From: John De Armond X-Source: The Hotrod Mailing list Date: Mar 1992 Subject: Re: Bolt torques > Last night I was reading the latest issue of Popular >Hotrodding and ran across an article on engine building. In the >article they suggested lubricating the main bearing cap bolts >with assembly lube before torqueing to avoid torque errors from >binding. I have very limited engine building experience, but I >believe that this is very wrong. Torque specs are given for >clean dry threads. Lubricants will cause the bolts to be >overtorqued. Please correct me if I'm wrong. Actually both are wrong but oiled threads are more correct. Torque has only an oblique relationship to the real parameter of interest - fastener stress. This stress or preload, measured by the enlogation of the fastener, is the critical value. SAE specifies a stress load for each kind of fastener in ... (oh hell, I can't find it now) anyway, the "bolt spec." Turns out that a fastener achieves its ultimate strength when stressed to about 85% of its yield strength. A good layman's summary of this subject can be found in Carrol Smith's "Engineer to Win" book. The problem with "clean dry threads" is that an unknown proportion of the twisting effort goes to stiction, dry friction and perhaps galling. Lubricating the threads and head gets the situation much closer to the ideal (coiled incline plane). The problem is many American engines have their torques specified dry. Since there is no mathematical conversion, other methods must be used. The best method is to simply measure enlogation directly. This is possible on rod bolts where both ends are available. A small dial indicator assembly is readily available for the purpose. Blind holes, such as cap and head bolts are more trouble. I specialize in one engine (the Datsun Z) and even though the japanese sensibly give torque figures as wet, I've gone to the trouble to develop my own. This is most easily done by making up an assembly of parts such that the bolt can be torqued in a similar environment but where access is had to both ends of the bolt. I carefully clean and polish the threads and the underside of the cap, lube them and then torque the bolt until the specified stretch is achieved. Averaged over several samples, this becomes my assembly torque. My assembly lube is STP or STP with moly disulphide mixed in when I can locate it (Dow Corning Moly 77 powder.) During assembly, I carefully chase the female threads, polish the threads and the underside of the head of the fastener using a buffing wheel and red rouge, use a hardened thrust washer and lube the whole assembly. Even then, if it does not feel right, the joint comes back apart and the trouble found. I never use "click" torque wrenches because the feel of the joint is destroyed just when you need it the most. I also don't use them because when I worked in a metrology lab, click wrenches were the most common instrument to arrive grossly out of calibration. This may seem like an excessivly tedious procedure but it is standard practice in the aerospace/nuclear fields, places where failed fasteners have a bit more serious consequences. I can proudly say that I've never had a bottom end failure on one of my engines attributable to fastener failure Interesting trivia: The 6" diameter studs that hold the head on a nuclear reactor are hydraulicly stretched while heated to operating temperature with a heater inserted in the hollow shank. the "nut" is then hand tightened down against the reactor lid and the hydraulic pressure released. No wrench or torque is used. John |