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Exhaust port opens under piston after gasket removal

popopboat

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Hello, its me... Again...
Tried deleting a gasket on my chainsaw and replacimg it with rtv.
And now as stroke bigger than usual on these chainsaws lower side of exhaust port is opened just a tiny bit below the piston on tdc.
( i did not touch at all lower side of the port)
So my question is how bad/serious it is?
 

Rob Stafari

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I think the biggest issue is that most RTV is not oil and fuel resistant. Should you have used one of that variety, there is a good chance it won't hold up to the test of time. As for the opening below the piston, I believe they call the 'freeporting.' I also believe most say it is not ideal, but not necessarily the worst thing in the world. Hopefully someone who actually knows what they are talking about shows up soon though. I do know the biggest things with the base gasket delete are 1) That there is enough clearance between the top of piston and the head. 2) You make a good seal at the base of the cylinder to the case.
 

Marshy

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Hello, its me... Again...
Tried deleting a gasket on my chainsaw and replacimg it with rtv.
And now as stroke bigger than usual on these chainsaws lower side of exhaust port is opened just a tiny bit below the piston on tdc.
( i did not touch at all lower side of the port)
So my question is how bad/serious it is?
Did you measure your squish clearance? Also, what saw are you working on?
 

popopboat

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Dont worry about rtv, i already used it for that purposes and it handled it very well, even as a headgasket.
Uhm cant really measure clearance too precise but i suppose around 0.7mm / 0.0275 inches
 

Canadian farm boy

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Most base gaskets are .015-.020 thick. Your freeporting is gonna be minuscule. As long as you have adequate squish clearance it will be fine.

Do you have any pics of the freeporting?

Easy way to check squish is to use thin soft solder through the spark plug hole. Position it above the wrist pin and turn the engine via the flywheel back and forth a few times to compress the solder. Then just measure the thickness of the compressed solder with a micrometer or calipers.
 

popopboat

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Oh my... i thought that i posted picture

Here it is on the bottom, dont worry about scratched piston, its zoomed in so its doesnt look that bad IRL (400grit sandpaper)74266007_389911078553275_1691251644630564864_n.jpg
 

popopboat

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I checked clearance with aluminium foil, folded it and squished, then mesaured with caliper... precise as it can be hehe
 

Canadian farm boy

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Oh my... i thought that i posted picture

Here it is on the bottom, dont worry about scratched piston, its zoomed in so its doesnt look that bad IRL (400grit sandpaper)View attachment 205882
I’d run that.
You can always put a base gasket back in if you don’t like it but I predict you’ll be happy
 

popopboat

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Thanks everyone for answers!

And one more question, can i replace a piston with another one which is about 10grams / 0.35oz ligther (on the same crankshaft) ?
How will that affect vibrations
 

cuinrearview

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The floor of that port looks really flat. Is the roof like that?
 

popopboat

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Ive widened it a bit, and then polished with sandpaper 400-->800-->2000 grit if you think on that, and ye top is also polished
 

Rob Stafari

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I believe he was asking about where the port meets the cylinder wall(shape of port opening into cylinder). The top of the port needs a certain amount of arch to it. The flatter it is the more likely it is to snag a piston ring and grenade.
 

popopboat

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Sorry then, didnt know about that so didnt understand question.
Yeah, top is also very flat, they made it that way, chineseums i guess :/
So what do you suggest? making a bit of an arch ?
*real ports have curves ( ͡° ͜ʖ ͡°)*
 
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