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Tip of the Day

Determining your Engine Overhaul Needs - Measuring Rod and Main Bearings
(22 July 2014)
These to items can be lumped in together since the methods used to measure them are identical. The measurement you are looking for here is the existing size of the crankshaft pin or Journal. With this, you can calculate which undersized replacements will bring the crank or rod journal tolerance back to factory specification. Wear on the journals or damage that requires they be turned by a machine shop, causes the journal to be undersize to its original specification. Your measurements determine just how much undersize it now is and tell you what size to order.

The first step in measuring the crankshaft and bearings is to look the surfaces over for scoring. If scoring is present on the crankshaft, it must be turned or replaced. After looking the crankshaft over carefully, use a caliper to determine if it has equal wear across the surface. If the journal has more wear on one side than the other it will also have to be turned or replaced. Lastly, check for see if it has worn unevenly around the diameter of the journal (oblong!). This can be done by taking a measurement, rotating the caliper around 1/4 turn and taking another measurement. Repeat this process a few times and you will quickly get an idea whether it is serviceable. The better manufacturers manuals will provide what are acceptable tolerances in this respect. If the crankshaft does exceed the manufacturers wear tolerances, you have two options. The first is to purchase a reground crankshaft. While this is more expensive, it has the benefit of simplifying your measurements since nearly always, new bearings will be provided with the replacement that are premeasured for the shaft you receive. In this case you are done with this job. The less expensive alternative is to have it ground by a local machine shop. If you can find a shop to do this, you will find their prices reasonable. The difficulty is that few places will grind a shaft now and those that do usually are very busy with specialty engines such as those for racing.

A word of warning about grinding the crank. Be sure you can get the right sized bearings to cover the amount of material the machinist will remove before you have the shaft ground. If you don't, you may have wasted your money on the machining.

The other journal and bearing measurement process is accomplished by putting the whole assembly back together with plastigage inserted between the bearing shell and the crank journal. Some manuals will talk about using shim stock to measure the clearance. This is not practiced anymore because it is so easy to use the plastigage. You purchase plastigage at an auto parts store. It comes in a few sizes depending on the clearances you are measuring. If you take your service manual, the parts person will help you find the right size to purchase. The cost is minimal. Plastigage is an impregnated string that squishes when you insert it between the journal and bearing shell and torque the bolts down. The width that it squishes to is measured against a scale on the package to determine your exact clearance. Once you find this value, you can determine how much oversize will be required to bring the clearance back to that required by your manual. To use plastigage, cut a strip wide enough to go across the journal, reassemble the shell and cap with the strip placed across the journal, torque the bolts, and disassemble. You can then measure the surfaces against the package that the plastigage came in. Never turn the crank during this process.

By now you must have noticed that we have measured the journals twice, once with a caliper and once with plastigage. In practice, with old well-used tractors, you will do both these measurements. First the caliper finds the irregularities and gross undersizes that mean crank welding, grinding or total replacement. The plastigage process is still necessary to determine any possible shimming that will be required in reassembly or may be used when there is very little or extremely even wear. If plastigage is the only measurement you use, you may find it difficult to find certain important facts like conical or oblong journals. On the other hand, you virtually can't get your final clearance right without plastigage.

Most manuals will offer another alternative for bearing shells that are not out far enough to justify the purchase of new bearings in the smallest undersize (.002). The process that is used is to carefully file the edge of the bearing shell to achieve the small difference between what you measured and what the factory manual requires. This can be somewhat difficult to maintain the perpendicular surface nessecary on the end of the shell. This would not be the recommended method of correcting the clearance but has been practiced for many years. Never file down the bearing caps as this permanently ruins the cap.

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