Calculating Head Squish Measurements - (REP)

[Aus_Rider_22]

Just wondering if anyone has had any work done by these guys?
Particularly cylinder head milling for higher compression.

Here's an extract

" milled for higher compression, the motor was built by REP (Racers Edge Performance) there was a spacer ring under the spark plug, which is included."

I am after just the head.

Also, if my head was milled for higher compression like stated above, would BP Ultimate suffice? It's a 98 octane premium fuel for high-performance engines. It's not an overly expensive "race gas" like VP etc, just BP's best premium fuel that they now have out the local BP.

Thanks for your help boys and girls

[Faded]

If you don’t mind donating a little time and being resourceful you can complete the steps necessary to know what is needed to set up your head, for your particular bike and your desired octane requirements. This is going to be more benificial then sending it to someone for a cookie cutter job on a "typical" bike.

First you’ll need to make a few measurements so that you can do the math and find out where you’re at, and eventually where you want to be. Production tolerances can leave quite a bit of untapped potential hiding in your engine especially in regards to the squish band. Taking the production tolerances out of the equation and fine tuning the squish and compression ratio(s) to suite your available fuel can make a difference.

If you choose to continue make sure to double check your measurements and once you’re confident be sure to record them.

I would first measure the squish clearance with the head on. To measure the squish clearance you need to first find yourself a roll of 0.125" diameter solder (look at an electronics store like Radio Shack). Next bend a small length of solder into and "L" shape with the leg being about as long as 1/2 the diameter of the bore. Insert the solder through the spark plug hole making sure that the end of the solder contacts the side of the cylinder bore, preferably the left or right side of the bore (as opposed to the front or rear of the bore which may give a false reading due to the piston rocking on the wrist pin). While holding the solder against the side of the bore slowly cycle the kick starter so that the piston rotates near TDC, crushes the solder and descends enough to get the solder out. Be careful to only cycle the piston once so that you don't pinch the solder multiple times which could skew measurements. Remove the solder and measure the crushed thickness. This will be your squish clearance! Accuracy does count so try to use a measuring tool that will read down to 0.001" (dial calipers).

Next I would remove the head to measure the squish band. The squish band has two physical dimensions, the height and the width. The height is the distance between the gasket surface of the head to the top of the squish band. Measure the height (actually it’s a recess) in the head and the head gasket thickness. Measuring the squish height and head gasket thickness in conjunction with your total squish measurement will help you determine what the deck clearance is (height from top of cylinder to top of piston, can be + or - or even 0). Total squish - (squish height + head gasket thickness) = deck clearance. This is only easily determined when working with flat topped pistons. Next I would measure the width of the squish band (the flat portion from the edge of combustion chamber to the opposing edge where it starts to recess into the combustion chamber). Be sure to record each measurement.

After these measurements I would then measure the volume of the head, otherwise known as "cc-ing" the head. Again, accuracy counts so being able to measure down to 0.1 cc is to your benefit. If you can beg, borrow or steal an accurate measuring device (like a burette, syringe, etc.) that will allow you to meter down to 0.1 cc and a small piece of Plexiglas with a hole in it then you can do it yourself. Yes, you will need to fully install a spark plug, turn the head upside down and seal the gasket area of the head to the Plexiglas with a small amount of grease. Use your measuring device to fill the head with the liquid of your choice (water works) until it's full and record the amount it took to fill the head.

This measurement will only be the volume of your head if you do not use a head gasket and run 0 deck clearance. If you have + or - deck clearance and/or a head gasket (and/or a domed piston!) this will have to be figured into the equation to come up with total head volume.

Since you have the head off I would take two more measurements on the cylinder. The first would be the distance between the top of the cylinder and the bottom of the exhaust valve (with the exhaust valve closed) and the second from the top of the cylinder to the top of the exhaust port (with the exhaust valve open).

