When you get right down to it, many of the tasks involved in building a race engine are mundane. It isn’t too difficult to drop a piston in a hole or bolt up an intake manifold. And neither is checking your valve lash or fitting your main and rod bearings. The trick is knowing how to do it right, taking your time to pay attention to the details and having the proper tools which can make your life a lot easier.

This month we’re going to take an in-depth look at how to measure for the correct clearances between your crank and rod journals, their corresponding bearings and how to install them.

One of the most important factors when it comes to the health and durability of your rotating assembly (crank, rods, and pistons) and the block itself is the amount of open space between the main and rod journals on your crankshaft and the bearing shells they are spinning within. This space is known as the bearing clearance, and it’s critical because that open area is filled with oil which provides a cushion between the journal itself and the bearing. When everything in the engine is right, the oil keeps the steel of the crank journals and the babbit of the bearings separated, and they never touch.

But problems can enter the picture pretty quickly when the bearing clearances aren’t correct. Too little clearance and you’ll have trouble keeping your oil temperatures in check, and a spun bearing is much more likely. On the other hand, if the tolerance is too loose you hemorrhage oil out from around all the bearings. Obviously, too loose is better than too tight, but hitting the center of the bullseye is always preferable.

As a general rule for a race engine, you want to have approximately 0.001-inch of bearing clearance for every inch of journal diameter. So for a 2.100-inch rod journal, it’s always a good idea to shoot for 0.0021 or 0.0022 bearing clearance. And once again, because too little bearing clearance can mean a spun bearing, we always aim to wind up too high rather than too low.

Adjusting the clearance is easy, but probably a bit more expensive than most of us would prefer. Most quality bearing manufacturers make three versions for all the popular styles. First, there’s the standard bearing. This is where you start. If you find you need more clearance, you can purchase an undersized bearing. This means the bearing shell is thinner, which creates 0.001 more clearance than a standard bearing. An oversized bearing is the opposite. It’s thicker and takes away 0.001 of clearance.

One one-thousandth of an inch is a big jump, but you can actually make adjustments in 0.0005 increments—that’s one-half of one thousandths of an inch. To do this you mix half the shell of a one-over or one-under bearing with half the shell from the standard bearing. This means you may have to purchase a couple sets of bearings to get everything just right in your race engine, but sometimes that’s the price you have to pay. Just remember when mixing shells to always keep the sizes in the same alignment. In other words, if you’re using one-unders with standards for your main bearings, put all the one-unders in the cap with the standards in the block side of the housing bore, or vice versa. So with that said, let’s get on with it.



One of the most important factors when it comes to the health and durability of your rotating assembly and the block itself is the amount of open space between the main and rod journals on your crankshaft and the bearing shells they are spinning within





Problems can enter the picture pretty quickly when the bearing clearances aren’t correct



SOURCE
Royal Purple
One Royal Purple Lane
Porter
TX  77365
888-382-6300
http://www.royalpurple.com
King Engine Bearings
327 Little Falls road, Suite 5
Cedar Grove
NJ  07009
800-772-3670
http://www.kingbearings.com
Powerhouse Products
3402 Democrat Rd.
Memphis
TN  38118
800-872-7223
http://www.powerhouseproducts.co
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Eagle
8530 Aaron Lane
Southaven
MO  38671
662-796-7373
www.eaglerod.com