discussion of head button squish angle

[LohringMiller]

We did the same experiments with Quickdraws. The piston just touched the head at .002" (.05 mm) clearance.

Lohring Miller

 

[ids987]

Thanks guys. I forgot we were talking about a 125cc motor

 

[Terry Keeley]

Calculate the area of the complete bore (pi x 1/2 bore squared) and subtract the area of the bowl to get the squish area. Divide the squish area into the total area to get the ratio. You can work the numbers backwards or just mess around until you figure out how wide to make the band to get a certain ratio (%).

You can use an online calculator to get the volume of the button:

http://keisan.casio.com/has10/SpecExec.cgi?id=system/2006/1223382199

To check it all you should have a syringe and some sort of plate, I used to get them at the drug store but got tired of the questions so I got a glass insulin needle off FleeBay. The plate is plexiglass with two small holes drilled in it.

Don't forget to include the volume of the squish and any dish in the piston.

The corner between the bowl and squish band should be sharp, not rounded or blended in.

terry,are you getting the bowl cc's first and then doing the squish? and when you do the squish are you working from the outside in or visa versa with the cutting tool?

You measure the bowl cc's then calculate the volume between the squish band and head ATDC.

Volume of a cylinder is pi x radius squared x ht. So pi x .5 bore squared x squish ht. = volume of squish area. Work in cm to get cc's then add that to the bowl volume. Use the total head volume for your C/R calculations, I'm sure Marty's EAP s/w works it all out but you can easily do it "old skool".

The squish volume is really an unknown tho at operating temps since it closes down as the piston heats up. I run about 0.012" in my 21's and 45's, much less and I start to see that band around the piston telling me it's hitting. My 90's need another couple thou...

 

[mark ham]

so what is better, flat top piston or domed?

 

[Chris Wood]

The corner between the bowl and squish band should be sharp, not rounded or blended in.

Terry

Curious why u like the corner sharp ?

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[dwilfong]

The corner between the bowl and squish band should be sharp, not rounded or blended in.
Terry

Curious why u like the corner sharp ?

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For the same reason you want the back of your boat sharp. It helps keep the stream running fast in to the chamber and not get turbulence. you want the coalition to happen in the middle of the chamber at the ignition source. Then the swirl will carry it to the outer rim. This is the theory why a toroidal chamber works so good it extenuates this swirling effect.

 

[LohringMiller]

Below is an article to give a better understanding of what really happens with squish.

Lohring Miller

Squish.pdf

 

[Michael Costanzo]

The corner between the bowl and squish band should be sharp, not rounded or blended in.
Terry

Curious why u like the corner sharp ?

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For the same reason you want the back of your boat sharp. It helps keep the stream running fast in to the chamber and not get turbulence. you want the coalition to happen in the middle of the chamber at the ignition source. Then the swirl will carry it to the outer rim. This is the theory why a toroidal chamber works so good it extenuates this swirling effect.

this makes perfect sense. now to the question of why do some people like 3* angle as opposed to flat on the squish? and to Terry's answer,he likes 65% angled and then flat,what does that do to the flame travel?

 

[Chris Wood]

i have heard that having sharp corner can cause detonation?

 

[dave roach]

The sharp edge at the bowl will cause to hold more heat and causing detonation.I like to put about .020 radi on that sharp edge.

Dave

 

[Terry Keeley]

The corner between the bowl and squish band should be sharp, not rounded or blended in.
Terry

Curious why u like the corner sharp ?

Sent from my iPhone using Tapatalk

It's been a while but IIRC the sharp corner increases turbulence which is good for combustion.

 

[Chris Wood]

The sharp edge at the bowl will cause to hold more heat and causing detonation.I like to put about .020 radi on that sharp edge.

Dave

That is what I have heard and what we do. 15 different ways to get it done. Opinions vary like most stuff I guess. I have learned along along time ago what works for me don’t always for others.

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[Timbo Whalen]

The sharp edge at the bowl will cause to hold more heat and causing detonation.I like to put about .020 radi on that sharp edge.