Knowing your head volume along with the two exhaust port measurements you can figure your actual compression ratio and see how close you are to the manufacturers advertised compression ratio. This will also allow you to crunch numbers and figure compression ratios for different octane gas.

[Woody_393]

Wow, I'm not even sure I understood half of what you wrote... not that you weren't clear, I"m just not familiar with all these terms... You one smart cookie Faded.

[yz250guy]

Originally Posted by Aus_Rider_22

I am after just the head.

Wouldn't it be easier to keep looking for the head you lost?

[Faded]

Originally Posted by Woody_CRF

Wow, I'm not even sure I understood half of what you wrote... not that you weren't clear, I"m just not familiar with all these terms...

W, no worries; it's a two stroke thing.

[Faded]

Originally Posted by Aus_Rider_22

"...the motor was built by REP (Racers Edge Performance) there was a spacer ring under the spark plug, which is included..."

FWIW, the "spacer ring" is more than likely an indexing washer used to index the spark plug. If you want one you should be able to get them at an auto parts store; they're just copper washers that come in a few different thicknesses.

[AKColby]

Originally Posted by Faded

If you don’t mind donating a little time and being resourceful you can complete the steps necessary to know what is needed to set up your head, for your particular bike and your desired octane requirements. This is going to be more benificial then sending it to someone for a cookie cutter job on a "typical" bike.

First you’ll need to make a few measurements so that you can do the math and find out where you’re at, and eventually where you want to be. Production tolerances can leave quite a bit of untapped potential hiding in your engine especially in regards to the squish band. Taking the production tolerances out of the equation and fine tuning the squish and compression ratio(s) to suite your available fuel can make a difference.

If you choose to continue make sure to double check your measurements and once you’re confident be sure to record them.

I would first measure the squish clearance with the head on. To measure the squish clearance you need to first find yourself a roll of 0.125" diameter solder (look at an electronics store like Radio Shack). Next bend a small length of solder into and "L" shape with the leg being about as long as 1/2 the diameter of the bore. Insert the solder through the spark plug hole making sure that the end of the solder contacts the side of the cylinder bore, preferably the left or right side of the bore (as opposed to the front or rear of the bore which may give a false reading due to the piston rocking on the wrist pin). While holding the solder against the side of the bore slowly cycle the kick starter so that the piston rotates near TDC, crushes the solder and descends enough to get the solder out. Be careful to only cycle the piston once so that you don't pinch the solder multiple times which could skew measurements. Remove the solder and measure the crushed thickness. This will be your squish clearance! Accuracy does count so try to use a measuring tool that will read down to 0.001" (dial calipers).

Next I would remove the head to measure the squish band. The squish band has two physical dimensions, the height and the width. The height is the distance between the gasket surface of the head to the top of the squish band. Measure the height (actually it’s a recess) in the head and the head gasket thickness. Measuring the squish height and head gasket thickness will help you determine what the deck height is (height from top of cylinder to top of piston, can be + or - or even 0). Next I would measure the width of the squish band (the flat portion from the edge of combustion chamber to the opposing edge where it starts to recess into the combustion chamber). Be sure to record each measurement.

After these measurements I would then measure the volume of the head, otherwise known as "cc-ing" the head. Again, accuracy counts so being able to measure down to 0.1 cc is to your benefit. If you can beg, borrow or steal an accurate measuring device (like a burette, syringe, etc.) that will allow you to meter down to 0.1 cc and a small piece of Plexiglas with a hole in it then you can do it yourself. Yes, you will need to fully install a spark plug, turn the head upside down and seal the gasket area of the head to the Plexiglas with a small amount of grease. Use your measuring device to fill the head with the liquid of your choice (water works) until it's full and record the amount it took to fill the head.

This measurement will only be the volume of your head if you do not use a head gasket and run 0 deck height. If you have + or - deck height and/or a head gasket (and/or a domed piston!) this will have to be figured into the equation to come up with total head volume.

