This is how I measured my 200Ts squish clearance

[Stubby]

CNC lathe.... surely?

 

bmp01

 

Yeas . Of course . Then the distance from dome to base and position of the ports would all be controlled accurately .

[MattG]

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

Yes, man. And that was my precise issue. They protrude (along with the rest of that crappy ring around the periphery of where the jolly old squish should be) by about ~0.15mm. Which scuppered my otherwise tasty measurement of 0.55mm I got for normal area (away from that stupid moulding artifact). Growl!

Edited May 2, 2017 by MattG

[MattG]

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

Finally, I found a 0.1mm (4 thou) thick flyer at work. Pinched it. And made me a replacement gasket.....with a razor blade and belt hole puncher as my cutting implements. Using this (instead of the stock gasket) should give me ~17 thou at the crappy periphery where those stupid bosses are, and about 26 in the area without that sh!te.

 

Ta da!

[bmp01]

Yes, man. And that was my precise issue. They protrude (along with the rest of that crappy ring around the periphery of where the jolly old squish should be) by about ~0.15mm. Which scuppered my otherwise tasty measurement of 0.55mm I got for normal area (away from that stupid moulding artifact). Growl!

 

So get a bit of sandpaper stuck on the top of the old piston, spin the piston with a bit of wood. 5 mins and those protruding lumps will be gone. Use the existing squish band to guide you in your progress. You're not afraid to get in there and clean up the combustion chamber, this isnt much different. In both cases very careful cleaning required. ...

 

bmp01

[MattG]

So get a bit of sandpaper stuck on the top of the old piston, spin the piston with a bit of wood. 5 mins and those protruding lumps will be gone. Use the existing squish band to guide you in your progress. You're not afraid to get in there and clean up the combustion chamber, this isnt much different. In both cases very careful cleaning required. ...

 

bmp01

That's a good idea. Any thoughts for holding the paper down though? I'm thinking superglue, since I'll probably want to start coarse and work down finer.

 

Do appreciate that those things you describe as "lumps" merely look that way when viewed on a screen, they actually protrude down no more than that thin ring, the very very outermost annulus - of about 2mm which you should be able to just make out by viewing my piccy. They are merely D-shaped bosses on the existing casting process artefact (for want of better terminology). The entirety of those things you refer to as lumps and that very very outermost ring on the dome, all do protrude down by about 0.15mm into the cylinder, repeating myself, into the available squish clearance space.

 

Sorry for waffle! But yeah, the paper idea is a good idea - and I'll keep you posted.

 

Matt

Edited May 3, 2017 by MattG
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[bmp01]

That's a good idea. Any thoughts for holding the paper down though? I'm thinking superglue, since I'll probably want to start coarse and work down finer.

 

Do appreciate that those things you describe as "lumps" merely look that way when viewed on a screen, they actually protrude down no more than that thin ring, the very very outermost annulus - of about 2mm which you should be able to just make out by viewing my piccy. They are merely D-shaped bosses on the existing casting process artefact (for want of better terminology). The entirety of those things you refer to as lumps and that very very outermost ring on the dome, all do protrude down by about 0.15mm into the cylinder, repeating myself, into the available squish clearance space.

 

Sorry for waffle! But yeah, the paper idea is a good idea - and I'll keep you posted.

 

Matt

 

Got any double sided tape? You want the stuff that is just a bit thicker than normal sellotape. I've got some made by 3M and some used for carpet laying, both work. When you need to remove it, use heat.

The key here is a uniform thickness of what ever you use to hold the paper.

 

Yeah, i understood how little you need to remove. I don't think you even need to go very coarse (200 grit?) but cleaning the paper and cylinder regularly to stop it "plugging" will help.

 

And of course none of this is worth it if you make a mess of the cylinder bore so keep the bore and piston OD clean. And keep the paper away from the edge of the piston, say 1mm minimum in from the edge.

 

Just my 2 pence worth....

 

bmp01

[MattG]

Got any double sided tape? You want the stuff that is just a bit thicker than normal sellotape. I've got some made by 3M and some used for carpet laying, both work. When you need to remove it, use heat.

The key here is a uniform thickness of what ever you use to hold the paper.

 

Yeah, i understood how little you need to remove. I don't think you even need to go very coarse (200 grit?) but cleaning the paper and cylinder regularly to stop it "plugging" will help.

 

And of course none of this is worth it if you make a mess of the cylinder bore so keep the bore and piston OD clean. And keep the paper away from the edge of the piston, say 1mm minimum in from the edge.

 

Just my 2 pence worth....

 

bmp01

Dude,

 

Missus has caught some double-sided taped she reckons.

 

I've got fine paper already, but I've requested that 800, 400, 240 is purchased, and I'll see how things pan out.

 

10-4 about keeping it clean!! I never ever want to repeat the exercise of my late teens, where not having any valve grinding paste, led me to mixing sand with grease, and unfortunately getting the odd grain between the valve stem and guide. You live and learn it seems

 

Your comment re. "1mm from edge" Hmm..... unfortunately the crappy moulding extends right to the edge, so it will be flying on the seat of my pants type of work. I've done worse in the past - so I'll just have to play to cool.

 

later fella

[spudulike]

Blimey:confused1:

[MattG]

I done the "machine the top of the dome" trick using a old piston with disc of emery stuck on top, as suggested by bmp01. Have to say, it was a lot easier than I thought, and I reckon I've got a pretty squish clearance without the base gasket now, rough measurements being between 0.45 and 0.55mm in the 4 different measurement points. Will try to get some piccies up in the next day or so.

[bmp01]

Blimey:confused1:

 

It's Shed tuning, Spud Not commercially viable granted, but fun for those that enjoy that sort of thing.

 

But in fairness if you can run the piston up close to the squish band it will reward you with a little bit of extra oomph but more importantly an all round nicer engine. The turbulence you can get from tight squish is underrated IMHO, a more dependable / reliable combustion process due to the extra gas movement, so even if the mixture isn't spot on (it never is, through the whole rev range) - combustion will be better. So hopefully a more crisp engine response. Just makes the saw nicer to use. Innit....

 

Starting it might need a bit more care though, especially a top handle saw like this one - you listening MattG?

 

bmp01