KEEPING ABREAST OF CHANGING TECHNOLOGY
Meaning: Read the Instructions!
BEEN THERE, DONE THAT. How often have you heard that expression? Fact is, the popular saying can be applied to many things related to assembling racing engines, but there should also be a big red caution flag waved because industrywide improvement in metallurgy, machining accuracy and technology have changed things. A process once held as gospel might be obsolete today.
The gravity of this situation was recently pointed out by the folks at fastener manufacturer ARP, who have made a concerted effort to call attention to the changes and encourage racers to make sure that they adhere to the most current engine-assembly practices put forth by manufacturers. To that extent, ARP has updated and expanded its instruction sheets and has posted them online for easy access.
A case in point involves installing head studs. Today’s cylinder heads are machined to much closer tolerances than in the past. The surface finish of the mounting pads is now very smooth, as opposed to the relatively rough machining of years ago. Accordingly, if you get any lubricant under the flat washer, it can turn it into a bearing of sorts and greatly affect torqueing accuracy.
Regarding surface finishes, there are enough differences between a polished stainless-steel fastener and a blackoxide-coated chrome-moly bolt or stud that instruction sheets may reflect varying preload specifications.
Thanks to developments in measuring actual fastener preloading, more attention has been focused on the phenomenon known as “preload scatter.” When a fastener is torqued, only a percentage of the energy is used to stretch the bolt or stud, which is where the clamping force comes from. Think of a fastener as a spring, with the rebound, when stretched, providing the preload. Much of the energy is actually usurped overcoming friction, which can largely be attributed to the thread quality and cleanliness, as well as the lubricant that’s employed.
ARP’s engineering team spent years carefully analyzing preload scatter using sophisticated computer-controlled measuring equipment and discovered huge disparities from one lubricant to another—up to 30% or more. This lead to the development of ARP Ultra-Torque Fastener Lubricant, which is so consistent, it delivers 95-100% of the desired preload on the first (and any subsequent) pull of the torque wrench.
Prior to this, the only way to ensure preload accuracy was to torque the fastener to the desired setting, back it off and retighten. Between three and six cycles were usually required to mitigate the friction between the threads, bearing surface and lubricant to ensure proper preloading.
Having the proper preload is important for a couple of reasons. Not having sufficient clamping force on a head gasket, for example, exacerbates its failure; and having varying clamping loads on adjacent studs or bolts can distort a cylinder’s bore and actually have an effect on piston ring sealing.
The recommended procedure for installing a head stud or bolt today is to thoroughly clean the threads using a special chaser tap on the block that won’t cut threads, followed by carefully removing any debris and properly lubricating the fastener threads, as well as the underside of the nut (or bolt head).
ARP recommends achieving the desired torque reading in three incremental steps but also cautions that torque wrenches need to be checked for accuracy. For years ARP has offered free torque wrench checks at its display on the Manufacturer’s Midway at all NHRA National events and discovered huge disparities in accuracy. A common problem, according to ARP’s tech reps, is employing a torque wrench as a breaker bar. Using it to remove a fastener is a
big no-no. Torque wrenches are delicate instruments and should be treated accordingly and calibrated periodically.
When torqueing the head stud or bolt, make sure the washer is not rotating. Washers with patterns machined into them must be installed with the pattern facing the block.
Another potential problem area concerns upgrading the quality of a fastener and relying on OEM torque specs. Oftentimes, fasteners with increased tensile strengths require additional preloading to achieve the desired clamping force. Rely on the data for that specific fastener.
In a related matter, when clamping force is increased, it’s vital to understand material will be affected. For example, having higher strength main bolts or studs that require increased preloading can affect the shape of the cap and pinch the bearing. It’s important to align, bore and hone a block with a fastener upgrade. The same type of thing happens with connecting rods, so higher preloading warrants resizing the rod.
There are many other instances where old practices can be problematic. Take head gaskets. For years, using one particular spray/sealer was the hot tip. Now some manufacturers of MLS (multi-layer steel) gaskets eschew the practice.
The bottom line is you should rely on the instructions and not do something because that’s the way it’s always been done.
As a parting thought, for many years, racing icon Smokey Yunick was a valued consultant to ARP and represented the company at trade shows. In his book, Power Secrets, Smokey notes, “I don’t care what kind of engine you’re building, before you even consider ‘final assembly’ you should do a complete mock-up assembly of the engine and all the engine components.” Smokey went on to advocate carefully checking all clearances, as bolt head height and the nut/ washer combination can affect how things fit overall.
After all, it’s the little things that count, and diligence can often spell the difference between a project that goes smoothly or winds up with major problems.
… it’s the little things that count, and diligence can often spell the difference between a project that goes smoothly or winds up with major problems.
Use a special chaser tap to clean threaded holes.
The cylinder head stud/bolt mounting pad on a popular aftermarket SBC cylinder head, and the other pad on a new Coyote engine. You can see how the newer heads are finished better and provide little grip for the washer.
Check your torque wrench for accuracy.
Use lubricant on the underside of the bolt head (or nut), not on the underside of the washer and on fastener threads.
Follow manufacturer specifications when applying preload torque. Use a stretch gauge on connecting rods.