Bolts/screws boke off, thread/pipe tap -size? Alpha One Gen 2

[carlmalm]

Okay, so while installing a new grounding/continuity circuit kit, two bolts/screws on my Mercruiser Alpha one Gen 2.

And, since we use metric threads in Sweden, I couldn't use my normal pipe tap-kit.
Do anyone here know what thread size, or pipe tap size I should use for those small torx headed screws?

They have about 7 threads per 6.7 mm (0.263779528 inches), and the (male) diameter of the screw (outer size) is about 4.2 mm (0.165354331 inches).

 

[carlmalm]

 

[carlmalm]

Okay, so while installing a new grounding/continuity circuit kit, two bolts/screws on my Mercruiser Alpha one Gen 2.

And, since we use metric threads in Sweden, I couldn't use my normal pipe tap-kit.
Do anyone here know what thread size, or pipe tap size I should use for those small torx headed screws?

They have about 7 threads per 6.7 mm (0.263779528 inches), and the (male) diameter of the screw (outer size) is about 4.2 mm (0.165354331 inches).

Two bolts broke off*

 

[Scott Danforth]

what is the diameter?

looks like a #10 screw. there would be 24 or 32 threads per inch

 

[Chris1956]

Pipe thread is usually tapered. You will want a straight tap for those.

If your diameter measurement is accurate that would be a #8 screw which is 5/32". Those can have 24 or 32 threads per inch. If your threads measurement is accurate, they are 24 threads per inch.

They look to be larger to the naked eye. Where exactly are they from? You might post this in the MerCruiser forum, as those guys are most familiar with your outdrive.

 

[Grub54891]

If those are the bonding wires, just drill a new hole next to the old broken one, being sure to drill where the wire will reach. Then use a ss screw as stated above. Drilling the old screw out never results in a good time.

 

[Wildey]

I'm not familiar with what Merc. uses, but that looks like a triangular thread, self tapping type screw. Typically used in aluminum or other soft (or thin) material for rapid production assembly. You just drill a hole slightly smaller in diameter than the screw and insert the screw. A little lube / cutting oil / grease helps to minimize the torque required. This type is prone to corrosion due to the small voids in the assembly ... unles sealed or filled with loc-tite or a corrosion inhibitor.

 

[Wildey]

If your diameter measurement is accurate that would be a #8 screw which is 5/32". Those can have 24 or 32 threads per inch. If your threads measurement is accurate, they are 24 threads per inch.

#8 Coarse = 32 Fine =36 TPI
#10(3/16") " 24 " 32 "
I they are not metric, they are most likely #10-24
The diameter measurement is likely a bit under .1875" due to the triangulation of the thread.

 

[carlmalm]

Thanks to all of you!
I believe it's a 10-24, so I've ordered a set of taps.

And interesting what you write Wildey, that you can force the screw in just by drilling a hole in the soft aluminum. Sadly, thers already a stainless screw stuck in there, but otherwize I would have tried. And the screws are a little triangular shaped in the bottom.