Reaming 13mm Holes..what Size Drills?

For a 13 mm reamer, that's a lot of material to remove, .030. That would put a lot of stress on a reamer
and shorten it's life I would think. I wrote to Yankee Reamer and I'll let you know what they write.
For now, I have a 12.9 mm drill bit that I'm going to test first. Thanks.
 
typically 0.015 is what was considered reaming tolerance on the automatics
 
Yankee reamer wrote back and suggested using a 12.5 mm drill. He wrote that most drills will
leave an oversize hole, or use a 31/64" drill followed by an undersize 1/2" reamer then finish with the 13mm reamer.
The 12.9 mm drill leaft the 13 mm hole oversize, 13.02 mm, which is fine for my project. It's a loose slip fit manifold type part.
 
Here are my results. I used a 31/64" stub length drill bit to open the initial starter hole.
Then I used a 1/2" AP undersize reamer and finished with an AP 13mm reamer. I got a nice smooth hole
.006 over. The 12.9 mm drill was giving me .020 over. In the future I will use a 12.5 mm drill instead of the
1/2" undersize reamer.
 
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