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B gearbox prop flange - drilling out M8 threads

  • Bruce Conway
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4 months 1 week ago #17480 by Bruce Conway
Bruce Conway created the topic: B gearbox prop flange - drilling out M8 threads
I own an old CFM Shadow 2-seat aircraft with a Rotax 582 engine, B gearbox and 3-blade Precision Prop.

Several Shadows lost their propellers when the M8 prop bolts, threaded into the prop drive flange, sheared off in flight, In June 2002, a mandatory service Bulletin was issued (CFM SB 16) and the advice is to follow the CFM factory practice of drilling out the threaded holes and fitting longer bolts with locknuts.

So, I’m about to drill out the threaded M8 holes in the Rotax ‘B’ gearbox drive flange.
I will fit longer AN5 bolts with nyloc nuts on the gearbox side of the flange.

Any tips for drilling the hardened steel flange?
Should I use a 5/16” drill (to match my AN5 bolts) or an 8mm drill?
Do I need to support the flange (to avoid any strain on the gearbox castings)?

Thanks in advance for your help.

Bruce (New Zealand)

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