Helicoil repairs

[ian Howell]

I notice that Keith is using helicoil repairs on a crankcase and is (rightly) concerned about thinning of base metals around tapped holes.

I used to work for a Main Distributor of this product (so I may be biased, but . . ) and we had a very convincing demonstration set up: -

Using a SOFT aluminium bar about 3/8" thick, a hole was drilled and tapped 5/16 BSF and an Unbrako high tensile socket head screw fitted with a 1/2" thick spacer.

Customers were then invited to tighten the screw until the thread stripped. The hole was then re-tapped to suit a 1 1/2 dia Helicoil and the Unbrako screw and spacer refitted.

The customer was then invited to repeat the prevous test. This invariably led (eventually and after a couple of scraped knuckles) to failure of the Unbrako screw.

THEN the fun - the broken-off Unbrako could be unscrewed by hand - so obviously no damage to the base metal. How many would you like Sir /Madam?

The reason is that the Helicoil - unlike the base metal - can move as the tension increases so the load is distributed throughout the (engaged) length of the Helicoil.

The same is generally true of other coil type inserts but Helicoils are made from rolled stainless steel so the thread finish is superior to most (all?) other types.

If in doubt - consult Ed Nankivell for whom I Helicoiled a Jupiter crankcase back in the 1960's and which is still in everyday user.

End of commercial.