Head Clearance

[Doug Moon]

What is the proper way to measure head clearance on a PA75, PA65.

I would like to compare my clearance to others around here and I need the procedure please? I have heard comparing head shims alone is not the correct way to do it.

Thank you
Doug

[BillLee]

What is the proper way to measure head clearance on a PA75, PA65.

I have a PA 75 that feels very soft above 45. It has across the three different planes I have had it in. When I do the hourglass at the top I am pulling the handle way down to keep tension. It really makes me sad when my flying partners arms are straight up and mine is not...

Its not trim.

I would like to compare my clearance to theirs and I need the procedure please? I have heard comparing head shims alone is not the correct way to do it.

Thank you
Doug

Bring it by, Doug, I'll show yo how it's done.

Bill

[Doug Moon]

My thought is measure the depth of the piston in the sleeve at TDC. Then measure how far down the lower edge of the squish band reaches into the sleeve. This is the tightest clearance can be. If you add shims you raise the clearance.

[Brett Buck]

My thought is measure the depth of the piston in the sleeve at TDC. Then measure how far down the lower edge of the squish band reaches into the sleeve. This is the tightest clearance can be. If you add shims you raise the clearance.

   Exactly.

     Brett

[Paul Walker]

How can a 75 get "soft" up top??

[Motorman]

Couple of tips,

Sometimes the piston will be tight enough to lift the sleeve up even if you're pressing down as hard as you can. In that case you need special clamps to hold it down.

Don't measure your head with the shims on it, measure them separately.

Piston in the hole + shims - head = clearance.

Might need more drive out of your prop (more pitch). Compression, prop, venturi, needle setting have all been talked about on here. https://stunthanger.com/smf/engine-set-up-tips/engine-tuning-tips/


MM

[Doug Moon]

How can a 75 get "soft" up top??

That’s what has been pissing me off for so long. The motor on he inside is clean a a whistle. Still very tight at the top. Hot starts with one flip. When I push the pipe in for some added boost it doesn’t do anything. Pitch up the prop and it gets too aggressive and charges.

I thought I would start messing with compression and see if there is more to be had.

[Doug Moon]

   Exactly.

     Brett

Thank you

[Doug Moon]

Couple of tips,

Sometimes the piston will be tight enough to lift the sleeve up even if you're pressing down as hard as you can. In that case you need special clamps to hold it down.

Don't measure your head with the shims on it, measure them separately.

Piston in the hole + shims - head = clearance.

Might need more drive out of your prop (more pitch). Compression, prop, venturi, needle setting have all been talked about on here. https://stunthanger.com/smf/engine-set-up-tips/engine-tuning-tips/


MM

Thank you

[Lauri Malila]

How can a 75 get "soft" up top??

It’s never a healthy practise to do it with a “squeky” healthy engine when cold, but there are a couple of helpful tips:
-turn it to TDC before removing the head. If you don’t do that, there is a big chance that cylinder sleeve will raise together with piston.
-Heat the engine a little with hairdryer, it’ll loosen up.
-Use a few drops of good old castor oil as lubricant, it’ll squek much less than with any synthetic.

Or, you can forget the squish clearance and measure the head volume instead, piston at TDC. Then all you need is to remove the plug and use a 2ml syringe (with needle) for measuring.
It takes some practise to get repeatable results, but it’s a better way if you are comparing similar engines.

Just for curiosity; how much is a typical squish clearance in a PA or Jett? L

[Doug Moon]

Made some changes last night.
Pushed the pipe in to 19 from 19 3/8.
Changed venturi from 12 to 7 kept air filter on to keep stuff out.
Removed .003 shim.

At 9am the air density altitude was around 2600 feet, actual altitude is 550. 91% density. ugh..

But me and Mike Scott forged ahead.

It was a very pleasant surprise to feel the plane flying again with lots of usable power. Plus it was about 400 less RPMs. Still had enough time to finish the pattern with the bigger venturi.

Lauri,
Depth of the piston at TDC was .042
Squish band .030
.013 total shim added.
That gives me a total clearance of .025

[M Spencer]

Someone who shall remain nameless used to mention ' P.S.I. ' in relation to heads . a adaptor to a ( small ? ) automotive Compression tester , might work .
With a lot of flicking . 

But B.M.E.P. & recorded compression & Comp. Ratio are all interelated , wherea , outside a certain range , a ' squish band ' / clearance - gets awkward .

Was talking 20 od , each side of here ( picture ) SO , a miniturised thing reading 100 to 200 would be the go , perhaps. If you could find one .
maybe he wound it over on a electric starter / .





Much better looking , suppose all youd need is a one way check valve in a pressure gague .



Or if you have digitaliteous .

We have presented the topics of Thermal Efficiency and Volumetric Efficiency as methods for estimating the potential output of a given engine configuration.

Brake Mean Effective Pressure (BMEP) is another very effective yardstick for comparing the performance of an engine of a given type to another of the same type, and for evaluating the reasonableness of performance claims or requirements.

The definition of BMEP is: the average (mean) pressure which, IF imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output.

Please note that BMEP is purely theoretical and has NOTHING to do with ACTUAL CYLINDER PRESSURES. It is simply a tool to evaluate the efficiency of a given engine at producing torque from a given displacement.

Thisiwhy toonedpipes do things .