Undoing stuck screws

[Mark]

I have to get the selector cover off from my QB Opsrey gearbox to reseat the selector rod (yes, like a fool I undid it....)

The three smaller screws came out no problem, as the head on these was for an allen key. The two larger screws have a conventional screw head, and they won't shift at all. I am worried about them breaking - and the consequent nightmare of having to drill them out.

So I have stuck the gear box in a bucket of diesel (WD40 didn't do anything). Will this work, and how how should I leave the gearbox in the bucket of diesel?

Any other ideas for getting these screws out?

Thanks in expectation

[CatiGull]

Gentle heat on the casing will help ease the screws - my preferred easing oil is 'Liquid Wrench' which is thinner than WD40 and will penetrate better, I think. I cant back this up with any facts though.

I cracked my EFNR gearbox screws with a hand impact driver which puts a sudden twist onto the screw whilst the impact keeps the screw bit engaged in the slot. Once they crack, can turn them off with a screwdriver using a ViseGrip (tm) to increase torque.

Keep sprinkling Liquid Wrench like holy water, especially after they crack for the first time.

I had a lot of experience slacking overheated screws on air cooled VWs over the years, you get some feel for how much torque is safe to apply and when you are near the breaking point and have to use heat/penetrating oil soak and time (I would often soak for few days!!) to avoid breaking hardware.
This experience is difficult to pass on, unfortunately.

[CatiGull]

Adding on....a vise grip applied directly to the head of a screw will help put a lot of torque on it, but this is enough torque to break hardware that is seized up....at least you wont round off the screw slot this way.

Dont forget to 'ring the screw' with light hammer taps whilst soaking it in Liquid Wrench, or equivalent, to help the loosening oil reach the threads.