Measure the squish & set the timing on the PA125

How to check the ignition depth is correct without having to disassemble the motor.

Without dismantling the engine, remove the spark plug and insert a length of solder to measure the "squish", or the distance between the piston at TDC position (TDC) and the cylinder head. This distance, called squish, must be between 1.3 mm. and 1.5 mm.

By inserting the solder and literally crush it with the cylinder head you can measure the thickness of the resulting solder crushed between the piston and the cylinder head. Remember that you must insert the solder until it reaches the cylinder wall and parallel to the pin and centered on the circle (see photos below)

Once satisfied that the squish is correct. See that the position of the flywheel is correct according to the scheme and provided with the piston at the point of maximum compression or PMS Measurement

Place the solder in the middle of the hole parallel to the piston bolt.

Measurement of solder after having crushed the piston against the cylinder head.

The distance between piston and cylinder head at maximum compression (piston at TDC) must be between 1.3 and 1.5 mm.


Gauge that is use to find the P.M.S. of the piston. It screws in place of the plug. It is the engine to manually rotate the piston up or down and so find out the PMS Tamper.











Once assured that the piston is at P.M.S. we see that the mark of the magnet and stator is 1.5 mm away from the coil.

Radio Squelch setting tutorial

Keith Pickersgill

I am constantly surprised that so many pilots do not understand what the Squelch setting on their two-way radio does or how to set it properly, so here goes a mini tutorial:

The purpose of the Squelch is to mute the speaker/headphones when there is no signal being received, to save you from listening to static noise and to save the radio batteries.

When the Squelch level is set correctly, the receiver will be silent until the radio receives a signal from another radio, then the speaker un-mutes in order for you to hear the call.

The higher your Squelch setting, the stronger the incoming signal must be in order for you to receive it.

If you set your Squelch to high, you could mute some weaker radio calls and will not hear them.

If you set your Squelch level too low, the constant static noise in  your ears will affect your ability to concentrate and causes stupid mistakes to be made in flight.

The appropriate Squelch level depends on the ambient RF (Radio Frequency) noise conditions, which varies from place to place and from time to time.

It should be set at the start of every flight, or at least checked that it is not too high.

How to set your Squelch:

First, set the radio to the frequency you will be using for takeoff.

1) Turn the Squelch down (anti-clockwise on the dial, or the down-arrow, depending on brand/model of radio), until you hear the static noise.

At this stage, you can use this noise to adjust your volume control, saving you from asking someone to give you a radio check for the purpose of setting your volume.

2) Now gradually turn the Squelch up until it just mutes the speaker and the radio goes silent (if no-one else is transmitting on this frequency).

3) Now start your engine and during the warm-up, listen to see if the increased RF noise from your ignition system opens the Squelch (the speakers emit Static Noise again). If this happens, increase the Squelch just enough to mute the speakers again.

4) You might find that when you switch on your GPS or camera or other electronic device, your Squelch opens again, in which case you increase the setting again just enough to kill the static.

Once you have an idea of how much you need to increase the Squelch to compensate for engine, camera, GPS, etc, then in future, Do step (1) above, then increase it by that amount (rather too little than too much).

In flight, once everything is on, you can check the squelch by adjusting it downwards and the Squelch should open immediately, indicating that you had the correct level.

If you set the Squelch too high, which is the dead lazy thing to do, you might miss some critical radio calls which impacts on your own safety and that of other aircraft.

Note: Your squelch setting has absolutely no affect on your transmitting, your microphone or how others hear you over their radio.

It affects only the receiving audio.

The next time someone tells me their radio is causing lots of interference in the air and it turns out to be merely their squelch setting was too low and they were listening to static noise the entire flight, expect a kick up the backside!

Listening to static noise in flight affects your ability to concentrate, causes headaches, and causes your hearing to eventually shut down,  so you do not hear when others call you.

Its not a difficult thing to get right.  Read this tutorial again if you are not sure.

If you still do not understand, now is the time to ask...

Prop repair

Keith Pickersgill

Q-Bond remains the best option for Carbon Fibre props. It is the same density as Carbon, hence no need to re-balance the prop if you do a careful repair. Easily worked with a file, sandpaper or my personal favourite, a Dremmel. In some cases, gentle application of an angle-grinder or bench-grinder can be used for the first prep before changing to something less drastic. Finish off with 1200 grit water-paper and polish on a piece of leather, delivering a perfect finish.

