Author Topic: Squish, is enough too much?  (Read 227 times)

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Offline KilliansRedLeo

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Re: Squish, is enough too much?
« Reply #10 on: January 14, 2014, 10:14:06 pm »
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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Offline 3000 FPS

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Re: Squish, is enough too much?
« Reply #11 on: January 14, 2014, 10:28:41 pm »
Maybe taking a little off the top of the piston.
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Offline farmboy

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Re: Squish, is enough too much?
« Reply #12 on: January 15, 2014, 07:32:24 am »
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

Offline KilliansRedLeo

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Re: Squish, is enough too much?
« Reply #13 on: January 15, 2014, 11:02:00 am »
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?
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

Thomas Jefferson
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Offline sharkey

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Re: Squish, is enough too much?
« Reply #14 on: January 15, 2014, 01:31:15 pm »
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.

Offline Al Smith

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Re: Squish, is enough too much?
« Reply #15 on: January 16, 2014, 05:05:28 am »
A couple thou won't make that much difference .If I were to err I'd prefer on the side of caution.

Offline rms61moparman

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Re: Squish, is enough too much?
« Reply #16 on: January 16, 2014, 05:46:18 am »

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

Offline aclarke

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Re: Squish, is enough too much?
« Reply #17 on: January 22, 2014, 11:37:54 pm »
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

 

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