In Model Info/Units you have it set to inches and 0 precision, so it can only tell you the rounded up dimension.
Adjust the precision and it will give you an accurate dimension.

Thanks Anton. Not quite right but I'll have a go myself now that I've up-graded to SP3.5.6
I notice that I have to modify some parts so will changing them, change your animation. ie, hide the control lever that now comes through the side of the fuselage due to a narrowed nose section, or adding the cut-outs in the floor to accept the wheel?
As for the mains, I thought I left an angled torque drive to show rotation axis. Never mind, as I said, I'll give it a go myself.
Once again, cheers.

Thanks Anton. I hadn't noticed that group conflict.Will fix it when I'm ready to add the rest of the system.
Again, thanks for your tuition, just hope my 72 year old brain can remember it.

Maybe the debug function is activated?

spdebug.jpg

Fixed. Made the under-carriage assembly 'magnetic'. Works like my dream now.

SP is tough, try some simple stuff first, for example, spin the prop on your engine, which is very well done by the way.
If you get that to work reliably then move on to another item. On the engine/prop. Start by exploding the groups then make one group for the prop assembly, and one for the engine assembly. In the warehouse I have a simple wind generator.
http://sketchup.google.com/3dwarehouse/details?mid=eace30d21a3521305c5e8df5dc8d3d85
Start there to get some ideas how to proceed.