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I'm sure it is but why would you want to? Not being too familiar with a 405 rad, is it a cap similar to the tyre valve sort of caps elsewhere in the system? If so I'd use either some silicone sealant, some ptfe thread seal or heat the plastic until it melts together. Or try another cap.

Thanks for the reply.what it is is the bleed valve on the right hand side of the rad. it just screws in and out when you are trying to get the air out of the system.so i am going to seal this thing once for all.as it is leaking even doing it up tight ,i am going to seal it up with silicone sealant.at the week end. thanks for the reply from chris Addlstone Surrey.