Honestly the plastic end cap might be appropriate for positioning the crank arm on the spindle prior to tightening the pinch bolt but it isn't sufficient to hold the crank arm on the spindle, it's only a soft piece of plastic. It appears to be a M18-1.0 equivalent bolt size and if I could find a steel M18-1.0 I'd replace the plastic cap with a steel bolt and washer. I have a friend who is a machinist and I might give him the cap and ask him to make a copy out of steel with twice the length in the threaded portion.

Hi Gaynier, there is no need to CNC out a copy of the plastic bolt. I did get one for 4€ from Amazon.. (if linking is allowed here: https://www.amazon.it/gp/product/B001AQX08M ). 

Agreed completely, and in fact that's what the customer support rep said too.  Having said that, one that plastic end cap gets stripped, I've been completely unable to keep the crank arm in place with the existing pinch bolt.  My assumption (for what it's worth) is that the repair kit mentioned above has slightly newer versions of the end cap and pinch bolt, with the pinch bolt likely being slightly longer. I can't say for sure though since they're out of stock and I'm getting a replacement unit.

I'd assume that mating between the crank arm and the splined spindle gets damaged if it's allowed to move around once the pinch bolt gets loose. My bike is still relatively new and since this discussion I've been checking the pinch bolt frequently. See the reply from Luca , the FSA crank bolt looks like the perfect fix to me. I'm planning to order one and replace the plastic cap with the FSA crank bolt.

Thanks! A simple and elegant solution, sprezzatura.

Do a search for "M18 crank arm bolt".  There are plenty of off the shelf ones available.  Also be careful with the length.  I don't know how far the internal threads of the spindle are supposed to be threaded in, but I had to "work" the bolt I bought a bit to get it snug.  Not sure if the internal threads were not clean of burrs or if I found the end of the threading.

If you look up Shimano Hollowtech crank arms you will notice that they use 2 opposing pinch bolts instead of one.  Personally I think the issue with the Tacx 1 bolt design is if not properly torqued the arm can lever its way off the spindle over time since you are point loading the center of the arm with 1 bolt centered vs. distributing the load evenly with 2 bolts spread towards the edges.

I would also guess they supply a new pinch bolt to ensure you are starting with a fresh bolt that hasn't been torqued past yield and/or had the 5mm hex rounded out.

Luca, the model that you indicated here worked fine for you? Tks

I can say that it worked fine for me. I still periodically check the pinch bolt but I haven't had any issues. I purchased the FSA crank bolt and it fit perfectly.

Ality, Please let us know if the larger bolt replaced solved the problem.. tks