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What Is A Spun Bearing?

Suspecting you’ve spun a bearing in your engine is a gut-wrenching feeling. The worst part of this type of failure is that it’s not exclusive to any one type of engine. It can happen in a completely stock engine, a race engine, a gas engine, or a diesel engine. The occurrence of a spun bearing doesn’t mean that the problem is isolated, either. More often times than not, a spun bearing seizes itself around its respective crankshaft journal. When you hear the term spun bearing, it’s typically referring to two types of failures: main and rod bearings. A spun connecting rod bearing tends to be the most common. A spun rod bearing will ruin the big end bore of the rod, damage the rod journal on the crankshaft and, occasionally, ruin the connecting rod itself.

Crankshaft Bearings

As you likely already know, a broken rod being slung around inside an engine can and will usually cause some collateral damage. On the other hand, a spun main bearing will typically tear up the main bore in the block but usually won’t cause the crankshaft to fail. Fortunately, with a main bearing failure the damage shouldn’t go any further than the main journal surface on the crank and the main bore housing. A spun cam bearing will damage the bearing bore in the block (or head in the instance of an overhead cam arrangement), and maybe snap a timing belt or chain (if so equipped), which could then lead to one (or numerous) bent valves if it’s an interference engine. If you’re really unlucky, the cam bearing seizure could even snap the cam.

What Exactly Is A Spun Bearing?

This section of the article is for those of you who couldn’t contextualize the above paragraph, or for you unlucky few who’ve just been told your engine spun a bearing and you’re looking for answers. Let us paint a quick picture for you…

Each bearing insert (C shaped halves) have a tab on the outer edge at one end which fits into detents that are designed to prevent them from turning. When a bearing insert wears down enough to allow the tab to jump out of its detent, it will spin over the adjacent bearing insert and become stacked, wedged, and/or seized. This means the flywheel will force the crankshaft to turn and the stacked, seized bearing will eventually come loose. Occasionally, the bearing will spin and seize when you’re trying to start the engine, yet the starter will be unable to power through the drag. Many spun rod inserts cause catastrophic damage—as in the block and crankshaft are damaged beyond repair.


A spun bearing is usually the product of other faults. These tend to be high operating loads, excessive heat, and lack of lubrication. Spun bearings typically result from a loss of oil pressure. However, there are many contributing factors that will eventually cause the bearing to seize up and spin.

Diesel Crankshaft Diagram

Normally, bearings allow the crank to glide on a hydrodynamic film of oil between the bearing surface and journal. So long as there is consistent flow of clean oil making its way into the gap between the bearing and journal, everything should be fine. The film only needs to be a few microns thick in order to be adequate enough to provide sufficient protection. However, if there is a sudden drop in oil pressure and flow at high rpm, that protective film of oil can vanish instantly. Once that happens, you get metal-on-metal contact between the bearing and journal surfaces, which leads to to scuffing and an increase in friction and heat.

Again, this problem is not exclusive to any sort of engine or bearing, be it aluminum or a Babbitt – facd tri – metal, because the fact of the matter is all types of bearings will begin to melt when reaching temperatures approaching 450° F. If there is a good amount of scuffing, friction and heat present from a lack of lubricant, the bearing will melt, smear and stick to the crank journal. Yes, that means the dreaded spun bearing along with whatever else it decides to damage. But all hope may not be lost, as a Babbitt overlay on a tri-metal bearing will provide a good amount of seizure resistance.

It’s most common for a spun bearing to result from an ongoing lubrication issue, which ultimately caused a bearing to scuff, run hot, and overheat. This can discolor a crank journal and blacken the big end of the rod. So, on the occasion you’re delving into an engine and see signs of overheating, it’s a good indication that the engine has an oiling problem, which needs to be identified and addressed as soon as possible.