Since you have the head off I would take two more measurements on the cylinder. The first would be the distance between the top of the cylinder and the bottom of the exhaust valve (with the exhaust valve closed) and the second from the top of the cylinder to the top of the exhaust port (with the exhaust valve open).

Knowing your head volume along with the two exhaust port measurements you can figure your actual compression ratio and see how close you are to the manufacturers advertised compression ratio. This will also allow you to crunch numbers and figure compression ratios for different octane gas.

Faded,
That's some awesome information there. Are you a self tought mechanic?or did you go to school, your are VERY knowledgeable

[Aus_Rider_22]

What an awesome post Andrew! Thanks for the PM also.

I've actually understood and could follow what you've typed.
What equation are you talking about with the head volume? I am guessing I will be needing it as I run domed pistons with a diameter of 53.93mm.

[Faded]

Originally Posted by Aus_Rider_22

What equation are you talking about with the head volume? I am guessing I will be needing it as I run domed pistons with a diameter of 53.93mm.

Okay, really only one way of doing it then, and that is to pull the head off and find absolute TDC. Use some grease and seal the piston crown to the cylinder bore so that nothing can leak past to the rings. Put the head/gasket back on and cc the combustion chamber through the spark plug hole. This will give you your total combustion chamber volume without having to worry about head gasket thickness, deck height, ect. but is a little less accurate.

Once you have the combustion chamber volume and the exhaust port measurements you can figure out your compression ratio(s).

The manufacturer’s list two compression ratios, a high speed CR (one with the exhaust valve open) and a low speed CR (one with the exhaust valve closed). You can determine both compression ratios once you find out your head/combustion chamber volume and your two effective stroke measurements (one with the exhaust valve open, one with the exhaust valve closed). The formula for finding a compression ratio is:

CR = (V1 + V2) / V2

CR = Compression Ratio
V1 = Cylinder Volume
V2 = Combustion Chamber Volume


The difference between high and low speed CRs will be the V1 measurement due to the difference in effective stroke (the piston covering the exhaust port with the exhaust valve open vs. with it closed). The formula for finding V1 is here:

V1 = [ 3.1416 * D2 (squared) * ES ] / 4000

D = Bore Diameter
ES = Effective Stroke
and V2 You'll have to find out.

[Aus_Rider_22]

Andrew, you've outdone yourself!

Invaluable info in this thread!

I vote this to be put into the knowledge base forum. I could change the name to "Calculating Head Squish Measurements"

[Dirtbikindad393]

Originally Posted by Aus_Rider_22

Andrew, you've outdone yourself!

Invaluable info in this thread!

I vote this to be put into the knowledge base forum.

I second that motion!

[Florida 393]

Well I am impressed . I am moving this to the Knowledge base forum. This took me back to engineering physics classes and labs. I think I'll print it out for my personal use too.

Faded you are a valuable person for this site. Thanks for contribuiting so much

Bill

[Aus_Rider_22]

Thanks for that Bill

[Faded]

Thank you. This is for those that are interested in getting more out of there motor for not much dough. The manufacturing tolerances really start to stack up in production and the compression ratio and squish clearance that your bike ends up with can be a bit off from the design and published specs. By tightening things up it will run as designed and the end user (you) benefits.

[Woody_393]

I renamed this and put it in the two stroke section.

Edited.

Actually, I moved it back to the two stroke forum so I don't have to approve every post. When this has run its course, we'll move it then.

[Aus_Rider_22]

Thanks Jonathan

Hopefully, soon as I get my stuff together, I can follow Andrew's instructions and report back with photos

[E-man811]

Great post Andrew.

First of all, you know that I have a great deal of respect for you.

And, based on part of your post, I might offer a bit of feedback that you may want to add further clarification on. I dunno, we may just be using different language but the statement below seems off just a bit.

Originally Posted by Faded

......... Measuring the squish height and head gasket thickness will help you determine what the deck height is (height from top of cylinder to top of piston, can be + or - or even 0)........

