How to safeguard an old-aged fathers property when the son is guarantor for the defaulter.

Will  of no use ,he can gift.

If you are a guarantor with no document executed by your father for guaranteeing the loan, property is safe until it comes to you by way of inheritance.   If it does not come to you either by virtue of gift to your spouse or through testamentary succession, the property could not be utilised for recovery in connection with the default that you described. 

Sir i am worried abount the way of inheritance since father is super senior citizen.

Let he gift it to your spouse and children .

Will and testament will not work ? since gift deed incurred more expense.

In some states  gift for blood relations nominal stamp fee  .

Have you taken your father into confidence? Ultimately, it is his decision; you cannot compel him to transfer his property to your wife under the pretext of an uncertain contigency incidental to inheritance.

By the way, are you the only legal heir to your father?

Is the property self-acquired by your father or ancestral?  If the property is his self-acquired no one has right over the property.