Squish Clearance

Squish Band

After seeing a great volume of questions dealing with how to set the head clearance (Squish Clearance) and raise and lower the head if the engine does certain things, I decided to address this topic in this Tech Note. These questions appeared on the RcBoat.com DISCUSSION GROUP.

I remember when I started racing; I asked the question of some of the more experienced boaters. What do I do if I continually burn glow plugs out? What do I do if my engine heats up? Well....I got lots of help and MOST answers said to raise the head some, to eliminate both problems. WRONG WRONG WRONG !!!

If someone tells you to do this, and you make the change, they are doing the following to you. Your engine will make LESS horsepower and LESS torque! This leaves less torque and less horsepower available to power your model, and you will need to either lean the engine or run higher rpm to compensate. Both of these put tremendous stress on the engine!

Solution:

Set your "Head Clearance" correctly! (The clearance between the top of the piston at the top of its stroke, and the lowest projection of the head button – "squish band")

What is the correct clearance? How do you determine the correct clearance?

The closer that you can run the clearance the better! The percentage Nitro you run, the type boat you run, the size prop, etc, etc, etc. DOESN'T AFFECT THE SETTING OF THE HEAD CLEARANCE! The ONLY things that determine how close this clearance should be are: the amount of slop in the rod/crank pin, the amount of slop in the rod/wrist pin, the amount of play in the crankshaft bearings, and how tight the liner/piston fit is.

What does having the squish clearance "close" do? It makes the compressed charge between the squish band and the top of the piston (at the top of the stroke) thin, thus allowing for quicker heat transfer between the piston and the squish band. This clearance change will produce MORE HP, MORE TORQUE, and the engine will not have to work as hard. You will be able to run a richer needle setting, come off the turns faster, launch better, run cooler and run faster! This is probably one of the 3 biggest failings of the Novice Boater! Help them out with this information EARLY in their RC Boating.

What clearance is BEST:

For a modern .21 engine .006-.007", for a .45 .009-.011", for a .67 .010-.013, for a .80/.90 .012-.014".

The K&B .45 Outboard requires a little more because of the way the crank bearings are mounted .012-.014".

Examples:

#1 You have a new .45 boat that is burning the heck out of plugs & running HOT regardless of the prop you put on and the percentage of nitro you run. What can you do to help the plug burning & heat?

Set your squish band clearance to between .009" and .011", richen up the needle and you will make MORE HP and the engine will not have to work as hard, will run cooler, launch better, run faster. It is important what the compression ratio of the engine is, BUT setting the squish band clearance correctly will benefit the engine regardless of the compression ratio.

#2 You want to be able to maximize the performance of your model by setting the correct squish clearance, AND having the best compression ratio for your fuel, weight, etc. How can you accomplish that? Three ways! You can measure your engine using The Engine Analysis Software and it will tell you the correct settings. Or your can set the correct squish clearance, raise or lower the compression ratio by changing the size of the head cavity.

#3 What is the best way to accurately measure the squish clearance? I use a gauge that Bill McGraw sells at Rossi Sales of America. You can reach Bill at Speedmaster@internetMCI.com The gauge is not very expensive and very easy to use AND very accurate! I don't know how you would measure your squish clearance accurately without this measuring device.

#4 Are there any other reasons to have the squish band "close" to the top of the piston at the top of the stroke? Yes...The squish band creates a very high velocity charge moving from the edges toward the center of the combustion chamber. This further atomizes the fuel/air mixture, creates turbulence, and makes the mixture burn quicker. All these phenomenon are desirable to horsepower generation.

Marty Davis