How to Measure Engine Clearances using Plastigage

What is plastigauge.

Hey Guys and welcome back to another video on the SimpleCarGuy Channel. If you have never heard of Plastigauge, you are not alone! Before I started rebuilding this engine, I have never used or even heard of this handy product. What Plastigauge does is measure the clearance between machined parts such as crankshaft and the crankshaft bearings or the crankshaft and the rod bearings. Since the clearance on modern engines is so small, it’s not like you can physically insert anything between the parts. All it really is a thin plastic thread that when crushed by two surfaces can accurately provide the clearance. Before you start, make sure you know the range that your will need for you application as there are many different scales. You can find the specifications for your engine in the workshop manual, technical documentation for the engine or TIS software for your brand of car.

Why use it.

Now that we know what it is, why do we need it? Well, modern engines are built on very, very tight tolerances and clearances that control oil flow to vital parts of the engine, such as the crankshaft. If your bearing clearance is looser than it should be, it will reduce flow resistance and lower the oil pressure and if it’s too tight, you are introducing access heat, which increases wear, which then increase a chance of a spun or welded bearing.

How to use it

Finally, how do you actually use it? Luckily, it is very easy to use but of course there are a couple of things to do before you start. First, clean all of the contacting surfaces so that the result is accurate. Second, make sure you know the correct torque specs for the bolts or screws. The reason that both of these are so important is that if you have debris between the two surfaces or don’t torque correctly, the results will not be accurate. Then, cut a small piece of the Plastigauge that’s about the width of your bearing journal. Place it on top and proceed to install the bedplate or rod cap. On most modern engines, you cannot re-use bolts on the crankshaft or other vital components; however, it IS ok to reuse the bolts for testing purposes when measuring clearances with plastigauge.

How to measure the clearance.

Once you have torqued you bolts to spec, you can release them and remove one of the parts to expose the crushed plastigauge. Now, you can just grab one piece of the packaging that has the scale on it, it can be imperial or metric, depending on how your specifications are provided and place it right on top of the line you see left by the plastigauge. Compare it to the scale to see which one is the closest and that’s the clearance between the two parts. If the clearance is what you are looking for, wipe the ruminants with some break cleaner, lubricate the bearings if needed and do the final assembly. That is all it takes to measure the clearance on crank bearings, rod bearings or any other parts that use this method. Thank you, guys, for watching and if this helped you understand how plastigauge works, hit that like button. If you’d like to see me rebuild a BMW N55 engine, check out the playlist in the description below. I’ll see ya in the next one!