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Author Topic: Squish clearance question.  (Read 837 times)

Offline Lauri Malila

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Squish clearance question.
« on: April 29, 2013, 04:22:31 AM »
 Hello.

 Well, I know I should give more detailed info and there is no universal rule, but:
 How much would be a safe minimum squish band clearance in my engine? I have noticed that it has a great effect on running stability so I want to go to minimum.
 Bore 24mm, stroke 28. Running temperature is very low thanks to monolithic cylinder construction and piston scavenging. No nitro.
 At the moment the clearance is about 0,7mm and to make it smaller I must make a new head.
 I know how much the head volume must be, so I don't want to play much with shims.
 I know that in high rpm engines there is a safety limit for squish clearance to prevent the piston touching the head, but I believe there is no risk of that with my 6700 rpm.
 0,3mm..?

Lauri
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Online Paul Smith

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Re: Squish clearance question.
« Reply #1 on: April 29, 2013, 06:31:07 AM »
In my old days of combat flying, using the English system of reckoning of course, .010" was considered safe and going down to .005" was risky business.  But the name of the game was setting up for all the RPM you could get for the temperature, pressure and humidity of the day.  Somebody running 6700 RPM (in the English system) wouldn't be all that concerned about every last revolution.

We never ran mufflers.  A muffler prevents all the exhaust from leaving the engine and thus effectively increases compression.  So the "rule of thumb" for a wide-open engine would not apply to a muffled one.
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Paul Smith

Online Brian Hampton

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Re: Squish clearance question.
« Reply #2 on: April 29, 2013, 09:29:55 AM »
Lauri
I have a self-imposed limit of .010" (.25mm) because I like to err on the safe side but I have run one engine at .005" without any problem.
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Offline Lauri Malila

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Re: Squish clearance question.
« Reply #3 on: April 29, 2013, 10:18:50 AM »
Yeah, I think I'll go for 0,25...0,30 with a 0,1mm shim. So it's still possible to try slightly less.
My muffler is not very restrictive and fuel consumption is quite low at about 3,7...4oz./flight but it's still a little too rich:)

L
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