Proper Squish measurement

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LC Cnd
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Proper Squish measurement

#1 Post by LC Cnd » Tue May 29, 2018 2:02 am

What is the proper standard top dead centre squish for an Elsie - 0.9mm?

LC Cnd
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Re: Proper Squish measurement

#2 Post by LC Cnd » Thu May 31, 2018 1:24 am

Found this bit of info but it relates to YPVS squish but not sure if it translates to a LC:

"The optimum squish gap has been found by trial-and-error over time. For the Yamaha water-cooled 350cc twins, a gap of between 0.9mm and 0.75mm is ideal."

"It pays to keep an eye on squish clearance too. Bend a short length of 2mm soldering wire at 90 degrees and thread it down through the spark plug holes so that it touches the bore. Turn the motor over by hand, squashing the wire between the piston and the head. Measure it, the thickness should never be less than 0.7 to 0.8mm."

So... it appears optimal is between 0.8-0.9mm for a LC.

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two-stroke-brit
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Re: Proper Squish measurement

#3 Post by two-stroke-brit » Tue Jun 05, 2018 2:14 pm

i was told by more than one "tuner" that .9 was a minimum.
:smt023
350LC,Athena 392 big bore,stock porting,swiss cheese air box,stock reeds,spec11 pipes.(now TSA big bore pipes)
She might not be pretty but she will always be a fun ride


1982 YAMA RD350LC
98KDX 220R
1967 TRIUMPH TIGER
2001 TRIUMPH TT600

evan_calgary
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Re: Proper Squish measurement

#4 Post by evan_calgary » Thu Jun 07, 2018 8:57 pm

I believe, but don't quote me, the 2-stroke tuner's manual by bell, which is basically the bible on tuning, that it is around .8 to .9 for any 2 stroke motor. The balance of the variables such as head volume are determined by compression which there is also an acceptable range. Our buddy MK may be able to provide some more info. I think your quotes are all relatively accurate. 0.75mm may be getting tight. You may want some room for error built into the numbers as well. If you shave the head or cylinders you are also increasing compression if you don't mess with the dome. My stock TZR came in around 1.8mm. My understanding is the factory targets the stock squish so if tolerance in all items were at the tightest you would be at minimum squish. My best guess this number is around 1.3mm for midpoint of tolerances.

LC Cnd
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Re: Proper Squish measurement

#5 Post by LC Cnd » Fri Jun 08, 2018 1:25 am

I went though my copy of Gordon Jennings tuner's handbook to see if there was a formula that I could calculate squish - nothing really. Mr. Jennings spoke theoretically about calculating squish for a 250cc twin with a 50% squish band would be 1.14mm. Pretty close to what your saying Evan when you consider another 100cc for a 350. So I imagine 0.9-0.8mm would be a bit tight as aready pointed out. Also learnt that you have to use Rosin Core or acid core Solder not solid or plumbing solder as solid core will not squish as well, and the reading will not be correct. I have to make a return.

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Re: Proper Squish measurement

#6 Post by evan_calgary » Sat Jun 09, 2018 4:11 am

Hey LC, the solid core works just fine. Turn the engine back and forth near tdc until there is no longer resistance. Haven't had an issue. I though he had mentioned a general squish but you are probably right it is dependent upon surface area for the flame front and potentially dome shape although modern ones typically all use the same rough shape compared to some of the really old stuff in that book.

LC Cnd
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Re: Proper Squish measurement

#7 Post by LC Cnd » Sat Jun 09, 2018 11:01 pm

Found a calculator for squish velocity (other intesting info also) but that's about it. The site mentions a Professor Gordon Blair (Queens University Belfast) & hints that their that their calculator is base partly on the professor's two stroke work. Prof Blair worked with Yamaha for several years. Too bad he did not publish a squish calculator.

https://www.google.ca/url?sa=t&rct=j&q= ... 1T0V5eMrOe" onclick="window.open(this.href);return false;

Bare
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Re: Proper Squish measurement

#8 Post by Bare » Thu Jul 19, 2018 11:37 pm

Solid Solder is Iffy although it does work.. mostly.
Rosin core is softer .. therefore better suited.
Also easy to find and Cheap enough so there is Zero reason not to use it exclusively.
Jennings info is decently Good /Reliable.
Bell ? I wouldn't trust to him under any circumstances .. too many odd views/ theories.
Many of which are superceded /wrong.
But hey! it's Your engine ..choose yer own poison :-)

Hooligan
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Re: Proper Squish measurement

#9 Post by Hooligan » Thu Aug 09, 2018 2:16 am

Measuring squish is one thing. Adjusting it is an entirely different proposition.

My LC is set up w O-ringed heads and so has minimal squish. My RZ runs an OEM head gasket and as a result I was unable to adjust the squish so it is huge. The two motors have a very different feel to them. The RZ feels a bit soft and squishy, while the LC feels a lot more crisp and responsive.
'82 RD350LC in '81 colours, OEM pipes, UNI filters
'85 FZ750

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