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Base Gasket delete question

TooLegit2Quit

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I’ve got a stihl 171 I want to play around with. Is a base gasket delete something I can do without porting?
 

Stem

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nope. not unless you do a bunch of machine work
 

drf256

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Looks like a clamshell. Most use sealant and have no base gasket.

It’s a design/geometry thing when you try to change squish on these. You need to line bore the bearing pockets higher in the case to tighten it up.

The cylinder and the top half of the crank case are one unit. The engine block has 2 parts, the cylinder and top crankcase half and a bottom cap. Pro saws have 2 case halves together and a cylinder mounted on top, so 3 parts.

Clamshells cheaper to produce.

Generally not worth the time and very difficult to get it right.

You need a different model if you want to start playing.
 

old guy

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I did it once with a 2900 poulan, I did the bearing pockets with sandpaper on a piece of pipe, I had good measureing tools & a lot of time, it took 9 hrs. to do the sanding,I don't remember if that was just the bearing pockts or the flat also.
It raised copression 138 to 164. the saw is now a little ripper.
I have another 2900, but it is still stock.
 

Al Smith

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You can do some improvements on a clam shell design but not as much as a removable cylinder engine .IMO it's not worth the time to do it .As they say you can't make a silk purse from a sows ear .
However an option rather than line boring could be an after market raised dome piston if you could find one .As I understand it there are some pistons on the market for 029-039 Stihls .I personally would not bother with them but it's not my saw nor my business .The only reason they might interest me is for the challenge to do it .
Then there's the possibility of welding a piston crown and making a pop up .Again though the skill levels it would require most likely is far and above the skill levels of most people .
 

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I’ve got a stihl 171 I want to play around with. Is a base gasket delete something I can do without porting?

You’ll get gains from a muffler mod and 1/4 key timing advance. Beyond that complex machine work is necessary…or short lived hackery.
 

old guy

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I would certainly not do another one like that, but I did have good depth gauges & mikes from my machineist days & an understanding that everything had to change the same.
That was 10 or 12 years ago, saw still runs good today, mostly i suspect the starter mech. will give out under that much comp.
 

Al Smith

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I could bore it out because I've got a model M Bridgeport model M milling machine with an indexable boring head .Instead of a boring tool I'd use an end mill ,much smoother .To bore it would not take long but the set up would take forever .
 

thompsoncustom

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I haven't got it back together to test it yet. but I took a clamshell and some aluminum braising rod and filled in a lot of the domed area in the head.

Not sure how it's gonna work out quite yet but compression gonna be high for sure.
 
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