Here are some factors that can contribute to oil pressure and lubrication problems:

  • Low oil level in the oil pan due to leaks, maintenance neglect and/or improper filling.
  • Oil pump cavitation and aeration at high rpm’s due to the design of the pump, restrictions in the pump inlet tube or pickup, or oil sloshing away from the pickup in the bottom of the pan due to extreme cornering, acceleration, or braking forces.
  • Blocked or partially blocked oil galley or oil filter.
  • Improperly installed bearings (oil holes do not line up with the gallery holes). This is important to note with cam bearings, which can easily be misaligned.
  • Bearing clearances are too tight for the oil viscosity being used. Later model engines like the Chevy LS or Ford modular V-8 (with tighter main and rod bearing clearances of .0015˝ to .002˝) will typically require a thin, multi-viscosity motor oil such as 5W-20. Fill the crankcase with a relatively thick racing oil and you’ll have problems right from the get-go. Tighter bearing clearances require thinner oils while looser bearing clearances (of say, .003˝ to .004˝) in older engines require thicker oils (20W-50 or 50W) to savor the protective oil film between the bearings and their journals.
  • Misalignment of the bearing shell and crank journal.
  • Mistakes made when machining housing bores or crank journals (incorrect diameters).
  • Cold starts, especially in tight tolerance racing engines that are running thicker oils.
  • Very high bearing-to-crank loads due to lugging, detonation, or high boost or nitrous.

Crush Fit

Diesel Engine Kit - Main Bearing 6 MCIf you’re looking for something to help hold the bearings tightly in position to avoid the hassle of a spun bearing, the answer is crush fit (or press fit for cam bearings). Crush fit is highly beneficial thanks to its ability to promote better heat transfer between the bearing and its housing, thereby deterring the bearings from overheating.

The amount of crush fit you may need does varyvaries depending on the application , but it’s generally within the .002” to .004” totalrange. According to one one major bearing supplier of bearings, around .003 003” will suffice for the majority of vehiclesengines.

The actual amount of crush fit that comes with a bearing applied by the manufacturer is usually based on their engineering analysis of the respective vehicle, along with the OEM main bore, connecting rod, and cam bearing specifications. The bearing shells are then designed to stand proud above the parting line. When the main cap, rod cap, or cam bearing is tightened down, the bearing compresses slightly in order to hold it securely in place. The small locating lugs on the bearing exist only for alignment purposes—they do not play a role in holding the bearing securely within its housing.

Diesel Engine Seal - Crankshaft Conversion 2 MC

It can be tricky to check crush fit, and most bearing manufacturers claim it can’t accurately be measured on a work bench, or in the field. When you place the half shell of a bearing in a main bore, main cap, connecting rod, or rod cap, both ends of the bearing will be a small bit proud than the parting line of the housing cap where they’re situated. Utilizing a feeler gauge to measure how far the ends of the bearing are beyond the surface won’t actually give you a very accurate reading of crush fit. This is due to the fact that the bearing has a certain amount of built-in spread that will warp as the bearing is tightened in its housing.

One way to accurately determine crush fit is by the bearing manufacturer using a block gauge to measure the bearing, post installment, with a known size the cap has been torqued down to. If crush fit proves to be correct, the bearing will be held tightly in its place without too much warp causing any sort of clearance problem.

As you may have guessed, due to the number of small measurements and tricky determination of the correct amount of crush fit, it has to be fairly precise. Too much crush fit could warp the bearing to such an extent that it grinds against its journal. On the other hand, too little crush fit means the bearing will be considered loose and could shift back and forth within its housing. This could then increase the risk of it spinning or seizing. Not enough crush fit could also cause the bearing to run hot due to inadequate heat transfer between the back of the bearing and its housing. Over time, this could cause the bearing to degrade and eventually fail.

Another factor to worry about when it comes to crush fit is thermal expansion. Aluminum blocks and connecting rods expand more so than their cast-iron counterparts. For example, an aluminum housing bore with a diameter of 2.25” will expand by roughly 0.003” over a 200° F temperature change, whereas steel will only increase by approximately 0.0015”. Consequently, if a bearing falls short of enough crush fit, the bearing could come loose when the engine gets hot, once again increasing the risk of spinning.

Here are some causes of insufficient crush fit:

  • The use of incorrectly sized or smaller OD bearings than the housing.
  • Insufficient or incorrect tightening of the main bearing cap bolts or rod bolts during the installation phase.
  • Rod bolt stretch – especially at higher rpm in a race engine.
  • Burred, dirty, or damaged rod or main caps that prevent correct assembly and loading of the bearing.
  • Housing bore is not within specifications (too large to hold the bearing shells securely).

