Chainsaw Repair
How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: KilliansRedLeo on January 13, 2014, 08:04:44 am
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OK, might as well get some thoughts on this. In my experience stock saws can have any where between 0.030 and 0.050 squish. When building I shoot for 0.020 or as close as I can get without going below 0.020.
I got a saw in Thursday last that was built a few years ago by an out of state builder, it had 0.018 squish, ran pretty well. Not my saw, I was asked to put new seals in it because the owner thought it had an air leak, it did.
Anyway I was wondering if anyone had opinions on what would be a good rule of thumb for minimal squish?
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OK, this forum is "How To Basics" so this non-engine building, non-mechanic will ask...
What is a Squish?
Can you describe it, show me a picture of it, and why is it important to saw builders, but not so much to the saw manufacturers?
Thanks for teaching.
Chuck
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Go here download parts VIII and VIIII
http://chainsawrepair.createaforum.com/tutorials/conversion-of-a-husqvarna-261-to-262xp-with-muffler-mod-and-other-mofifications/
Squish is the clearance between the piston crown and the top of the cylinder.
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.001 will run good...
.100 will run okay...
.015 on small saws works great...
.020-.030 on work saws...
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Awesome, thank you.
Chuck
Go here download parts VIII and VIIII
http://chainsawrepair.createaforum.com/tutorials/conversion-of-a-husqvarna-261-to-262xp-with-muffler-mod-and-other-mofifications/
Squish is the clearance between the piston crown and the top of the cylinder.
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Awesome, thank you.
Chuck
Go here download parts VIII and VIIII
http://chainsawrepair.createaforum.com/tutorials/conversion-of-a-husqvarna-261-to-262xp-with-muffler-mod-and-other-mofifications/
Squish is the clearance between the piston crown and the top of the cylinder.
No problem, that is what we are all here for to help and learn. You might want to have a look at the other sections as they cover the top end rebuild and modification of a saw. You are the perfect person to give feedback! The tutorial is not quite finished but when done it should take a person through the entire process step by step. If you think things are not detailed or clear enough post a message or PM me.
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I've heard of some porters using .018 that are pushing it for every last dime and worked fine for them. Most I seen on mine were .020-.022
What saw?
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Saw was a Stihl 038Mag
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I had an 084 that was .110...it ran ok. I got it down to .020 and it runs better but also a lot harder to start.
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I usually shoot for .020 myself but I do have a Poulan 335 that I took down to .018 and it ran just fine with no problems.
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Well, I'm messing around with an experiment turning a HVA 261 into a 262XP. I do not want to cut a new band and the 262XP does not provide enough clearance to run without a base gasket. Nominal squish with the gasket is 0.03275 base gasket is 0.345. If I were to run a 0.5mm gasket I might get away with it. The experiment is to try to use a 365SP piston, which is 1.5mm 0.060 taller than the stock 262XP piston. So I'm trying to figure out if there is a way to get close to 0.020 or even a little less using standard HVA or even AM gaskets without cutting a new squish band. In the past I would cut a custom gasket from 110 annealed precision copper shim stock but don't want to do that for a single experiment.
After all that krap, if any of you guys have any thoughts let me know.
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Maybe taking a little off the top of the piston.
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IIRC there is a guy in Oz running .018 on a Dolmar 7900 for a couple of years now. Mweba did .018 on a 562XP and it hit squish band while running. He caught it before any major damage was done. I try to stay as close to .020 myself on everything. I've got a 44mm Stihl 026 (play toy)I may pull it down to .015 and see.
Shep
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OK, to the best I can figure
The 365SP piston is 1.5mm (.060) taller (compression height pin center to crown) than the 262XP piston.
If I want to accommodate for the piston and a squish of 0.020, I need to use a gasket that is 0.080 thick e.g. raise the cylinder off the case 0.080. Well I cannot get exactly there!
If I use 1/64 & 1/16 (0.015625 & 0.0625) that will get me to 0.078125.
If I then subtract from 0.078125 the increased height of the piston (0.060) I m left with a nominal squish estimate of .018125: not 0.020 but that is as close as I am going to get with standard gasket material.
Have I forgotten anything in my calculations?
So what do you guys think, am I chasing my til on this or what?
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Your numbers look correct. Consider that the connecting rod can grow a little when hot. Older engine may have some wear on the crank pin bearing etc. May want to stay around 22-25 thsdths.
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A couple thou won't make that much difference .If I were to err I'd prefer on the side of caution.
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I know of one 346 that is running at .013 and it runs great and has for several years. Not a toy either, it was a production firewood saw for at least two years.
My 346 is at around .020 and I wouldn't want to go any lower, I just don't feel like it gives any margin for safety. A couple thou. of carbon and bad things are starting to happen.
Mike
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Most folks crank the squish down tight in an effort to gain compression on otherwise stock cylinders. optimum squish velocity typically occurs way before you get the squish to .018 range. so in other words, most guys who build "woods port" saws are tightening up the squish to gain compression, not optimize squish velocity