On the W407B and the upper W407C, each had two countersunk #40 holes which matched #40 holes in the W407D doubler. AN426AD3 rivets held the three pieces together as a unit. I guess it was for ease of installation. There were no corresponding holes in the rear spar because the assembly of these three pieces happened before riveting the whole thing onto the rear spar. When I drilled them out, I drilled through into the rear spar for 3 of the 4 holes. As far as I could tell, when installing the new parts, there is no need for those 4 ''keeper'' rivets because all the match drilled holes place the parts exactly into position. I thought about just leaving the 3 #40 holes unfilled in the rear spar. Vans Aircraft suggested I should rivet those 3 holes so it would not spook some future owner seeing three holes without rivets in them some day. There is no safety issue, just a perception issue. I decided to use countersunk pull rivets through existing spar holes and not drill through the new work.
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