Reducing The Risk

Crankshaft Single

If an engine has succumbed to a spun rod or main bearing, a resurfacing of the crank journal will be required, with the possibility of having to re-match the main bore or big end rod bore (or replacing the rod), depending on the amount of damage inflicted. Note that this is only if the engine can be rebuilt. The replacement bearings will usually have to be correctly sized to make up for any changes in the diameter of the bore in order for them to fit properly when they’re installed. It is also advisable to get new main and/or rod bolts. Never reuse torque to yield bolts, as they actually stretch when initially tightened, meaning you run the risk of breaking them if you attempt to reuse them.

It’s a good idea to follow some basic engine assembly rules. For instance, pre-lubing the front of the bearings with assembly lube or oil and assuring the backs of the bearing shells are clean and free of moisture.

If you’re building a race engine, use performance bearings. Race bearings consist of tougher materials that will be able to endure higher loads, and they have more eccentricity to aid the oil wedge that provides hydrodynamic lubrication—especially at higher engine speeds.

It is also wise to make sure your engine has no oiling issues and can maintain a sufficient amount of oil pressure. Just to be safe, check the oil pump for wear and tear, damage, or excessive clearances and replace if necessary. Make sure the oil pump pickup tube is securely in place and that the pick-up screen is unobstructed and placed at the correct height above the bottom of the oil pan. If you’re building a race engine, always ensure the oil pump can provide the oil flow required by the engine at high rpm. Make sure the oil pan is filled with the proper amount (and viscosity) of oil before you start the engine. Also, make sure the engine is seeing sufficient oil pressure once it’s up and running.

Coated bearings

There seem to be varying opinions as to whether coated engine bearings can help prevent or reduce the chances of spinning a bearing. Anti-friction coatings will indeed help stop any metal-to-metal contact in engines that are subject to frequent stop/start cycles. Small displacement engines are also known to be pretty tough on bearings, hence why coated bearings are often utilized in these smaller (bearing) applications. Some suppliers who offer their customers special anti-friction and anti-wear coatings say that bearing life can be extended considerably with the use of them.

Diesel Engine Kit - Main Bearing 7 MCHowever, iIn a performance motorengine, a coated bearing isn’t really always a necessity necessity, but but it may add some additional protection against dry starts starts, along with metal-to-metal contact in case of a a sudden drop in oil pressure during a race or what not. In that instance, the coating would act as a temporary replacement for the oil film that is no longer there. Acting as a martyr, it will hopefullycan prevent any scuffing, seizing seizing, and spinning of the bearing, . how How long this will be effective effective nobody knowsis anyone’s guess, so so it’s best to avoid it. altogether.

During all-out racing conditions, a single bearing coating alone is not to be considered a reliable substitute for oil and will eventually succumb to the conditions it’s placed under. It could only take a couple seconds or a couple minutes, but eventually it will cease to provide you with any protection. However, if the driver knows of the problem (for instance, a blown oil line spraying oil and a zero oil pressure reading on the oil gauge), he or she may act with haste and cut off the engine before any real damage can be done. In such a situation, the extra bit of cash spent on a set of coated bearings would be a small price to pay as opposed to a highly damaged engine.

What happens if this occurs while you’re driving?

An old story floating around online has it that a seized bearing in a man’s classic ’52 pickup caused a slight drop in power followed by a rapid climb in coolant temp. The man described the experience as being pretty scary due to the fact that one seized bearing could become so hot it caused engine coolant temp to climb. Upon inspection, he also found molten bits of bearing material in the oil pan. Luckily, the engine was only running in this condition for a short period of time and wasn’t damaged.

Another man says his experience with a spun bearing resulted in an immediate rod knock, followed by a rapid decrease in oil pressure. An engine is often on borrowed time after this type of failure, and it’s usually less than 50 miles before it throws the rod.

Is anything salvageable after this happens?

  • Unless it’s a main bearing that has spun, the block is usually salvageable. The rods that did not spin are also salvageable. Any rods that did spin are to be discarded.
  • All bearings: rods, mains, cam are goners. The process leading up to the bearing welding itself to the crank sent metal filings downstream into the engine oil.
  • The crank is also now trash. You will usually see a rainbow discoloration around the area of the spun bearing—that's a specific type of heat treatment that means the metallurgical structure of the crank is now weakened.

If you’re wanting to fix a spun bearing issue, it’s best to have a knowledgeable builder perform the work unless you’re well-versed in engine assembly.

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