Miniplane style starter for the Polini Thor 100
by Catalin Atodiresei - France
Catalin Atodiresei is a French Romanian born in Romania. He is a very experienced pilot and likes to improve his equipment. This time, Catalin submitted and interesting article about improving a Polini engine
hand start mechanism.
Thank you Catalin for submitting this interesting and well written and illustrated article.
Alex L. Varv
I have owned my Polini Thor 100 for over four years now and when I think of this engine, the first 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 TOP80 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 engine and are subject to the extreme vibrations of the engine. They will slowly grind at the
starter and fail. The flash mechanism only adds to the unreliability of the starter, since it comes with its own series of failure points.
On the other side, the Top 80 starter has its moving parts (the pawls) on the flywheel, and the centrifugal forces keep them in place
while the engine is running, so no vibration induced wear will occur. The main pulley is a one piece part, and experience shows 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.
To achieve this, the following components must be created or adapted:
- new ratchet type spool
- starter pawls with springs
- pawl fingers
- pawl plate with holding pins.
The starter case and the rewind spring are reused from the original starter.
1. The new spool :
For the design of the part I first used a 3D Modeling program (SketchUp).
- The face which receives the rewind spring is identical to the original part.
- The ratchet gear is composed of four teeth which have been studied to function with the pawls (see below).
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 :
For these parts I just bought the parts from à Miniplane dealer. For the Polini Thor you will need four pawls, so you will most likely have to buy two sets of three,
which will leave you with two spares.
3. The pawl fingers :
For this par I used thin head M5 screws, to reduce the total height. The screws are only partially threaded. The body length (non threaded)
is exactly 12mm to accommodate the pawl and the spring. If you don't 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 to them in the mounting nuts on the flywheel.
You should cut them to around 9-10mm.
The separation between the pawls and the mounting plate is done with thin hex nuts, which also allow to hold the plate in place.
The partial assembly is done in reverse when compared to the Top 80. The pawl comes onto the screw, then the spring,
then the thin nut is treaded to the end and locked in place (I used a drop of SuperGlue to lock it in place). This whole assembly is then used to keep the plate in place.
4. Pawl receiving plate :
This plate is modeled to keep the exact 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 thick enough piece of metal sheet. For my project I used 2mm thick aluminum plate. The plate can be cut using a laser cutting machine, or a CNC router (in my case)
Final assembly :
The final assembly is done in a rather intuitive manner:
- On the engine side, you need to remove the original plate ring and replace it with the pawl receiving plate described above, using the pawl finger screws.
- On the starter side you have to place the rewind spring in the same way you would on the
original part. The spool is then mounted in the original casing. The spool should be rewound with about 1,5m of 3mm dyneema cord.
- When mounting the starter on the engine, you can use a small cord to keep the pawls tensioned, as shown in many videos and articles, or you can use clothespins to hold them while putting the starter in place. The cord or the clothespins can easily be removed afterwards.
1. Spool diameter:
With the original starter, on a normal pull, I could barely manage one and a half - two compressions. This made the engine very difficult to start,
because I couldn't give the engine enough initial momentum.
One of my objectives was to create a smaller diameter spool, in order to achieve more engine 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.
If you experience a particularly difficult start and constant kick-back from the engine, please consider cleaning the decompression hole in the cylinder,
as this has already been the case for me. This also came in handy for restarting the engine in flight. Il the original configuration, it was sometimes difficult even to get one compression
with a full length of arm. After this upgrade, I can get at least two compressions 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 (spool and pawl plate), you just need to write me an email at
If you have any questions you can also use the same email address.
If you can't be bothered or you don't 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).
I have used this new starter for over 100 hours now, and it hasn't shown any sign of wear whatsoever.