Hi All. Brand new to sketchup and for all my efforts i cannot seem to make an arch flush on an incline. It always seems to distort and bulge out or other crazy things happen. Tried to make it a component but didnt work. I imagine i missing something fairly simple to achieve this. Cheers for your help and knowledge 
Do you mean ‘arc’ instead of ‘arch’?
I’ve read your post four times and I don’t understand what it is you actually want to do. Maybe you could show us an illustration?
If you mean ‘arc’ I think I can see what problem you are having.
Make sure that each time you click with the 2-point arc tool you are first seeing a tooltip ‘On Face…’, then drag the mouse also on the face for the bulge radius. Or try the Arc tool, instead of the 2-point arc tool, to show first the centre point (the protractor will turn black when it is on the face), then click or type to define radius and angle of the arc.
It’s very easy (as I have just been confirming for myself) to make one or both clicks for the two point arc tool off the plane. Then the bulge direction can jump all over the place.
I mean arch, as in archway. i don’t know how to take images from sketchup
and post them here. I want to put an archway through an object but the
design has the walls at an angle,(like an entrance to a pyramid) sketchup
does not appear to like archways at angle. hope this clarifies. Thank you
for your input and knowledge. cheers 
You most likely need to make an object that has the profile of the arch push-pulled out to some length then move it into position through the sloping surface. Select both the arch and the surface, right-click and select Intersect faces with selection. Then delete the extra lines and faces you don’t need.
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EXCELLENT, that’s exactly what I’m trying to do, thank you for your help.
Take it easy!
Works for a single surface but not blocks, see attached picture.
probably doing something stupid, but your guidance is greatly appreciated.
cheers once again.
did follow your methodology from above but results in this for this
application:
It looks like you drew the arch on the surface of the block and then pushpulled it. Pushpull always extrudes perpendicular to the starting face. To cut horizontally through the block, draw the entire block first. Then use the method I animated above to cut all the way through the block, not just one face. You will have to make the cutter long enough to go completely through of course.
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