On 05/06/2021 11:08, Murmansk wrote:
OP here

Well, the bolt would only turn a quarter turn, I flooded it with oil and worked it for half an hour but no joy so with the help of my neighbour and a metre long bit of scaffolding pole attached to the spanner I eventually got the bolt out.

Every turn of the bolt took a huge amount of effort even with the scaffolding pole, the thread is knackered. Only question remaining is what the thread in the nut that's welded to the mechanism is like - probably knackered too!

I'll have to see if Witter will sell me a new mechanism


Not surprised to hear that this is your problem, from the original
description. If Witter are unhelpful nut condition can be estimated by
trying with a brand new bolt. I would strongly suggest using moly
disulphide grease on it, as that will reduce the risk of further damage.
If nut threads are bad, worth buying a tap and seeing if they will clean
up. Chinese carbon steel taps from eBay are available in a wide range of
sizes, much cheaper than "proper" HSS ones and OK if you only need to
use them a few times. I've bought a few for "odd" things (e.g. when I
wanted to fit "seat belt" threaded screw eyes into some angle iron).
Invest in a tube of Trefolex or similar "cutting" compound for tapping.

And use Moly grease on the bolts in future.