'CC Quickies
Hemi Parts
What's this, Hemi wheels? Yes sir. The specific 15x6 stamped steel wheels that were factory-fitted to many Street Hemi B-Bodies between 1968 and 1970 have become highly sought- after and can trade for big dollars if they bear the correct stampings and date codes. Look for part number 2944169 stamped near the valve stem or on the inside of the hoop.>>>
Jeff Smith: Let's see if we can eliminate some of your confusion, Ron. The whole process of attaching components with a fastener is really about creating a sufficient clamp load to produce either a proper seal, as between a cylinder head and the block, or to secure two components, as with a connecting rod and cap. To accomplish our task, we need a fastener-either a bolt or a stud-that is strong enough to secure these components without failing under load. This involves tightening the fastener with sufficient torque to create the necessary clamp load. We all know that insufficient tightening may create leaks or failed parts when stuff grenades. But overtightening can also cause fastener failure, which is just as bad.
Here's what happened when an ARP bolt was ground to clear the camshaft on a 383ci small-block Chevy stoker motor. This bolt was tightened to 0.0061-inch bolt stretch when it failed because too much material had been removed from the bolt head. Luckily, this happened on the workbench>>>
Frankly, anyone who works on mechanical devices that rely on fasteners to hold them together should have a working knowledge of bolts, nuts, and studs. The most common method of measuring clamp load is with bolt torque, so let's look at what it really measures. Let's take the process of applying a measured amount of torque, say 70 lb-ft, to a head bolt. The applied load that is measured by the torque wrench is actually the combination of several factors. First and foremost, it is a measurement of the resistance to movement created by the friction between the threads in the block and the bolt. But at the same time it measures the even greater friction between the bolt head and the cylinder head. Finally, we're also measuring a load applied to the bolt required to stretch the fastener a given amount. When it comes to creating sufficient clamp load to ensure that the head gasket does not fail, the only variable among the three we just mentioned that is of any value to create gasket sealing is the amount of stretch applied to the bolt. But because one end of the bolt is buried deep in the block where we cannot access it, we cannot measure the amount of bolt (or stud) stretch necessary to create the clamp load we need, so we're forced to substitute torque as a unit of measurement.
'CC Quickies
We spotted this freak sitting on a workbench. The oddball carrier-bearing pinch-bolt setup looks superweak compared to the beefy cap-and-bolt design used on most modern diffs. This weirdo is from a '64 Buick Riviera.>>>
You may already have figured out that if we apply oil to the threads of the bolt and underneath the bolt head, it will reduce the amount of friction created as we tighten it. That means that more of the effort put into the torque wrench will be applied to stretching the bolt, which will increase the clamp load. But wait, what if we use that slick moly paste instead of oil? The paste offers far more lubricity, so that the stretch value increases slightly. In the ARP catalog, the generic torque spec for a 71/416-inch bolt using 30-weight motor oil is 82 lb-ft, while substituting ARP moly paste changes the spec to 65 lb-ft. Thus the paste removes 17 lb-ft worth of friction to create the same tension or stretch on the bolt.
Just to scare you even more, what makes you think that your torque wrench is actually creating the torque value that it claims? We've seen torque wrenches off by 10 lb-ft and more. And even if you have your wrench calibrated, that can only be spec'd at one load setting, such as 65 lb-ft. As you can see, there are dozens of variables involved in the process of tightening a fastener. This is why any bolt manufacturer will publish specific torque recommendations based on the tensile strength of the bolt and the kind of lubricant used on the fastener. But even these are not completely accurate. There are too many other variables such as whether the bolt is brand-new or has been used, since creating a wear pattern in the area between the bolt head and the part affects the frictional load. ARP calls for burnishing the underside of the bolt head by applying full torque with lube a minimum of five times to establish a common wear pattern before the friction is normalized. This is also why most head-bolt manufacturers like ARP supply precision-ground head-bolt washers to improve this interface.