unfastening set screws marine gearbox top

[JonE]

Trying to dismantle a marine gearbox casing. Under the top, there are four setscrews fastening the apparatus for holding the end of the gearstick selector. They go into a particularly thin piece of the gearbox casting. 
Two will not shift. 
Tried kitchen flambe gas torch, tried WD40, tried a sharp but not too hard tap, but now the screw tops are starting to wear at edges of the slot.

Don't want to disturb/damage the casing - what next before I have to drill them out?

[Colin Morgan]

Plusgas?

What was the condition of those that came out?

Colin

[bob46320]

get a screwdriver where the blade passes right through the screwdriver handle. Give the screws a good bash with a hammer on the end of the screwdriver whilst trying to do the screw up (ie clockwise). If you are lucky, they might just do up a wee bit. Now try the same but undoing. I think an impact screwdriver may be a bit too heavy a bit of kit, but you never know.

https://www.ebay.co.uk/itm/Pro-Impact-Sc...SwgmJXzSDA

[JonE]

thanks both. the two on the right had no corrosion but they were under no load - its only a two speed with one selector seemingly.

What does an impact screwdriver do? do you hit it and it moves round a bit?

[Colin Wilks]

Assume there’s no chance there is a nut inside the casting?

I use 3 in 1 penetrating oil. Leave to soak overnight and then use a socket set driver that fits the screw head as exactly as you can. This lets you put downward pressure on the screw head whilst giving you lots of torque. I would give the screws a tiny nip up to start off with as this can sometimes break the crud that is locking the threads.

I would put copper slip on the threads on reassembly.

Good luck.

[Erich]

Jon, there are different types of impact screw drivers. The simplest is a rather heavy piece of metal, about an inch diameter, with a spring inside. Generally there are two settings, one for tightening and one for loosening(why one would need the former is beyond me). When you whack the end with a hammer, the spring compresses and the screw fitting on the end(blade, Philips etc.) converts the whack to a twisting motion. Bob's method is similar, though in that case your hand does the twisting. I would tend to put some penetrating oil on or WD 40 and let it stand for a good bit of time before I used an impact driver. As it has been in marine environment, go judiciously as metals don't do well there.

Erich in Seattle

[JonE]

I think for the sale of 7 quid, I probably need a Chinese impact driver in my life anyway...
The slot is actually wider than my biggest flathead screwdriver, so it might out on that front too.

[John Mason]

Hi Jon, Have you considered this type of heat treatment. Put the gear box in a moderate oven for about half an hour and then on removal give it a try. Be careful though as it will be hot. Also which is even more important be careful that she who must be obeyed does not catch you doing it.

John Mason

[JonE]

John - I've managed to do it! I'm wondering whether trying to get the bottom part of the slot deeper with a vibrating engraving tool was the eventual cause, or a combination of everything people suggested about tweaking and pressing.

I couldn't reasonably do the oven thing - its covered in two layers of green paint and still full of oily nasties... might taint our food production for months to come. Weird that the little kitchen blowtorch thing didn't seem to have effect though. Perhaps the heat wicks away into neighbouring metal too quickly.

Have bought copper grease, Colin!