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390xp squish.


Gerbutt
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Hello all

 

I’ve got the cylinder off my 390xp and I thought I’d have a go at measuring the squish. I’ve taken the rings off, bolted the cylinder back down, then poked some solder through the plug hole and rocked the flywheel until it’s turned all the way round. I’ve tried this in a few different places and the smallest measurement I can find is 0.33mm without a gasket. I’ve measured the gasket and it’s 0.50mm. What would be the ideal squish for this saw? I’d rather have some gasket paper so I’ve ordered some flexoid paper in 0.15 & 0.25mm.

 

Thanks for any help.

 

Ger

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  On 21/01/2019 at 10:13, Gerbutt said:

 

Hello all

 

I’ve got the cylinder off my 390xp and I thought I’d have a go at measuring the squish. I’ve taken the rings off, bolted the cylinder back down, then poked some solder through the plug hole and rocked the flywheel until it’s turned all the way round. I’ve tried this in a few different places and the smallest measurement I can find is 0.33mm without a gasket. I’ve measured the gasket and it’s 0.50mm. What would be the ideal squish for this saw? I’d rather have some gasket paper so I’ve ordered some flexoid paper in 0.15 & 0.25mm.

 

Thanks for any help.

 

Ger

 

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Why did you take the rings off the piston ? 

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One thing I am noticing is that the squish band isn’t flat. It’s consistent around the cylinder but is smaller beside the cylinder wall and then gets wider, 0.36 to 0.6. Is the squish band normally 3mm wide and then drop off?

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  On 22/01/2019 at 08:46, Gerbutt said:

One thing I am noticing is that the squish band isn’t flat. It’s consistent around the cylinder but is smaller beside the cylinder wall and then gets wider, 0.36 to 0.6. Is the squish band normally 3mm wide and then drop off?

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Probably the piston rocking without the rings on it ?

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  On 22/01/2019 at 08:46, Gerbutt said:

One thing I am noticing is that the squish band isn’t flat. It’s consistent around the cylinder but is smaller beside the cylinder wall and then gets wider, 0.36 to 0.6. Is the squish band normally 3mm wide and then drop off?

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If I'm understanding you correctly that sounds pretty good to me. The idea of squish band is to squish the gas mixture towards the centre of the cylinder, to promote good turbulence and rapid combustion. 

What you don't want is "end gas" stuck down the end of a narrow passageway where the combustion struggles to get to, that would be a waste of energy and won't do much for performance. You will get this if the squish band is too deep (radially).

Edit: whether 3mm is the right number is one for the experienced tuners, I think I would punt for a slightly bigger number,   say 5 or 6mm, depends somewhat on bore size and the combustion chamber shape.

Edited by bmp01
As per edit note.
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