Check Motor Bearing Clearances Without Breaking the Bank


When building or servicing your hot rod’s engine, checking your engine’s bearing clearances is a great practice while you’re “all the way”. Not only is it advisable to check the visible condition of your main and connecting rod bearings, but it’s also good practice to see how the tolerances hold.

The budget option, mostly known by its product name, Plastigage, is made from a special extruded wax plastic with precisely controlled crush properties. This fine plastic wire crushes with precision to repeatable dimensions according to the tolerances between the bearing and the crankshaft.

Reasonably priced tool

For high horsepower applications, typically defined at 800 horsepower and above, working with your low end bearing clearances suggests using very precise micrometers and dial bore gauges to ensure proper clearances. While these tools can be great for an engine builder, they are generally not considered “average” tools that every enthusiast has in their arsenal.

MAHLE Aftermarket Incorporated recently took over American production of Plastigage and Dan Begle of MAHLE gave his opinion on its practical applications with performance engines.

“Referring to your 800 horsepower fundamentals for Plastigage, that’s probably a good number,” says Begle. “I don’t know if the power levels are a huge issue; it depends on which engine builder or machine shop you trust. If you talk to someone who builds a massive number of performance engines, they probably use precision gauges.

With our engine stripped down to the short block, we visually check the condition of all connecting rod bearings and main bearings for wear or damage. With no visible issues, we can comfortably use Plastigage to confirm our bearing clearances without the use of micrometers and dial bore gauges.

Begle continues, “If you’re thinking of performing checks and balances on your engine, this is the one. It’s not as accurate as a dial bore gauge, but if you’re conservative and not building something completely crazy, you’re probably good.

For around ten dollars, the home engine builder can use Plastigage to get the job done, especially when combined with a proper visual inspection of your running surfaces. Currently, I have the big-block 489 Chevy from our Camaro race rack, and this is a great opportunity to carefully check out the engine that came with our unfamiliar turnkey purchase some time ago.

The Mahle/Clevite Performance Engine bearing catalog (number EB4016) is readily available online. It contains a wealth of information regarding bearing clearances as well as visual bearing inspection practices.

The typical rule of thumb for an OE or moderate performance application is 0.001 inch bearing clearance for every 1 inch of crankshaft or connecting rod journal diameter. With a Chevy big block using a 2.75 inch main journal diameter, our tolerance is calculated at 0.0027 inch. A similar calculation for the connecting rod journal indicates a necessary clearance of 0.002 inches.

These tolerances may vary depending on engine design, power levels, viscosity of oil to be used, and many other variables. In fact, I called the original engine builder, who advised that a stand-up race engine like this is built in their shop with slightly tighter clearances than the previously mentioned standard. Be sure to do your research before looking at your dimensions.

“I’m also a bit old school on the 0.00075 to 0.001 inch per inch newspaper rule set, but you also have to consider the viscosity of the oil you’re using,” Begle comments. “With light viscosity and/or synthetic oil, I would definitely lean towards the low end of that release spectrum. In the same consideration, if you are using a higher viscosity oil, definitely lean towards the higher side of your clearances.

The first step in checking the bottom end of your engine is to carefully inspect all bearings for any damage or uneven wear. Only then can you measure bearing tolerances. It is also essential to test with the Plastigage when the motor steel is at a minimum of 65-70 degrees for proper metal expansion. While extreme cold will make Plastigage somewhat brittle and heat will soften the material, Plastigage remains usable and accurate through all normal variations in temperature and air humidity.

Each pack of Plastigage comes with a 12-inch color-matched formulated piece of plastic. These pieces are usually called “threads” because they are equivalent in size to a piece of sewing thread in the uncompressed state. With a standard V8 engine, this will test all connecting rods and main bearings with some reserve. The supplement is handy because, if you ever drop a small section of this thread, in my experience, you’ll never find it again.

Plastigage is offered in three tolerance ranges for typical automotive use. Sizes range from green: 0.001 to 0.003 inch (0.025 to 0.076 mm), red: 0.002 to 0.006 inch (0.051 to 0.152 mm), and blue: . 004 to 0.009 inch (0.102 to 0.229 mm).

I first inspected each bearing for any signs of damage. If damage is evident, the tolerance measurement is a moot point until you have the crankshaft checked and possibly polished or machined for future use. If you are installing new bearings, using Plastigage at this point would be a good step when reassembling.

When I bought the Camaro, the salesman told me the engine hadn’t had more than 25 runs since new. This bearing inspection indicated that it was true with no indicators of damage or significant wear.

Careful tightening of each cap to specification with the Plastigage in place is necessary to achieve accurate thread crushing.

I started testing bearing tolerances one bearing cap at a time. Removing one rod cap at a time to install the Plastigage test seems like a reasonable precaution to avoid switching. Be sure not to place the test thread near any lube holes as the tolerances may differ around this area of ​​the crank journal.

Remove, tighten and repeat

With the bearing clearance recommended by the engine manufacturer, I used Green Plastigage. To gauge the results, use the printed ruler-like measurements marked on the yarn package. The larger the thread when compressed, the tighter the bearing clearance.

I use heavy white card stock to carefully cut each piece of yarn. With the Plastigage sections cut narrower than the width of the journal, we also use a narrower piece of card stock to place the tread on the face of the journal. This prevents accidentally crushing the wire if you try to pick it up and place it.

It is absolutely necessary to thoroughly clean the bearing surface and the face of the trunnion before the test. Plastigage is intended for use in a dry state. Any oil or residue on the surfaces may affect compression results. Once installed, tightened to specification, then loosened, carefully remove the caps straight outward from the face of the crankshaft, so as not to damage the compressed threads. Sometimes compressed Plastigage attaches to the crankshaft journal or bearing face.

If your engine uses a main sheath or internal oil pump, be sure to perform the proper installation, tightening, and removal process to ensure any changes in bearing clearance measurements. We thoroughly clean all bearings and journals with brake cleaner as Plastigage is intended for use on oil free surfaces.

Trust, but verify if necessary

With the bearing cap removed, I matched our compressed wire with the width ruler on the wire wrap. The thread has compressed to a little wider than the 0.002 inch indicator. This measurement puts the bearing clearance at just over 0.002 clearance or close to our target of 0.0020 to 0.0025 inches.

To evaluate the results of Plastigage, compare the crushed Plastigage to the wire wrap marks. The wider the thread is compressed, the tighter the bearing clearance. When you carefully remove the bearing caps after the crushing process, they may stick to the bearing or journal.

Ultimately, Plastigage is a “trust, but verify” device for me. Many engine builders heavily downplay the accuracy of plastic threading. On the other hand, many home builders swear by using it for the job, like the ones I performed on our support engine.

I recorded each of our individual rolling clearances for our Camaro logbook. It’s handy for future reference, and you can also spot trends like a bent crankshaft. If the clearances change gradually towards the center or at each end, it is wise to have the issues checked by a trusted crankshaft shop.

With friends watching my Plastigage work in my own workshop, a buddy asked me if coated bearings played a role with clearances. I forwarded the question to Begle. He replied, “Clearance is clearance, regardless of the type of bearing. While coated bearings in a performance application are an entirely different topic, I would still set up my games the same, coated or uncoated.

If you have a knack for tearing down, building, and rebuilding an engine in your shop, Plastigage is a great tool to help you check for “red flags” along the way. If the bearings don’t pass a visual inspection or you find they’ve fallen out of tolerance, it’s time to visit your favorite engine machine shop.


About Author

Comments are closed.