Dave

That is what I have heard and what we do. 15 different ways to get it done. Opinions vary like most stuff I guess. I have learned along along time ago what works for me don’t always for others.

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Amen that, Chris!

 

[David Ashcroft]

I have always done that using a sharp razor taking of the edge.

 

[Jim Allen]

OK,first off NOT A PISSING MATCH! everyone has there own opinion and many ways to skin a cat.

i would like to know what the effect of having a flat squish band VS a tapered squish band one. i have a head that has way to much piston to head clearance and have to mill it down which will also lower my vol#'s which is what i am looking for. thanks,mike.

Mike,

The angle of the squish band should be determined by the shape of the piston's crown. Flat top pistons should use a straight squish band of 1* to 2* according to the bore's size. A bigger bore needs less angle & a smaller bore needs more angle. One thing that can effect how well the squish band is working is where the incoming charge is being directed. Another thing is the amount of deck clearance should be held to the minimum, which is the mechanical limit of the engine (piston just touching) at WOT. Pistons with a dome also use straight squish bands. The angle is determined by the height of the dome. In this case the curvature of the piston's dome can cause the incoming charge to attach itself to the dome, helping to cool the piston's crown from the combustion side. This may not be necessary in a nitro engine because of it's much lower operating temperature compared to a gas engine. In both cases, the squish band's total area should be approximately 50% of the bore's total area. As previously stated a very sharp edge should be left at the intersection of the squish band & whatever bowl shape is used.

Jim Allen

 

[Michael Costanzo]

OK,first off NOT A PISSING MATCH! everyone has there own opinion and many ways to skin a cat.

i would like to know what the effect of having a flat squish band VS a tapered squish band one. i have a head that has way to much piston to head clearance and have to mill it down which will also lower my vol#'s which is what i am looking for. thanks,mike.

Mike,

The angle of the squish band should be determined by the shape of the piston's crown. Flat top pistons should use a straight squish band of 1* to 2* according to the bore's size. A bigger bore needs less angle & a smaller bore needs more angle. One thing that can effect how well the squish band is working is where the incoming charge is being directed. Another thing is the amount of deck clearance should be held to the minimum, which is the mechanical limit of the engine (piston just touching) at WOT. Pistons with a dome also use straight squish bands. The angle is determined by the height of the dome. In this case the curvature of the piston's dome can cause the incoming charge to attach itself to the dome, helping to cool the piston's crown from the combustion side. This may not be necessary in a nitro engine because of it's much lower operating temperature compared to a gas engine. In both cases, the squish band's total area should be approximately 50% of the bore's total area. As previously stated a very sharp edge should be left at the intersection of the squish band & whatever bowl shape is used.

Jim Allen

thanks jim for the explanation. i have to ask while we are talking about heads.. i just noticed that on this OS Max VRM .81, the head button from the factory swims around the bore of the sleeve?? there is space between the side of the button and the sleeve?? when the bolts go thru the head and into the case,everything lines up.. i have never seen anything like this..any motor i have worked on the head button fits tight in the bore. what is the story here?? thanks,mike.

 

[Michael Costanzo]

BUMP ^^^^ for the question in the above post ^^^

 

[bermudadude]

"thanks jim for the explanation. i have to ask while we are talking about heads.. i just noticed that on this OS Max VRM .81, the head button from the factory swims around the bore of the sleeve?? there is space between the side of the button and the sleeve?? when the bolts go thru the head and into the case,everything lines up.. i have never seen anything like this..any motor i have worked on the head button fits tight in the bore. what is the story here?? thanks,mike."

simple- u got a bad one. i have os81/91's they all tight. also ive never seen a loose one on any engine of the dozens n dozens of all brands of marine racing nitro that i have

 

[Danny King]

Or maybe its a .67 button.

 

[Jim Allen]

When we build high performance nitro racing engines there are different diameter head buttons machined in .0002" increments. The reason is because we do not attempt to hone cylinders to any exact size, only to an exact taper & very round. A loose fitting head button (.0005" +) will cause a nitro engine to go into detonation when running at WOT. This would also apply to a gas engine that has the head protruding down into the cylinder.

Jim Allen