Piston Deck Height Measuring and Its Importance in Head and Engine Design

    What is Piston Deck Height?? Also known as Piston Deck Clearance.


    How do we determine Piston Deck Height??


    Why is it important when designing performance items for an engine??


    Is there a benefit to having a smaller or larger Piston Deck Height??


    Can the Piston Deck Height be easily changed??

 
 
    Above are some of the questions I will try and answer in this technical article.
 
What is Piston Deck Height?? or Piston to Deck Clearance?
 
    The Piston Deck Height is the distance the piston EDGE is in relation to the top of the cylinder deck. NOTE: This
distance can be negative (the piston edge is recessed in the cylinder deck) or positive (the piston edge is sitting above the cylinder deck). Please remember... we are talking about the relationship of the EDGE of the piston not the center. Of course, if the piston is of flat-top design then the edge and the center will be the same.
 
 
How do we determine Piston Deck Height??

 

    1.  Remove the cylinder head.
 
    2.  Grab your Dial Indicator and Magnetic Base (very accurate), Flat-Blade Depth Micrometer (accurate), Dial Caliper(less accurate), or straight edge and feeler gauges (much less accurate) OR your Step Micrometer (needed for pistons with a positive deck height   .
 
    3.  Place piston at TRUE Top Dead Center (TDC) NOTE: Since every crank has dwell at TDC, determining TRUE TDC can be a bit tricky. Using a custom piston stop is the most accurate method for finding true TDC. Since some of you may not have a custom piston stop, finding true TDC can be determined using a dial indicator and a mag base. WHAT!! you don't have a spare mag base and dial indicator laying around??
 
Hmmmm... OK, Here is a fast way to determine true TDC without using a piston stop or dial indicator. This method, if done with finesse and time, will yield an accurate, or fairly accurate, measurement. This will require a very thin, but rigid, piece of material with a very fine point (if the piston is crowned).


  a. Rotate the crank to what you believe is true TDC.
 
  b. Now, take your rigid piece if material and place the sharp end in the cylinder to where it touches the piston. Be sure to hold this material in such a manner where it can be held in place without the backing of the piston.
 
  c. Now, while holding the material on the piston, very carefully rotate the crank (by hand) forward and backward while keeping your material STATIONARY. If at any point the piston raises the material, even just a little bit, then you were not at true TDC. So... with this in mind continue this method until the piston no longer displaces the rigid material.
Once, this occurs, you will be very close to true TDC.
 
OK.. Now, you have found true TDC.. you are half way there!
 
Now.. you know when you are at true TDC, so, all you have to do is measure the distance from the piston edge to the cylinder deck.
 
How this measurement is done will be based on the measuring equipment you have chosen to use.
 
Please keep in mind... when measuring the piston deck height on a crowned piston, it is nearly impossible to get to the actual edge of the piston. This is because your measuring device does have mass and will hit the piston crown before it hits the actual edge. The finer the measuring device.. the more accurate the measurement. So.. if you are using the butt end of a dial caliper on a crowned piston, please keep in mind that this butt end is very wide and flat and will surely hit the crown way before the edge. Since the dial caliper is the instrument most commonly available for this measurement, I have determined an approximate offset factor for measurements obtained with the dial caliper. Of course, the accuracy of this offset will be determined by the accuracy of the initial measurement given. So, PLEASE take your time in measuring.
 
For pistons with a positive piston deck height, a step micrometer seems to be the most accurate tool for this measurement. It is nearly impossible to accurately measure positive deck without a step micrometer.
 
WITH ALL THAT SAID...MATH, along with measuring tools, can be used effectively to help determine piston deck height. If you know the piston's crown height, then this can be used to aid in determining deck height. For example.. if you can determine how far above the deck the center of the crown is at true TDC, you can subtract the known crown drop from this number to determine your piston deck height.
 
Why is it important when designing performance items for an engine??

 
 
Piston Deck Height is a very important piece of information when determining engine parameters and designing combustion chambers.
 
Let's start with its role in determining engine parameters.
Many performance shops utilize software and / or math equations as an aid in determining engine parameters and upgrades. For example, it is important to know the position of the piston, in inches or millimeters, at different points along the stroke ie. exhaust opening, transfer opening etc.. This information can be calculated using software or by punching the numbers in the mathematical equation (software is much easier). TSR
www.tsrsoftware.com has a nice program for this calculation.
OK, you may be asking yourself, "What does the piston deck height have to do with the piston position?" Well, not much from an initial design point, but for the person porting your engine, it can be very important. For example... many people send their cylinders out to be ported. Once the shop gets the cylinders they have the pleasure of determining how to modify it in order to yield performance gains. Many performance shops determine what to change based on the port timing of the engine. Now, if they only have your cylinders, not the entire engine, how can they determine what your port timing actually is??? Well, you may tell yourself that they have seen that engine before and know what the porting arrangements are and your engine will be exactly the same. Well, if you are convinced this is true then, you have nothing to worry about. But if you are like me, and KNOW that this is not always true, then you should be concerned if the shop you are sending your cylinders to does not ask for a piston deck height measurement or a base gasket thickness. There are often variances in cylinder castings, piston heights, connecting rod lengths, and base gasket thicknesses. ALL these things effect port timing and piston deck height. Knowing these parameters will assure that the shop has the necessary engine measurements to do the best job they can in modifying your cylinders.
 
OK, How about combustion chamber design? How does piston deck height effect the design of the combustion chamber?
 
 
Piston deck height is a VERY important measurement to consider when designing a combustion chamber.
 
Piston deck height is a major player in determining the squish clearance of an engine. While it is beyond the scope of this article to discuss squish band design (maybe later), let's just say the squish action within a combustion chamber is very important in the combustion process and power making process.

