Rotational Rationale

So, where does this leave those of us building our engines at a home workshop with less-than-professional shop tools? Well, there is Plastigage. This stuff is about as cheap as it comes, but it is looked down upon by almost every Cup engine builder I've ever mentioned it to. Why? Because it is not as accurate as the more correct (and expensive) tools for the job. However, I think there may be an element of techno-social climbing going on here. Having used all three of the methods mentioned, here is my take on it. First, Plastigage accuracy is not as bad as some pros would have you believe. Testing this for yourself is easy. All you need is two machine parallels and two 0.002-inch feeler gauges. Just place the two feeler gauges side-by-side on one of the parallels and place a strip of the Plastigage between the feelers. Next, place the other parallel on top and squeeze the pair in a press or vice. When you measure, the now-spread Plastigage will have a reading that is really close to the 0.002 inch that it should read. The not-so-good news is that when applied to a curved bearing and journal, things are not quite that close. Generally, the results are within about +/-0.0002 inch (i.e., 2 ten-thousandths of an inch). Maybe this is not perfect, but it is good enough for most of us if the bearings are about middle limit. Now we come to the best part of using Plastigage. If the bearing being measured is out of tolerance, it will be obvious. If the bearing clearance is wrong, especially if it is on the tight side, it will instantly show up, thus preventing serious engine damage.

Now we know how to measure bearing clearance, but how much of it should we have? For most V-8s, a good working figure is 0.002 inch (2 thousandths) for the rod journals and 0.0025 inch (211/42 thousandths) for the mains. Going up half a thousandth on this is no big disaster. In fact, I have used as much as 0.004 inch (4 thousandths) on the mains when it has been a case of "use the crank or don't race." If you are building an engine that has good components, then these nominal figures can drop by half a thousandth (0.0005 inch). If you are building for a small-engined four-cylinder class, the clearances can also stand to be about half a thousandth (0.0005 inch) less than V-8 clearances.

The amount of crank-to-bearing clearance used also influences the weight of oil needed. The closer the clearances, the lighter (thinner) the oil needs to be. This is good news if you are reducing windage and crankcase scavenging losses are a priority.

Conclusions We have seen the advantage gained from coated aero cranks with reduced MOI in wet-sump engines, but there isn't necessarily a direct carryover to dry-sump engines pulling a lot of crankcase vacuum. The more vacuum pulled on the crankcase, the greater the tendency for oil to drop out of suspension. Sure, coated aero cranks are still an advantage for dry-sump engines, but not necessarily by such a margin. One direct carryover from wet sump to dry is the reduction in MOI. This is good, regardless of what type of engine you are running.