hydraulically assisted fastener comprises a body with a central bore to engage a connector element and with an annular recess opening outwards to an end surface . annular thrust member fits into and seals the recess . charging medium is injected under pressure into annular chamber defined by recess and member and moves body relative to member to tension element . medium sets in chamber to maintain the tension in element . element may be a bolt or stud , body can be a nut and member can be a plan washer . medium can be a curable viscous paste , a suspended solid in a self - setting compound or a particulate solid which behaves as a fluid . suitable materials include graphite , and lead , copper or steel balls .