5 Make or Break Clearance/Tolerance Checks
Make a list and check it twice. Give your car something nice.
1. Valve Lash
There's no better to way to memorize your engine's firing order than by setting and checking valve lash. That's because it's the fastest and most foolproof order to follow when setting lash—the measurement of the distance between the valve tip and the rocker arm tip. If that distance is too great, it will cause premature wear on your valves and you'll lose some of the lift your high-performance cam wants so badly to give you. If it's too little, your valves will stay open longer than they are supposed to, leading to increased wear on other parts of the valvetrain, and the heat transfer from the valves into the head. Necessary even on engines with hydraulic valve actuation, it's essential to check valve lash soon after the first few heat cycles on a new build, and it should be part of any pre-race regimen. Periodic checks are also important because valves and valve seats wear over time, and keepers can even wear into retainers. Checking lash, and inspecting your valvetrain as you go, helps spot other problems like bent valves, worn or damaged valve seals, damaged retainers and keepers, rocker-to-retainer clearance, and broken valve springs. Think of it like getting your blood pressure checked every time you go to the doc.
2. Valve Spring Coil Bind
You can usually count on new valve springs being within the manufacturer's tolerance for spring weight, but it's up to you to make sure you install them correctly. A change in any part of your valvetrain can affect the amount your spring is compressed, and if it bottoms out—known as coil bind—you'll damage your camshaft and bend or break the weakest link in the system. A professional valve spring testing device is the easiest and most accurate way to measure the difference between your spring height at full lift and again at coil bind. All engines need some clearance between these two numbers, check the measurements you get against those recommended by the manufacturer.
3. Piston Ring End Gap
Too much piston ring end gap and you're throwing power away. Too little gap, and as your engine heats up the ring will close until the ring buckles and cracks a piece of piston off. Disaster. Piston and ring manufacturers all provide a ring end gap spec for your intended application. Nearly all high-performance rings are designed to be ground to fit, the good news is that it's simple to do, and the tools to do it aren't expensive. You'll remove material and then insert the ring into the cylinder bore and measure the gap with a feeler gauge. Tools are also available to make sure your ring is square to the bore when you measure it, but a piston dome will also work. Make sure you take the measurement far enough down in the bore if the taper is an issue. Use a diamond stone on the ring ends after grinding them to take away any sharp edges that could scratch your cylinder bore, or give you an incorrect measurement, before checking clearance or installing your rings. Don't get greedy, the mythical put-back tool still doesn't exist if you take too much material off.
4. Timing Chain Stretch
It's easy to forget about your faithful timing chain. It's under a cover or covers, some cars keep it tight and quiet with a tensioner, and it's not sexy—messing with it won't squeeze a few extra horsepower out of your engine. But neglect it just a little too long and it's like taking your whole engine and twisting it—nothing will work right anymore. A worn timing chain can be noisy, but worse, it will throw your cam and ignition timing out of whack too. Check your timing chains by measuring deflection and/or droop and inspect them for obvious wear.
5. Crankshaft Endplay
We saved this easy-to-do, but not-so-easy-to-access, check for last. Crank endplay is the fore/aft movement of your crankshaft. It's measured with the crankshaft, main bearings, and thrust bearings installed and main caps fully torqued down, and best done as part of a test assembly prior to the final assembly of your engine. Use a dial indicator to measure the movement of the crank as you carefully pry it, using prybars or large screwdrivers, to its forward and rearward extremes. Pry until it stops moving but don't reef on it, you're not trying to bend anything, just make sure it reaches the endpoints of its travel. The measurement you come up with needs to fall within the safe range given by your manufacturer—there's not a lot of room to mess around with this one.
There you have it, a few dozen checks of a few thousands-of-an-inch each and you're thousands of times less likely to hear the 'strange metallic noise' that puts frowns on the faces of too many or our engine-building brethren. Happy measuring!
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