Squish clearance is the distance from the edge of the piston, at TDC, to the outer edge of the combustion chamber's squish band. So, one can easily see how the piston deck height effects the squish clearance measurement.
 
So, you may ask yourself..." How can one design a proper combustion chamber without knowing the piston deck height??" Well.... the answer is simple..
ONE CAN'T!! Sure, they can get close. All I am stating is that they can get a lot closer if they have a list of engine measurements, like the piston deck height.
 
It has already been determined that there are many factors that effect the piston deck height.. cylinder casting, piston casting (or forging), con rod length, and cylinder base gasket to name a few. So, with this in mind, how can a head designer
properly modify your head, or design a new head, for your engine without knowing the piston deck height of your engine? THEY CAN'T!
 
You might be saying to yourself " So, what if I am off on my piston deck height measurement .008", how big of a deal can that be?"
 
Well... While I will not go into the difference .008" has on squish action within a combustion chamber, let's look at what .008" does to the compression ratio and volume of an engine.
 
Volume of a cylinder: PI * R^2 (radius = 1/2 bore) * H (Stroke)
 
So, let's take  the 800 Rotax twin engine with an 82mm bore and a 76mm stroke. We will convert the .008" to mm so the numbers in the equation coincide. .008" = .2mm
 
What effect will .008" have on the compression ratio?
 
Let's  do the math:
3.14 * (82mm/2)^2 * .2mm = 1.05cc of volume change. So, what does 1.05cc of volume increase or decrease on this particular engine? Well.. 1.05cc equates to a 0.4 change in un-trapped compression ratio. OK.. 0.4 change in un-trapped compression ratio will change a 12:1 engine to a 12.4:1 or a 11.6:1 engine. Well..may be that is not so bad. so, lets change the engine by .015" or .38mm
 
3.14 * (82mm/2)^2 * .38mm = 2.0cc 
of volume change. So, what does that do to the un-trapped compression ratio? Well it changes the un-trapped compression ratio on a 12:1 engine to 12.79:1 or to 11.21:1.
 
So, you can see that the compression ratio is effected but what is also effected is the squish action within the head. Squish action is important in determining power characteristics of an engine. The squish band acts as a cooling layer to help cool the end gases as they are being rapidly compressed. By keeping these gases below their combustible temperature, one can prevent undesired combustion of these end gases in the squish band area. If these end gases are allowed to combust before the oncoming, spark initiated, flame front chooses to combust them, then you have the receipt for detonation and engine damage.
The squish action also creates turbulence within the combustion chamber. This turbulence has a direct effect on the flame front speed so ,in actuality, it effects ignition timing.
 
OK, another measurement for the squish action is the Maximum Squish Velocity (MSV). In short, this is the max velocity of the end gases as they are be compressed. It is actually a lot more complicated than that, but I will leave it at that for now. It is measured in meters/sec (m/s).
 
Squish velocity has a very large effect on the heat release and rate of burning in a two stroke engine. Hence power output and engine reliability.
 
Software exists to give a close approximation of this velocity but.


Let's take our above examples and see how squish velocity is changed by a small variance in squish clearance.
 
The first example showed a change in squish clearance of .2mm. Using 2 stroke software this .2mm change in total squish clearance will increase the squish velocity in a 13.5:1 head by 7.4m/s if this .2mm is removed from the total squish clearance. If this .2mm is added to the total squish clearance, then the squish velocity is decreased by 5m/s.
 
The 2nd example, showed a squish clearance difference of .38mm
This .38m change in total squish clearance will change the squish velocity by 18m/s when this .38mm  is removed from the total squish clearance. If this .38mm is added to the total squish clearance, then the squish velocity is decreased by 8.4m/s.
 
NOTE: The above MSV calculations were taken from a specific head design. Since MSV has many determining factors, the changes in MSV could be much less or much greater than the ones listed above. The overall head design and cylinder port timing determines the magnitude of the MSV changes.
 
One can see that the relationship between adding and subtracting squish clearance is not linear and does have pronounced effects on squish action.
 
So, you can see how one needs to be careful when purchasing a new aftermarket head or modifying a stock head. Next time you are talking to a head maker or a shop that may be modifying your head, ask them what the piston deck height is for that engine. If they do not know, how can they design a head for that engine that will have acceptable squish velocity, squish clearance, and compression ratio? OR when speaking with these people, tell then that you have added an extra base gasket to your engine. Ask them how that will effect the "on the shelf" head they want to sell you. I have already shown you what a .008" and a .015" difference in deck height will do to compression ratio and maximum squish velocity.
 
So... the old adage of shaving off .015" of a stock head can get you into excessive squish velocities in a very big hurry. Excessive squish velocities can lead to piston breakage and severe engine damage.
 
 
Is there a benefit to having a smaller or larger Piston Deck Height??

 
There maybe some benefits to having a large or small piston deck height.
 
The one that comes to mind first is in the cooling effects of the engine. For example.. if the piston deck height is large in the cylinder, then there may be an argument for the end gases retaining more heat due to them being trapped in the cylinder vs. the head. One may argue that end gases trapped in the head portion of the squish band would be subject to the better cooling properties of the head. This would be a hard theory to prove, but it does have merit.
 
Can the Piston Deck Height be easily changed??
 

Yes, it can be easily changed.
 
Below are several methods of altering piston deck height, which, as I have shown, also alters MANY other operating factors.
 
1. Changing base gasket thickness
2. Decking cylinder base
3. Decking cylinder top
4. Changing piston
5. Changing crank
6. Changing rod length
7. Changing stroke
8. Altering piston crown

 
 
 

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