Getting the right crankshaft bearing clearances

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Crankshaft rackLast month we were rebuilding an engine where the customer had provided us with a crankshaft that had just been ground .010/.010 on the main and rod journals.  That’s great, right?  It saves us the hassle of having to get the crank cut.  Well, not so fast!

 

We had to get 2 sets of main bearings and 3 sets of rod bearings from different manufacturers in order to end up with the bearing clearances we wanted.  Why is that?  Because in spite of what most people think and what common sense would dictate, the same size bearing from Manufacturer A will consistently run a little bigger (or smaller) than a comparable bearing from Manufacturer B or C.  And if you plan on running a race or high-performance bearing, that adds another variable as well since those clearances are usually different even within the same manufacturer line. Finally, OEM’s and aftermarket crankshaft manufacturers have an acceptable range for the finish sizes of the crankshaft journals (usually a tolerance of .0005 to .001).  So if the crankshaft is cut to the high or low end of that range, it will affect bearing clearances as well.

 

Here is the tried and true procedure we use at our shop to get the perfect bearing clearances on the first try.  Let’s assume you have a high mileage standard size crankshaft that needs to be cut .010/.010 on the main and rod journals.  First, order the brand and type (standard, high-performance, or race) of bearings that you want to run.  Next, align hone the block if needed and resize the connecting rods if needed.  Finally, install the new .010 under bearings and measure the actual main and rod bore housing sizes.  From those numbers deduct the bearing clearances you want to run and that will give you the exact size for your crankshaft grinder to cut the main and rod journals. We often want a different size for a specific main journal (say the rear journal on small or big block Chevrolets).  This method gives you the flexibility to do just that, as opposed to grinding all the main journals to the same size.

 

To illustrate the math, let’s say you are building a street engine and want to run .0025 bearing clearance on the mains and .002 on the rods.  The bore housing sizes with the .010 under bearings installed measure 2.4425 on the mains and 2.092 on the rods.  You would then instruct your crank grinder to cut the main journals on the crankshaft to 2.440 (2.4425 – .0025) and the rod journals to 2.090 (2.092 – .002).  Assuming your crank grinder does accurate work (you should always double check), your bearing clearances will be exactly where you want them.

 

This procedure is not new, it’s been around since the dawn of the engine rebuilding industry.  But it’s surprising how many people don’t follow these simple steps.  It does take a little more time to complete an engine rebuild this way since you have to obtain the bearings first, then measure the housing bores with the bearings installed, and then send the crankshaft out for grinding and wait to get the crankshaft back. But this method will provide many years and many miles of dependable service – so it is definitely worth a little extra time and effort to do it up front.

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