When measuring bearing clearance, always measure vertical clearance. The horizontal clearance is always larger, depending on how much eccentricity is designed into the bearing.
The back-and-forth thrust clearance measurement, without the center main cap installed, should read the same as when the center main is torqued in place. If it is not the same, the cap is not in the proper location or the mating surface of the main cap is not square to the thrust surface.
The only way to measure exact bearing clearance is to use micrometers and dial-bore gauges. Don’t stress out over a few tenths here or there. If you have enough clearance, those few tenths won’t mean anything.
If the clearances are too tight, and the bearing touches the crankshaft journal, the spinning engine parts grind against each other and machine themselves. If your bearing clearance is a little loose, there is no negative. I will not build a high-performance Oldsmobile engine with less than .0035 inch of bearing clearance on the rod journals and .0040 inch on the main journals. The more power the engine produces, the more the components flex, and the more clearance is required. My personal 1,200-hp, 8,800-rpm, nitrous-enhanced Oldsmobile small-block engine has .0049-inch vertical clearance on its 2.500-inch main journals and .0043-inch clearance on its 1.888-inch (factory Honda-size) rod journals.
To make sure you didn’t install the incorrect undersize bearing, or if you simply do not have access to micrometers and dial bore gauges, use a feeler gauge to measure the vertical clearance. This has to be done with the bearings dry; oil takes up some clearance. Forget Plastigage.
Experience has taught me that when a main bearing is tightened to specification (torqued) in the block, or a rod-bearing cap is torqued on a connecting rod, the bearing bore diameter will be pretty close to .0020 to .0025 inch over the nominal spec. For example, torque a big-block Oldsmobile rod bearing in a connecting rod, and it measures 2.5020 to 2.5025 inches for a standard-size bearing. Similarly, torque a big-block main bearing in a big-block housing bore and the inside diameter measures 3.0020 to 3.0025 inches on a standard-size bearing. The actual bearing clearance is the inside diameter dimension of the bearing in its housing, minus the dimension of the journal’s outside diameter (OD). In general, the inside diameter of the bearing in the connecting rod and main journal is .002 inch over the nominal dimension. On undersized bearings (meaning .010-inch, or .020-inch, etc.) you simply subtract the undersize you are using from the nominal dimension.
As-manufactured or machining size errors can and do happen. Machinists sometimes make errors when sizing journals or cylinder bores, and there is a plus-or-minus size tolerance to the actual thickness of the manufactured bearing shells. Occasionally, mistakes are made in packaging or labeling manufactured parts. It never hurts to torque up a bearing in a rod or main journal and measure it yourself. The best way to achieve what you want is to torque up your bearings, measure with a dial-bore gauge, subtract the amount of clearance desired, and give that dimension to the crank grinder.
What should your bearing clearance be? See sidebar “Bearing Clearances” for my recommended bearing clearances. Keep in mind that non-rigid components require more main bearing clearance than more-rigid components. For example, an engine with a relatively heavy piston teamed with relatively soft factory connecting rods requires more rod bearing clearance than engines with lighter pistons and ultra strong connecting rod. You have to look at all your components to determine what clearances are best for your application. If you lack the experience to make these critical determinations, I must advise you to consult an expert who has this experience.
Rear Bearing Clearance and Number-4 Main Failure
If you have measured the bearing clearances in the main journals, you know that the rear main (also known as number-5 main) has an extra .0010- to .0015-inch bearing clearance if the crankshaft mains are all ground to the same dimension. The rear shells are, by design, thinner radially than the first four shells. The GM factory initiated this design and the aftermarket bearing companies simply copied them.
I don’t know why the GM engineers gave that extra clearance to the rear main, but I can only guess that it is because of the additional width. I am absolutely convinced this additional clearance in the rear shells is the main reason for the notorious number-4 main bearing and number-4 main web failure. I have never seen a rear main wasted in an Oldsmobile engine, but I have seen plenty of number-4 main and main bearing failures.
If you think about what is going on here it makes perfect sense. If the rear main bearing clearance is .001 or .080 extra, and the rear of the crankshaft, which is attached to the flywheel, moves around as it starts to transfer power to the drivetrain, where do you think the load is transferred? You guessed it. It transfers the load to the next one in line, which is the number-4.
Chevrolet engines do not have additional clearance in the rear, and neither do Ford engines. In fact, most engines do not have that additional rear main clearance. The Oldsmobile is the only engine that has a notorious failure rate for number-4 main bearing shells and breaking blocks in the number-4 main webs.
You can easily fix this issue by simply machining the rear (number 5) main larger than the number-1 through -4 mains so that it takes the load. This is simple with undersize applications such as .010, .020, .030, etc. In crankshaft applications that have standard main journal sizes, the only way to help this situation is to purchase a main bearing set with .001 less clearance than you are working with and use the rear shell to tighten it up.
MAHLE Clevite manufactures a bearing set (PN MS 804 H-1) for big-block Oldsmobile applications that yields a .001 tighter clearance. In a typical standard-size main journal application, you can use the MS 804 H on the number-1 through -4 mains and use the rear shell from the MS 804 H-1 set to tighten the rear clearance by .001, which forces the rear journal to take the load off the number-4 main. You could also use the MS 804 HX bearings in numbers 1 through 4 to tighten the rear by .002 compared to the front four. With all of these shell choices, you can set the main clearance to whatever size you want if you cannot machine it.
One of the most important machining operations you will do in your high-performance Oldsmobile engine build is the preparation and grinding of the crankshaft. Forget shopping around for the cheapest price for a crank grind! Always remember, you get what you pay for. Every machining operation takes time and care. Just because you stick the micrometer on the journal in one spot and its dimension is what you intended, the grind job may or may not be suitable. I have seen many improperly ground crank journals on re-grinds and, shockingly, on brand-new crankshafts. A properly ground crank journal should measure exactly the same everywhere on that particular journal. With improper or no dressing of the crankshaft grinder wheel, you may see taper or out-of-round (different measurements) from one side of the journal to the other. This means that clearance is tighter in some spots and the oil wedge is improper, lacking two perfectly flat surfaces.
Bearing Clearances
The information below represents the minimum amount of bearing clearance, and should keep you out of trouble. When checking your bearing clearances at the time of assembly, don’t stress out if the actual measured clearance is more than anticipated. In the case of clearance, a little more is acceptable; any less is not.