- Location
- central florida
M10-1.25 Helicoils are available in 10,15 and 20mm length . It is generally assumed that 1.5 times the diameter of a fastener is it's max. holding strength in steel, more for aluminum. You don't have room for a 20mm insert- nor would it be needed. The 15mm insert would be just fine- if too long, snip some off. Consider using cutting oil (or even just reg oil if you don't have it). That will give you a smoother hole and cleaner/neater threads, when drilling or tapping into aluminum.