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Miniplane Top 80 style starter for the Polini Thor engines

by Catalin Atodiresei

Introduction

I have owned my Polini Thor 100 for over four years now, and, when I think of this engine, the first thing that comes to my mind is the way car enthusiasts describe owning an Alfa Romeo: when they work, they are a real joy - playful and reactive, but also quiet and smooth. That’s when everything is ok and they work properly.  But, every now and then, something will go wrong, followed by the pain and the frustration of trying to find the fault and then a solution.

It’s a mix of emotions which gives them an almost human face and makes you grow even more attached to them. And yes, I can say that over these years, I have become very attached to this engine.

One of the most common points of failure of these engines seems to be the pull starter. Compared to a Top 80 starter which may outlast the engine, the Polini starter will fail after only a few hours of use. As it has been explained in a number of internet articles, the main reason for this is the fact that most of the moving parts are located outside the moving parts of the engine and are therefore subject to the extreme vibration that is present in all 2 stroke paramotors. The starter parts will slowly disintegrate and fail.

The flash mechanism also adds to the unreliability of the starter since it comes with its own points of failure.  Thankfully, this has been remedied in the later models of the starter.

stock Polini Thor starter

On the other hand, the Top 80 starter has its moving parts a part of the flywheel.  The centrifugal forces keep them in place while the engine is running, so vibration induced wear will not occur. The main pulley is a single piece and experience shows that it will most likely last the entire life of the engine.

The idea behind this project is to create a TOP 80 style starter for the Polini Thor.

Note:  If you normally experience a particularly difficult start or constant kick-back from the engine, check the decompression hole in the cylinder and clean it, if required.  Do not attempt this modification unless you are certain that your engine is in perfect running order!

Components

To achieve this, the following components must be created or adapted:

The starter case and the rewind spring are reused from the original starter.

1. The new spool

new spool for the Thor starter

For the design of the new spool I used a 3D Modeling program (Sketch Up).

For the first prototype I used a solid block of PEHD and a small hobby milling machine. The final iteration seen here has been 3D printed, using PETG.  Other strong materials may also be used (ABS, Nylon, etc.)

2. The starter pawls and springs

I purchased these parts from a Miniplane dealer.  You will need four pawls which require the purchase of (2) sets of the Top 80 pawls because the Top 80 only uses (3) pawls on their starter.

3. The pawl fingers

modifed Thor starter

I used a panhead M5 screw in order to reduce total height.  The screws are only partially threaded. The non-threaded length is exactly 12mm which will accommodate the pawl and the spring.  If you cannot find the right type of screw you can buy a different type and thread the body yourself in order to leave exactly 12mm of non-threaded length.

The threaded length is limited by the depth available in the special mounting bolt on the flywheel.  You should cut the threaded length to around 9-10mm.
The separation between the pawls and the mounting plate is done with thin hex nuts, which also hold the plate in place.

The partial assembly is done in reverse when compared to the Top 80. The pawl goes onto the screw, the spring, and then the thin nut is threaded onto the end and locked in place with a drop of Superglue or blue threadlock.  This whole assembly is used to keep the plate in place.

4. Pawl receiving plate

modified Thor starter

This plate is modeled to keep the same spacing between the screws as the original plate.  It has the pawl and spring holding pins which are made from the same piece of metal sheet and then bent upwards. The plate should be constructed from a 2mm thick aluminum plate.

The plate can be cut using a laser cutting machine or a CNC router (in my case).  For the handy, it can be cut by hand with a jigsaw and a drillpress.

Final assembly

The final assembly is done in a rather intuitive manner:

Other considerations

1. Spool diameter

With the original starter on a normal pull I could barely manage 1.5 or 2 compression strokes.  This made the engine very difficult to start because I could not provide enough initial momentum.  One of my objectives was to create a smaller diameter spool in order to achieve more turns on a typical pull. This, of course, comes at the expense of the force required to pull the starter.  But I am more than happy to pull a bit harder rather than having to yank the starter every time in order to give the engine enough energy for it to start.

This modification also came in handy for restarting the engine in flight. In the original configuration it was sometimes difficult to get even one compression stroke with a full length of my arm.  After the upgrade I can get at least two compression strokes with one pull.

2. Availability

If you want to do this modification yourself, I can send you free of charge the 3D files I used that include the spool and pawl plate.

If you cannot be bothered nor have the time or the means, I can create these parts and send you an upgrade kit (spool, pawl plate, pawl fingers and thin nuts, pawls and springs, plus 1,5m of 3mm Dyneema cord).

You may contact me at catalinatodiresei@yahoo.com.  Any questions are also welcomed.

I have used this new starter for over 100 hours now, and it has not shown any sign of wear whatsoever.

Good luck!

vulture