5 More Basic Mechanical Mistakes that can Plague Any Project
Earlier in the week, we published The 5 Most Basic Mechanical Mistakes That Can Plague Any Project. It wasn't hard to find five more. Read on to prevent problems, or re-live your pain.
1. Too Tight, Part 2: Tightness Strikes Back
This ain’t about torque—it's about space. I know it kills us to read the directions but read them anyway—every engineer designed a tolerance into their creations. Metal grows when it gets hot and holes expand at varying rates. Forget to file those piston ring end gaps and the rings will grab your pistons and stop them while the rest of your engine keeps going, or else pop a nice chunk of piston off. Leave those valve guides a thousandth of an inch too tight and they'll grab your valves and keep them open as your pistons race toward them at thousands of feet per second. Valve lash, bearing clearance, valve spring bind—the list goes on—if it moves there is a clearance spec for it. Carpenters say, "Measure twice, cut once." For us, it's more like, "Measure-cut-measure, repeat as necessary."
2. Too Bent
Just like every part has an acceptable clearance, every cylindrical or rotating part has a specification for straightness or runout. After a racing crankshaft is ground out of a billet of forged steel it's straightened before it's shipped to you, and what's the first thing you should do when you open the box from the shipper? Measure it. That goes double for cams or valves. Measure your nice rigid cast engine blocks, too—100 lbs of cast aluminum is more flexible than you think and it changes over time and through heat cycles. Never assume anything is straight. Oh, and assume wire wheels are never straight.
3. Too Painted
Think of paint as just another form of dirt when it's not where it's supposed to be. Pretty, colorful, shiny dirt. The factory doesn't use paint to bear loads or seal gaps and neither should you, even accidentally. Especially accidentally. Mask mating surfaces off when you are painting anything and make sure those important surfaces are paint-free when you get things back from the paint shop. That goes double for the flavor of the moment: powder coating. That stuff is only thin plastic, don't tighten your wheel lugs against it, and don't count on it staying adhered to anything metal forever. It's plastic, not magic.
4. Too Much Air
Air is a little like water—you need it but only in the right places. Air likes to get in where it's not wanted though, so we have to keep pushing it out. Like in our brakes, obviously, but also in our cooling and oiling systems. Put an oil pan on that's just a little too deep, or an oil pick-up that's not deep enough, and next thing you know there's air where your oil should be. Get that all sorted and go around corners a little too hard and there's air again, ready to get sucked in when your oil is sloshing around. In the words of the Talking Heads, "Air can hurt you too."
5. Too Dry
Cousin to Too Much Air is Too Dry. We all know how important good lubrication is, that's why we cringe every time we hear someone start up their car and immediately rev the crap out of it. We know to slather on special assembly lube when building our engines and many of us are using pre-oilers before initial start-up. But don't stop there. Treat every part like it needs protection from harm, because it does. Put a high torque thread lube like ARP on your head nuts/bolts and spray dry moly on any metal that moves inside of metal and doesn't already get packed with grease or fed with oil. Putting Anti-Seize corrosion-resistant paste on your street car lug nuts won't make your wheel fly off. (Be careful though, because delicate sensors typically don't work correctly if lubed.) However, if you're pressing parts together, or screwing studs into aluminum you better be using B200 press fit lubricant or something just as good to prevent galling. If it's under load, exposed to the elements, or takes a fair amount of pressure to assemble, there's a good chance it needs some kind of coating—if it doesn't need a retainer it probably needs lube. Remember, you're going to have to take it apart sooner or later—if you lubed it going together then you won't be crying when it comes apart.