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OS 120 Surpass Mk 111, rocker cover bolt, rounded head


Don Fry
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I acquired a second hand OS 120 Surpass Mk111 some time back. It was as cheap as chips, nice condition, undamaged, little used, and runs well. Now it’s going in a Spacewalker, 1/4 scale, I plan to build. Next up on the bench. One problem. The front, deeply buried rocker cover bolt is rounded, it’s a hex, shallow head. Not me, as received. I can’t cut a slot in it without cutting the rocker cover

I would prefer to resolve this problem before mounting in an airframe.

How to extract. Good workshop, access to lathes, industrial puller drill, tools.

 

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I’ve tried a torx ( that comes out as Tory on the spell checker). Brand new bit, Bosch, heated bolt, knocked in, faintest grip, then fail. 
Martin, is a left hand drill different to an screw extractor? I didn’t think extractors came small enough for this thing.

Kens’ system is favorite at the moment, especially is he says there is enough left to get pliers on. 
 

BTW, what is the size (diameter) of these bolts.

 

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Be careful what you order. I bought a set reasonable priced on eBay described as "left handed drills" they turned up as a set of easy outs and normal drills ! Tracy tools sell proper left handed drills and you probably won't need to buy a set .

Pretty sure they are 2.5mm bolts. 

Edited by Engine Doctor
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Don you could try this...

Take an Allen key the next size up and carefully grind a shallow taper on each face until the tip just engages with the head of the bolt. Then tap the key into the head of the bolt with a hammer. If you can force it in to a reasonable depth, you may find that you can now get enough purchase to unscrew the bolt.

 

A.

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Heating the screwhead with the tip of a soldering iron can sometimes help too.  I'm surprised the Torx bit didn't work, I've never had it fail - touch wood!  If I can't find a suitable size torx bit I've sometimes found that an imperial key will tap in tightly enough to get the screw moving.  I think that the impact of hammering it in helps to break up any crud holding the thread.  

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Cheers Andy, I will have a go with the grinding option. Bob, it’s a ruined hex, I think the problem started when the previous owner didn’t clean the socket out before use. I’ve tried heat, I’ve got a tiny gas torch, used for small workshop jobs, and caramelising  things in the kitchen, with a torx, and while it gave a bit of grip, nowhere near enough to shift even a screw just nipped up.

I’ve got a set of (proper) left hand imperial left hand bits coming, of use as I really should have some imperial bits, even after this job is sorted. 
Doc, you are right, 2.5 metric course, 15mm. Idiot here remembered that the motor was boxed, with instructions, then idiot also remembered the other cover bolt is undamaged, and measurable.

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Andy, just tried the tapered hex. Mounted motor in a test clamp, so I could mount it in a vice, so it does not move, and tapped it in with a dead blow hammer. Didn’t work, really is a ruined head. I think I will be patient. Drill it out with a left hand drill, in the pillar drill. It either shifts or the screw head cuts off. If the later, I revert to Ken Anderson’s method, or carefully cut a slot in the headless stump. No hurry, as I won’t  the need the motor for a couple of months.

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Yes, I’ve also tried gluing a hex into the hole. But, silver solder will, at least mess the rocker cover finish up.
I’m going for the patient approach.

When the new left hand drills arrive. Heat, then, left hand drill, will either get it out, or cut off the head. If the head comes off, revert to Ken’s plier extraction, or I can cut a slot in the top of the stub. 
or even, heathen like, hold the cover down with one bolt. No one will see it under the cowl. 

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Arrrrrrrr, Ken, thank you. I did not realize there is that much stump. I’m  off flying any minute, maiden of a 2 meter Stik.

I will have a go when I get home.

For information, slotting a bolt is achieved by slitting a slide on brass tube, (Dremel, might not get it first time, continuue til you get it right), The slit then holds a junior hacksaw in place, central,  until the job is done. I’ve got an old Laser 75, broken main needle, so slotted, has been for years. Much smaller than 2.5 mm.

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The maiden is done. It’s new to me, but a second hand airframe. Usual list of programming alterations to be done, and happy. Not great, as I left the batteries at home for the other aircraft I took.

Filled car on way home, bought whiskey.

And my considered opinion is Ken Anderson is perfect. Job took no more than a minute. Thank you gentlemen.

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