This is how I measured my 200Ts squish clearance

[bmp01]

I thought ( wrongly perhaps ) that the squish band was just the area around the periphery ?

 

It should be, it's a machined band, cut at the same time as the boring opertion.

Disappointing to hear the combustion chamber of MattG's 200T has lumps in it, presumably in board of the machined squish. Time for Dremel work?

 

Equally disappointing the squish is in the 1.0 mm range, poor effort.

 

bmp01

[MattG]

Well yeah, it is. It's the outer annulus of the combustion chamber......However the way that lots of cylinders are manufactured is that the bosses from which the casting are left and protrude into the depth of the chamber by up to 0.15mm which perturbs the squish (you can see it on the very outermost part of cylinder combustion chamber). You wont see this typically - unless you polish the roof of the dome prior to measurement.

 

A higher quality of manufacture would machine away this detail leaving a consistent band.

[Stubby]

Well yeah, it is. It's the outer annulus of the combustion chamber......However the way that lots of cylinders are manufactured is that the bosses from which the casting are left and protrude into the depth of the chamber by up to 0.15mm which perturbs the squish (you can see it on the very outermost part of cylinder combustion chamber). You wont see this typically - unless you polish the roof of the dome prior to measurement.

 

A higher quality of manufacture would machine away this detail leaving a consistent band.

 

And then it would cost more . Another CNC machining opp and tooling to hold it .

[openspaceman]

 

How about shooting for 0.45mm (18 thou) as a starting point for your mods

 

bmp01

 

Seems a bit tight. I guess you can get away with a tighter clearance on a 2t where each time the piston comes up it is under compression but on a 4 stroke with the piston flying up on the exhaust stroke all the bearings and conrod will be under tension as they slow the piston to tdc.

[MattG]

It should be, it's a machined band, cut at the same time as the boring opertion.

Disappointing to hear the combustion chamber of MattG's 200T has lumps in it, presumably in board of the machined squish. Time for Dremel work?

 

Equally disappointing the squish is in the 1.0 mm range, poor effort.

 

bmp01

Dremelling it would be nigh on impossible I think to get it right. A pro tuner would lathe it away I guess.

 

I'm not that perfectionist....and of course after losing the stock base gasket and then shimming out, with a custom paper/tin-can gasket, we probably take me to about 18 thou under the molding flaws and 26 in the regular part of the band.

 

But yeah, you're right bmp01, 1.0mm for a STIHL oem cyl is rubbish, and I've expected them to machine away the molding bosses.......

 

I'm glad I cleaned the dome first now, else I'd have never known.

[bmp01]

And then it would cost more . Another CNC machining opp and tooling to hold it .

 

Tiny bit more. But can be done at the same time as the existing squish and cylinder bore machining. No reason why the CNC cutter path has to machine the squish flat either.

 

bmp01

[bmp01]

Seems a bit tight. I guess you can get away with a tighter clearance on a 2t where each time the piston comes up it is under compression but on a 4 stroke with the piston flying up on the exhaust stroke all the bearings and conrod will be under tension as they slow the piston to tdc.

 

Agree on all counts. MS200 has a nice little connecting rod - not much stretch in the two bearing bores, unlike say a 1 1/2" or 2" (38 to 50mm) bigend ...

 

bmp01

[bmp01]

Dremelling it would be nigh on impossible I think to get it right. A pro tuner would lathe it away I guess.

 

I'm not that perfectionist....and of course after losing the stock base gasket and then shimming out, with a custom paper/tin-can gasket, we probably take me to about 18 thou under the molding flaws and 26 in the regular part of the band.

 

But yeah, you're right bmp01, 1.0mm for a STIHL oem cyl is rubbish, and I've expected them to machine away the molding bosses.......

 

I'm glad I cleaned the dome first now, else I'd have never known.

 

Yeah, Dremel isnt a precision tool, i was assuming you'd blend to surrounding to create clearance not an exact squish number.

Have a look at Mastermind's squish machining technique on Arboristsite. He uses a single profiled cutter mounted on a sustitute piston that closely matches the bore size. .... hand operation IIRC. Or for the material you need to remove - sandpaper mounted on the end face of a piston / sustitute piston (end face or piston not flat).

 

By the way, are the 4 lumps lower than the squish (that really would be carp) or is the piston domed?

 

bmp01

Edited May 2, 2017 by bmp01
better choice of words.... "substitute piston"

[Stubby]

Tiny bit more. But can be done at the same time as the existing squish and cylinder bore machining. No reason why the CNC cutter path has to machine the squish flat either.

 

bmp01

 

You would have to do a tool change from a flat endmill to a ball nosed cutter .

[bmp01]

CNC lathe.... surely?

 

bmp01