Measuring Head Clearance - The EASY and ACCURATE Way


By Marty Davis - 5/8/2015
http://1nitrorc.com


With some of the newer engines the manufacturers have decided that a domed piston is the "hot setup". That's fine, but it makes it difficult for us to accurately measure the head clearance.

Going back to the "old days" of working with automotive engine bearing clearances, a product called PlastiGage seemed to be the answer. It was for sure. This product is a round piece of plastic that is easily "squased" between two surfaces to show the clearance. The two sizes that you should have are the one in the Blue Strip (.004" to .009") and the one in the Yellow Strip (.009" to .020"). The clearance ranges for these two strips should handle all the engines that we run. You can go to your local full service automotive parts store and ask for Clevite PlastiGage MPY-1 and MPB-1. If you can't find it locally, Summit Racing has it and sells it on Ebay.
Here is a picture of the product:



I buy 2 different sizes in order to find out where the clearance is from the factory. Once I know that, I can easily change the clearance with shims or if I must, I turn some brass off of the top flange of the liner. It is difficult and hard to accurately deepen the projection on the head button. It is easier and more accurate to just turn some off of the top of the liner. For ready made shims for almost any engine, check with woodrcproducts.com or the original engine manufacturer online. Steve Wood has all sizes and are precision and available.

The way to use the PlastiGage product is to cut off a small piece that is just about the width of the bore (diameter of the piston). A little longer is OK since you can bend it a little and that way it will go all the way out to the edges. Put the head button back on the engine and rotate the piston over the top several times to squash the PlastiGage. Take off the head button and check the width of the widest part of the PlastiGage. The package that the strip of PlastiGage comes in has reference widths printed on the strip. You will easily be able to see the clearance.

Here is a picture of the squashed PlastiGage showing the clearance of this engine:



Now use the PlastiGage package to see the exact clearance:



Sometimes the PlastiGage sticks to the top of the piston and sometimes to the head button. Really doesn't matter as either are easy to read.

That's it, easy as can be and super accurate. I found the most recent .21 engine that I bought had .013" of squish clearance and I like to run .006". The change to .006" was amazing as I could pull MUCH bigger props after setting the clearance correctly.

Some other images that show the process:





Until Next Time......

Don't forget to consider one of the most important tools for the serious model boater. The Engine Analysis Software. Available at: http://1nitrorc.com