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Old 07-04-2023, 06:03 AM
CnC for me CnC for me is offline
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Reversing Gear Casting Damage - Drill & Tap questions

Hello All.
I have a C&C 29 with a rebuilt Atomic 4 from*Moyer Marine (circa 2020ish) and this is my second season with the Atomic 4 and I am having a great time working with it and learning from all the forum / Moyer materials.

Unfortunately, I was not careful enough while adjusting the forward drive in the reversing gear assembly and I inadvertently broke the segment of the casting holding the adjustment lock screw/pin. Luckily I did not lose any fragments into the gear case (phew!).

After I stopped using my sailor words and calmed back down, the forum community educated me that there are 2 additional flats on the cast collar which has given me hope this can be fixed without engine removal and gearbox disassembly. I rotated the casting and sure enough there is a thicker flat just waiting to be drilled and tapped.

So I am working up to drilling and tapping one of the flats for a new lock screw/pin and I am posting today to ask after any cautions, ideas, suggestions around this repair prior to tackling it.
I am at the disadvantage that "Firefly" is 3 hours away so I am attempting to be as prepared as I can be to repair it in one trip (if at all possible ).

In addition to general advice on the repair process, I do have a couple of questions that I would welcome comments on:

- In the reversing gear video, the bench sample appears to have a thicker and a thinner flat, do all casting have these ? (I have not rotated the casting completely and only found a thicker flat)
If thicker and thinner are on all castings, do you recommend using the thinner or the thicker for the new drill/tap location ?
Does anyone happen to have a measurement on the thicker & thinner flat vs the milled original lock screw/pin location as I would like to determine if I need a longer screw/pin or if the original will be sufficient.

I know some castings can be rather fragile to shock, do you think the casting will tolerate just enough center punch to allow a dimple for smooth drill start ? If not, I am open to suggestions on safe drill start.

I am planning to guard from drill tap filings dropping into case by sliding a paper sheet between the casting & adjusting ring as well as rigging up a vacuum at the drill point. (As once suggested by Don in an older thread). Open to additional creative suggestions.

I'd love to chat with anyone who has done this so I can leverage the experience, please PM me if you would be open to a call.

Thanks in advance for the input.

Respectfully, Karl

Last edited by CnC for me; 07-04-2023 at 06:07 AM.
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TimBSmith (07-04-2023)
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Old 07-04-2023, 10:50 AM
Dave Neptune Dave Neptune is offline
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Karl, yes indeed you need to be careful tapping, drilling and sucking out the chips.

I would use a #1 center drill, a drill designed to not drift and leaves a chamfer to start the drill. The center drill will require a minimum dimple from the center punch a good thing. Then I would drill a "small pilot hole" for the tapping drill minimizing drift off center. Another possibility is to drill a slightly larger hole for the tap which will ease the tapping force required. Once tapped install the locking screw with a minimum of torque and a drop of Blue Locktite as it will hold and still release easily for future adjustments. The "Red Locktite" is far more locking power than required. A center drill is something you may need to order from McMaster or a similar supplier. I suggest buying HSS (Hi-speed steel) or Cobalt and not carbide as it is strong, hard and very fragile. The HSS or Cobalt are tougher to break off the small "pilot point" of the C-drill.

Advice, spend a lot of time in set up and positioning as patience is required for what you are doing.

Dave Neptune
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TimBSmith (07-04-2023)
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Old 07-04-2023, 08:11 PM
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Al Schober Al Schober is offline
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I would hesitate to use a center punch on those ears. If a small divot is need to get the drill started, I'd use a Dremel tool with a diamond burr.
I second the use of cobalt drills. They're tough and stay sharp much longer than HSS. Left hand ones are very handy for broken fasteners.
I also would go oversize on the tap drill size - make it easier to cut the threads. I'd use a drill about half way between the measured crest and root of the fastener. A drill bit set that goes by 1/64ths may or may not do it. You might need a number size drill - a bit more expensive in cobalt, but not outrageous.
As for catching the chips, the full shield is the best. Think of draping in an operating room. Vacuum won't hurt. Another trick is a dab of grease. Vacuum and grease are good for the top stuff. You'll also have stuff breaking loose on the bottom side as the drill breaks through.
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