Plain Bearings Are Anything But

Words by Richard Kratz photos courtesy MAHLE Clevite

Whether an engine was built at an OE factory for duty in a daily driver or it was built for a high performance or race car by a specialty engine builder, the internal parts of the engine are basically all the same. You have pistons, connecting rods, a crankshaft, etc. If you’re building a high performance or racing engine for yourself or having one built you put time and attention into researching your connecting rods, pistons and your crankshaft. There are a lot of designs, materials and brands to consider. You know that your high output engine is going to put incredible forces on these rotating parts and you want the best parts for your money.

We’re no different than you. When we began planning for the new engine build by Arrington Performance for our project Maulin’ Magnum street/strip car, we hit the ‘Net
and starting searching.
We talked to various
suppliers at trade
shows like SEMA. We
talked to various racers
and engine builders like
NHRA Stock racing legend
Bob Lambeck. We ran our
finalist choices past Arrington
for their input and then
choose our crankshaft,
connecting rods and pistons.

We decided to upgrade our
new 6.4L Magnuson blown motor
to Arrington’s billet steel main caps.
We opted for MAHLE’s 2618 aluminum
alloy pistons instead of 4032 for the same reason we upgraded the main caps—to retain options for pushing the engine to more radical output at some point in the future. But we have to be honest; it wasn’t until late in the game that we put some thought into the main and rod bearings. Yet if you think about it, the plain bearings in your engine have to be just as tough and durable as any of your other parts, after all you engine’s bottom end is only as strong as its weakest link.

The crankshaft in our engine will spin up to 6,600 RPM. At that speed without lubrication the crankshaft would weld itself to the engine and main caps very quickly due to friction generated heat. That’s what engine oil is for, to create a film acting as a barrier between the crankshaft and the engine block/main cap and connecting rods. This film not only prevents metal on metal contact, but it also absorbs and carries away heat created by the friction forces involved.

But if that’s what engine oil does, why do we need bearings? Why can’t we just leave a little clearance between the rods, the engine block and caps and the crankshaft?

Good question and the answer is in the materials. A crankshaft is optimized to withstand the tremendous twisting forces imparted to it by the connecting rods as well as resonating harmonics and vibration. The steel in the crank is very good at its job. Likewise for the connecting rods, these are optimized for the extreme compression forces imparted to them during the power stroke and the tensile forces trying to pull the rod apart when it reverses direction after top dead center. The engine block and main caps utilize materials that allow them to resist distortion and forces acting upon them in all three planes.