Hello

I bought someone's CH750SD project. The left wing was partially built with bottom skin riveted on.

When I I prepared to drill spar, I had every possible cleco installed in the leading edge and many clecos on the top skins (but not all).

My holes lined up fairly well but there was some mismatches in the leading edge skin and the top skins - the mismatching was inboard to outboard not leading edge to trailing edge. Roger Dubbert thought I would be OK with the mismatch that I photographed and sent him.

I drilled all my #40 holes and was planning to upsize with #30 and then #20. Then I found an old threaded core drill that I had (still very sharp) that had a .098 pilot and a .160" finish drill. I tested the core drill on some scrap and it made a better hole than upsizing with the #30 and #20. 

I decided to used the core drill with my pneumatic right angle drill. The holes seemed to come out OK. When I took the skins off to start deburring - to my horror I saw that I had gouged the .250 strengthening bar at top of mid section of spar.

I thought I was drilling straight, but the pneumatic drill is so fast and powerful that I must have been angling a little towards spar.

I checked the top of spar and my holes were about .030" closer to the leading edge from the midline of spar than they were from the trailing edge.

Every single hole gouged the strengthening bar.

I think if I had used regular drill bits it would have come out better, but I think some of the holes are so close to bar that even if I had not gouged bar, the final rivet in skin might not come out right. 

I have emailed Roger Dubbert but he has not gotten back to me yet.

1) I don't know what caused my spar holes to be closer to leading edge than trailing edge of spar.

2) If I could get a pre-drilled spar from Zenith, I think I could drill out 62 rivets, buy a big pneumatic squeezer and replace bar, but if my holes in top of spar are not going to work - not sure where to go because my skins are already upsized.

3) Am I the only person that has encountered this? Misery loves company.

Thanks for any help you might be able to provide.

Jim Gould