390xp squish.

[Gerbutt]

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

[spudulike]

0.5mm and make sure all the carbon is off before measuring!

[Sawchip]

12 hours ago, 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

 

Why did you take the rings off the piston ? 

[Gerbutt]

0.5mm and make sure all the carbon is off before measuring!

Thank you, Spud. Yes I’ve done that now but hadn’t on the first measurement!

[Gerbutt]

Why did you take the rings off the piston ? 

It makes it easier to take the cylinder on and off a few times to confirm measurements.

[Gerbutt]

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?

[peatff]

19 minutes ago, 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?

Probably the piston rocking without the rings on it ?

[Gerbutt]

Probably the piston rocking without the rings on it ?

After looking at other photos of cylinders I think it’s just the squish band is 3mm wide and then drops off.

[Gerbutt]

I’ve read about matching the transfers to the case using the gasket as a template. Is the bit marked in the photo the bit that needs blending in?

 

[bmp01]

9 hours ago, 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?

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 January 22, 2019 by bmp01
As per edit note.