Tactics and Techniques - Measuring Head Spacing

In this short article I'm going to document a technique for measuring head spacing that Dad taught me. This is a quick method to determine where your motors are. This method takes into account the compressed shim thickness to give a truer measure in my opinion.

Knowing how your engines are set up helps with consistency between power plants in terms of needle settings and performance. Consistency is the goal as no one wants to deal with one motor that eats plugs and another that wants to be set off 1000 rpm rich.

Before I get started I should probably answer the following question. What is head spacing?

Head spacing is the distance between the top of the piston and the head. This distance controls the volume between the piston and head which in turn directly effects combustion. Too tight and the engine may fire too soon and cause serious problems. Dad’s initial work with the Irvines resulted in a broken wrist pin due to being too tight. The tuned exhausts common in today’s combat engine makes this more critical since a tuned exhaust effectively increases compression and therefore more head spacing is needed. The simplest way to determine if you’re “too tight” is by the plug. If you’re burning plugs or sucking the element out of your plug then it’s likely you need more head spacing.

If you are burning plugs I recommend putting a .005” shim in the engine and try it again. I’ve actually had RPM increases after putting a shim in an engine. Again, too tight can cause problems and reduced engine life. I’ve had plugs last for 3-4 contests once the engines were set up correctly.

This article is going to outline the method I use to get a number for head spacing. I’m the nerdy engineer type that enjoys numbers and comparisons and things like that. If you’re not interested in buying a dial caliper to get a number, just use the plug as your indicator. You should NOT be yanking elements or burning plugs on a consistent basis. If you are, you’re “too tight”. Okay, I think I’ve made my case, let’s get on with the measurement.

I purchased this dial caliper/depth micrometer combo from www.darrolcady.com. I had been looking for a dial caliper and depth mic for checking engines and Darrol’s was reasonably priced at $65. It also takes care of both depth, thickness measurements, and it’s digital so you aren’t playing math wizard trying to get a reading.  Here's a shot of one of my favorite toys.

The first step in the process is to determine distance from the top of the head to the piston.  I set my engines up in my test stand so the engine isn't moving around. 

Rotate the crank till the piston is at the bottom of the stroke (BDC).  Set your dial caliper on the top of the head and extend the slider through the glo-plug hole.  When you feel contact between the slider and top of the piston stop.  Rotate the crank counter clock wise until you reach top dead center (TDC) and continue on.  The caliper will indicate the distance from the top of the head to the piston at TDC.  I typically do this 4-5 times just to assure everything is repeatable.  Write this depth down on a sheet of scratch paper.

The next step is to measure the distance from the top of the head to the bottom of the head.  The image below illustrates how to get this measurement.  Again, measure this a couple times at different locations to assure your value is accurate.

Now, simply subtract the head distance from the distance of your first measurement.  Congratulations, you have measured your engine's head spacing. 

After determining, the current configuration I also measure the shim thicknesses.  I document these values in a journal for future reference.  I set my Irvines up between .028" and .035" depending on the shims I have available.  Each engine will be different but I believe .025" on a .25 with tuned exhaust is probably tight enough.  Any lower then this and you risk pulling elements or having overheating issues.

I hope this article has been informative for you.  This article can also be viewed in the TEAMseaholm forum.  If you have any questions, don't hesitate to drop me a line.

seaholm@teamseaholm.com

Fly hard, have fun!

 
A.J. Seaholm
Publshed: 03.10.2004

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