5 Essential Straightness/Runout Checks

1. Crankshaft

These checks are essential anytime you are assembling an engine, particularly one that you are planning to use for any sort of motorsport. Though these tips are general in nature, we attempted to address the differences in engine design and construction. We're also assuming your camshaft and crankshaft journals are the right size and not damaged.

So you got your trick racing camshaft delivered from the parts house today and you can’t wait to clean it up and install it. Even if it was perfect when it got loaded onto the courier’s truck, it might not be perfect by the time it gets to your door. Rest your crank's outermost main bearing journals in V-blocks on a flat, solid surface and check the runout (distance from the centerline) of all the main journals using a dial indicator. The manufacturer will have supplied you with a specification for runout, typically it’s very near zero—in the few .0001s-of-an-inch range. If your runout exceeds spec your crank will need to be straightened or replaced. However, if everything is fine, remember: always store your crankshaft on end, not lying down. It may sound silly to the uninitiated but crankshafts are meant to be supported and will warp over time if left sitting horizontally. Turns out metal is a lot more like wet wood than solid stone. All the more reason to do this check.

>>Join the conversation on Straightness / Runout Checks right here in the forum.

2. Main Bearing Bore

Now that you know your crank is straight, it's only logical to check your crankshaft bore to make sure it's straight too. This is particularly important if your crankcase or block is aluminum. We're assuming that you've checked each of the individual main bearing bores to make sure they are to size, but just because those are right it doesn't mean that they are in a perfect line. With the cases/main caps/girdle torqued, and anything else that would distort the block assembled, lay a precision straight edge across the bearing bores and check to see if you can slip a .001" feeler gauge past the straight edge. Ideally, nothing should fit under the straight edge, it should be flush. If you can fit more than a .001" feeler gauge under the straight edge then you'll have to send the block out to be checked with a precision-ground checking bar to see how far out of whack is. If it's too far out it will need to be replaced or line-bored. You'll thank yourself for doing this check now because if you are in the later stages of assembly and find out your crank doesn't turn, or turns hard, you're going to have to take everything apart again.

>>Join the conversation on Straightness / Runout Checks right here in the forum.

3. Camshaft

You probably saw this one coming. Yes, you need to check your cams for straightness too. The procedure is the same as with your crank: V-blocks and a dial indicator. Mark any high spots you find on your cam with a marker, including the measurement, so you can see if any deviation is a simple bow, or more of a complex twist. If your crank is too warped, send it back and get a replacement. If that's not possible, or it's only a few thousands out, it can be straightened, but we aren't going to tell you how to do that because it involves a punch and a heavy hammer and our lawyers told us to keep our mouths shut.

>>Join the conversation on Straightness / Runout Checks right here in the forum.

4. Camshaft Bore

Yep. You called this one too. A straight cam needs a straight bore to live in. Again, this applies even more to aluminum engines. Extremely common in overhead cam engines that have seen numerous high-temperature heat cycles, cam bore deviation can cause you to wipe out your cam bearings/bearing surfaces and cause other valve train nightmares. Assuming the bore isn't so distorted that a camshaft simply won't turn, cam bore straightness can easily be checked using a cam you know to be straight. Apply a very thin coating of blue transfer dye, like Permatex Prussian Blue, to your camshaft and spin it several times in either its cam bearings or torqued-down caps/journals if your engine doesn't use cam bearings. Take it back out and check the caps/journals or bearings for an even pattern of blue. No dye indicates a problem, as does dye that has turned dark grey. If you identify a problem, don't fret because there a number of ways to repair cam bore misalignment, from light lapping all the way down to over-boring/using bearing inserts in some cases.

>>Join the conversation on Straightness / Runout Checks right here in the forum.

5. Throttle Shafts

Wipe the oil off your hands, we get to check the intake side of things now. If you're running fuel injection congrats, you are maximizing your potential—but even fuel injection uses butterfly valves mounted to a rotating shaft inside a housing cast of soft metal. Typically fuel injection throttle bodies are fewer in number and larger in diameter than carburetor throats and less susceptible to warpage due to high heat and over-torqued castings than carbs are. Whether your engine is breathing through carbs or injection, its butterfly valves need to seal completely when closed and open completely at wide open throttle. Those things are easy enough to check, and though a bent or distorted throttle shaft can be the cause of a problem in this area, it's more likely to be caused by a worn or misaligned butterfly valve plate. The real problem with throttle shafts is air that leaks past them when they, or the bores they rotate in, are worn. This causes huge problems in a system were precisely metered air flow is a must. Remove shafts if you suspect a problem, being careful to mark the location and orientation of every part you remove. Measure the shafts with a micrometer and the bores with gauge pins. If you are working with rare or expensive parts don't worry, shafts can be replaced and bores repaired. The bores are a little trickier to fix but usually can either be bored oversize (for use with oversize shafts) or trued/bored and then sleeved back to the correct size. Do all these checks and we guarantee you'll sleep better at night. We know, we checked with our lawyers and they cleared us to say that.

>>Join the conversation on Straightness / Runout Checks right here in the forum.