First. Deck height is actually the measurement from the centerline of the crank to the top of the cylinder bore. (need special tools to get this exact measurement)

Second. The measurement of how far the piston is in the hole (below deck height) or out of the hole (above deck height is) is called deck clearance. Expressed as + or - and then the measurement or .00x in the hole or .00x out of the hole. Deck clearance can be determined a few ways. If you have flat top pistons you can use a deck bridge and a dial indicator, or you can add the stated stroke, stated rod length and the stated compression height of the piston and subtract this from the "known" (measured) deck height. The "Actual" deck height is what seems to vary the most in maufacturing. You'd usually do this with domed pistons. You'd still have to cc the dome to help determine the combustion chamber volume.

Third. Measuring the height of the squish band in the head (above the gasket surface of the head) and adding it to the gasket thickness does not tell you a thing about the deck height (relation of piston top to the top of the cylinder). It tells you part of the total Quench or squish as we commonly call it.

A more accurate statement would be "Measuring the head's squish band height and head gasket thickness will help you determine some of the total quench or squish."
or
Measuring from the centerline of the crank to the top of the cylinder will determine the deck height."

Which of these did you mean?


If you measure the height of the squish band in the head, add the measurement of the compressed head gasket and add the + or - deck clearance, now you have your total quench or squish.

Am I off hear at all?

Ride safe.

Ian

Edit: changed "stated" deck height to "measured"

[E-man811]

Oh, and I'll add this thought.

The tighter you start to make the compression in your engine, the more important it becomes to have the proper quench. There's a balance that the builders could tell you more about.

This is how a lower compression engine can sometimes be more prone to detonation than a high compression engine with the proper quench.

Ride safe.

Ian

[Faded]

Originally Posted by E-man811

First. Deck height is actually the measurement from the centerline of the crank to the top of the cylinder bore. (need special tools to get this exact measurement)

Second. The measurement of how far the piston is in the hole (below deck height) or out of the hole (above deck height is) is called deck clearance. Expressed as + or - and then the measurement or .00x in the hole or .00x out of the hole. Deck clearance can be determined a few ways. If you have flat top pistons you can use a deck bridge and a dial indicator, or you can add the stated stroke, stated rod length and the stated compression height of the piston. You'd usually do this with domed pistons. You'd still have to cc the dome to help determine the combustion chamber volume.

Third. Measuring the height of the squish band in the head (above the gasket surface of the head) and adding it to the gasket thickness does not tell you a thing about the deck height (relation of piston top to the top of the cylinder). It tells you part of the total Quench or squish as we commonly call it.

No arguments here, just a wording difference/mis-use on my part. I will edit my original post to clarify.

Originally Posted by E-man811

A more accurate statement would be "Measuring the head's squish band height and head gasket thickness will help you determine some of the total quench or squish."
or
Measuring from the centerline of the crank to the top of the cylinder will determine the deck height."

Which of these did you mean?

Explained below.

Originally Posted by E-man811

If you measure the height of the squish band in the head, add the measurement of the compressed head gasket and add the + or - deck clearance, now you have your total quench or squish.

Am I off hear at all?

You're not off at all. Since deck height/clearance is not easily determined by the average DIYer without the proper tools I've tried to supply an alternative method albiet a little more confusing at first.

If you measure total squish first (first step in my post) prior to pulling off the head then you have a number to work from. After pulling off the head you can measure the squish height in the head and the head gasket's compressed thickness. By adding these two numbers together and subtracting them from your total squish you can get a fairly good idea of deck clearance. Not the most precise way to do it, but close enough without having to buy costly equipment. I hope this clarifies things a bit.

[Florida 393]

Faded and Eman,
Thank you both for taking time to explain this topic so well. I hope y'all will finish it out with some fine tuning and answer a few member's questions. I am going to duplicate this in the KB soon so it is in here for all to use. You guys rock!

BTW, Faded, according to your profile you are 86. You look good in your profile picture for an old guy.

Bill