Thanks for all your comments, if not closed I would have expected a broken face error, redrawing the sketch cured the problem, it is that I couldn't see the error.

Further to your post, attached file of joined tubes and screenshot of helix thread form if the helix is made longer than required it can be positioned to give actual length needed. I have left the thread form visible FYI. Again unfortunately the complete file is to large to include here. When comple...

Sorry forgot to include here it is

Hi Don't know if this helps, First of all your original file contains several error if you check with Dependency graph you will see them. Here is one way of doing it and no doubt there is a better way, I have not generated the correct thread form just to same time. In this example I created 2 pipe f...

OS: Windows 10 Word size of OS: 64-bit Word size of FreeCAD: 64-bit Version: 0.18.16110 (Git) Build type: Release Branch: (HEAD detached at upstream/releases/FreeCAD-0-18) Hash: f7dccfaa909e5b9da26bf50c4a22ccca9bb10c40 Python version: 3.6.6 Qt version: 5.6.2 Coin version: 4.0.0a OCC version: 7.3.0 L...

Horizontal 4 cylinder rotary valve steam engine.

Very nice modeling

Model of gas fired tram loco named Ceasar 64mm gauge.

Solved.
Thank you "easyw-fc" very much for your input which has solved the issue I was having.
I have to confess I did not look in the new A2+ preferences.
Attached resulting snips.
Thanks again.

Thanks KBWBE for having another look at this, has anyone else reported having a similar issue? I even tried running V0.18 as Administrator as V0.16 will only run that way on my PC , still no luck. Attached snip of the same exercise in V0.16 with Assembly2 no problems encountered........... This is m...