

That allowed air to be sucked back into the cooling system from the cylinder, thus if you check the radiator in the morning, it would need topping up, and the bottle will be full.

BUT....stop leak can work on a head gasket that is leaking coolant outside, and down the side of the block. been there done that on several cars (but not supras, cause thats not where they leak).

AND...it can work if the leak is into an oil passage, and getting coolant imto the engine oil. They sure sign of this is ...more oil on the dipstick than you should have, and a brown 'milky' look to it, as it will mix with the oil.

ok
 

ok well if i dosent work do you no of anything i can put in the car to stop the head gaset from leakin i just need it to run good for about 3 months till i can buy a new motor

Nothing, aside from changing it. Once its out of place you have to pull the head off and replace it.