If no one has seen the Youtube link I put up here it is again.

 

That clip is more for the threads; in my case it was definitely the o-ring hence why the innards stayed within the bottle.

(Edit: To be fair, having now watched it all the way through I see that they do wipe the inside too)

what happens if the grease get in the coolant will it do any harm

Silicone is very inert so is unlikely to react with anything in the coolant.

5 hours ago, Ian Lanc said:

If you can get the old cap off without damaging it, then grease it up and re-fit it.  If not buy a new cap. 

Personally I'd just buy a new rad cap, not like it's £50.

Well had a check on mine and ford must of done it on last service,nice chap as ok now.

And yes been a few posts before on this,if this one didn't pop up again I wouldn't of checked it.

Hope you get it sorted MjNewton as it is your post 👍

6 hours ago, dezwez said:

what happens if the grease get in the coolant will it do any harm

Won't do any harm, years ago it was quit the norm to put a few drop of oil into the water which mixed with the water then clung to the metal moving parts within the engine.

13 minutes ago, Ian Lanc said:

Won't do any harm, years ago it was quit the norm to put a few drop of oil into the water which mixed with the water then clung to the metal moving parts within the engine.

you learn something new every day

Got my new cap today (genuine Ford) fits perfectly right up until you get to the small 'stop' at the bottom left of the cap as seen in the photo. It was a little tight to get it fully home but trying to turn it back to unloosen I had to wrap a cloth over it to get it started then it came back out easily! 

As discussed a smear of silicone grease may help with seating (and removal!).

1 minute ago, MJNewton said:

As discussed a smear of silicone grease may help with seating (and removal!).

Yes I know about the silicon grease.....that isn't the issue! 

I think the fact the cap is new the 'locking' mechanism of the cap has nice new square corners to it and I think it needs to be rounded slightly or put on and off a few times to bed the mechanism in. 

Ah, yes, I see what you mean. As you say a bit of rounding off from repeated usage may make it a bit more compliant.

I've fitted my replacement cap - a Gates RC240 - and whilst it fitted well I would expect that from any cap from new. I smeared on some silicone grease for good measure and will see how it fares over time.