For wooden props, Epoxy is best. Either the two-part quick-set Clear epoxy (Prattleys, et-el), or the UV activated version such as Anthony's Dura Rez (and many similar brands).

Unfortunately Epoxy is a VERY different density to Carbon, so most epoxy repairs would require extensive re-balancing on Carbon props, though its good for filling very small cavities in Carbon.

Some tips: With the UV activated resin, work indoors (or in the shade under a tree if outdoors), then when you have the resin just where you want it, step out into the sunlight to cure.  You could partially cure in sunlight for say 30 seconds, then step back into the shade to check and adjust, and repeat as often as required if you want a perfect job.

For Q-Bond, the liquid part is ordinary super-glue. Even the cheapest superglue from Chinatown outlets works just as well. For tricky jobs, get the slow-set Gel-type superglue, which allows you to form and re-form the carbon black powder as it cures gradually.

You will always run out of the superglue before running out of the Q-Bond Carbon Black, so buy a few very small tubes (3g) of superglue to add to your toolkit, at about R5.00 each from Chinatown or similar outlets. Once opened, you usually can discard a bottle of superglue unless you keep it in a fridge or keep it perfectly upright in storage.

To fill a big hole in a hollow carbon prop, take the thinnest tissue paper you can find (one layer of 2-ply works well), stuff it into the hole and tease it to form the shape you want, then very gently wet the tissue with superglue. Wait for it to dry and become rigid, then re-wet small parts at a time and apply the Q-Bond Carbon Black in very thin layers.

Build the Q-bond to slightly proud, then grind/file/sand down to shape then polish.

The Q-bond bottle usually has a HUGE hole, too big for prop repairs. Instead of screwing off the cap, I drill a 2mm hole in the cap and spinkle gently out of that, covering the hole after the job with some Gaffer's Tape (or Duct Tape or even Masking Tape will do).

A quick and easy way to re-balance spanwise, is to add a sticker (decal) to the lighter blade.

Remove the backing paper and lay it upside down on the blade (i.e. sticky side away from the prop). While the prop is on the balancer, slide the decal along the blade until it is balanced.

Closer to the hub if the decal is too heavy, or closer to the tip if you need more weight. Once it looks level, flip the decal over and stick it down in that spot. Now rub the decal very hard around the edges, especially the leading edge, with the back of your fingernail to activate the pressure-sensitive adhesive, to prevent it from lifting off in flight.

If a prop needs balancing chord-wise, i.e. it rolls over to one side on the balancer, then you have a tough time ahead to balance it, as you have very little radius to work with. This is why you want to repair very carefully, with a product of similar density to the original prop.

In this case, the best is to use clear spraypaint, and lay down a layer all down the lighter edge, which is down one blade's leading edge, and the other blade's trailing edge.

Wait for it to dry, check the balance, and repeat as required.

Unfortunately you affect the airfoil a but, but that is the lesser of two evils, as an unbalanced prop causes much damage to your engine and framework.

Vibration eventually makes aluminium and stainless-steel brittle, then the frame cracks on all the welds. So take your time to balance the prop as accurately as possible.

On wooden props, you can drill a 10mm hole on one side of the hub, roughen the surface inside the hole, fill it with molten lead, then re-check the balance. If too heavy, center-punch the lead plug, then start drilling the lead away with a 6mm bit, until the prop is balanced.

I have seen some pilots drill and cut a 10mm thread into the hole, then screw in a short bolt or capscrew as a weight, until the find the perfect balance, then epoxy the bolt into place.

It looks a bit strange but it works, but I prefer molten lead, as a wood thread is not robust enough for my liking.  If you don't want to work with hot lead, then use fine lead buckshot, and epoxy into place in the hole.

Make sure your choice of propellor balancer can check the balance in both the span and chord

directions.   This requires that the prop lays flat in/on the balancer, not edge-on.

Either the popular old Prop-Top

see:   http://xplorer.co.za/proptop 

or my new favourite, the Gadgeteer Benchtop Balancer

see:   http://ppg-guru.co.za/?p=116

Those knife-edge types or any that holds the prop edge-on, can check only the span and not the chord balancing.

Both Q-bond and Epoxy can be carried in your harness pocket, along with a sanding sponge for emergency field repairs.

Keith Pickersgill