The amalgamation procedure is actual when using structural methods of reliability. This may apply as the case of the action of a complex of damaging processes on a structural element and the case of the action of a certain damaging process on a system of elements. Classical methods of reliability are poorly adapted for powertrain mechanical systems with a series structural scheme of elements subjected to the gradual influence of several degradation processes. The problem of amalgamation of individual indicators is exacerbated at the stage of operation when diagnosing the technical state of the mechanical system. The application of the classical rule of amalgamation by multiplying the probabilities of survival leads to the effect of overmaintenance. New rules of amalgamation have been developed, which deprive the assessment of the reliability of excessive conservatism. The complex index of the technical condition is offered - the resource safety index (RSI). Its use determines the remaining lifetime. The algorithm for searching the RSI contains an assessment of the criticality of the failure. The search of RSI under the influence of the complex of damaging processes on the element of the powertrain system is demonstrated. The efficiency of the RSI method is shown by the example of the reliability assessment of aircraft bolts. Application of the RSI method increases the guaranteed lifetime by 4-10 times compared with traditional methods.