I need help in figuring out why my radiator keeps cracking at the joint between the metal and plastic part just below the cap. I know it gets heated so I replaced the thermostat and have let the air go out of the system while replacing the radiator each time but it keeps cracking or bursting within a week of driving.

 

First verify that its actually cracking. If the cap is leaking then coolant will seep along the radiator and look like the radiator is leaking.

Second, when you replace the radiator you should do a few things:
Flush and replace the coolant with Honda compatible coolant.
Replace the radiator cap
Use tightening hose clamps on the hoses instead of the stock clamps
Make sure there are no other leaks in the system
Make sure that the pressure generated in the system is within spec. A blown head gasket can introduce combustion pressure into the coolant system and cause blow outs.

If all else is really failing then buy an all aluminum radiator.

 

A new radiator cap each time the radiator was replaced?

Edit: what year/engine/trans/trim?

 

Thanks for the reply, RobinsonRicer and Roader. I did all you suggested to do, now again the 4th radiator cracked and spilled coolant while the car was running and thick fuse started coming out form the hood. When I removed the radiator the coolant was thick black, it seems like oil. I think as per Robinson I may have a blown head gasket. How do I confirm that and what is the procedure to fix the problem. All your suggestion is really appreciated.

 

It sounds like a bad head gasket. There are only to ways to get oil into a coolant system so... First check the ranks mission fluid assuming you have an automatic. If the ATF is clean and free of coolant then you know that the radiator did not fail internally which is one way to get an oil into the coolant, and two, if the ATF is clean then it is a bad head gasket.

You can do a simple check for the head gasket by... With the engine cool, remove the radiator cap, assuming the fluid level is fine start the car and watch for air bubble in the coolant. Yeh I know people will say that it might be just air working itself out of the system, but often times with a bad head gasket either intake air or exhaust will be pushed by the gasket and into the coolant system and create a tell tail sign of air bubbles. Other then that a compression and/draw down test can be done.

Until you know please be careful running the car. If by chance the head has not been warped yet by over heating then you should simply need a gasket. Otherwise you may need the head resurfaced, and if really bad, although rare the block may need to be resurfaced which in my book would mean recycle the engine and put something newer in. Just a little insight and food for though. Good luck.