Replacing the thermostat, gaskets?
 

When i flushed my system, running the engine up, i noticed a loud click/knock sound at about the time i guessed the thermo was opening - now the weather is a little colder, i *think* it's taking longer than expected to warm up to temp.

Now the system does get up to temp and regulates itself ok, but i'm wondering if the thermostat is sticking somewhat and i should just replace it.

I'm hoping i don't have to drain much coolant out, it's new, but the pipe has several inches of head over the thermo housing so i can't get away without it.

I also see there are gaskets available, i hate messing with RTV, so would rather buy gaskets, i found the one below, do i need one or two? and if two, are they the same one?

https://smile.amazon.com/dp/B000C823...d=RQGMTWMVW13Q

It needs one gasket, and that gasket in you link probably has adhesive on one side to keep it in place while you mount the thermostat.
No need to use RTV either...

If you think your old thermostat is faulty, test the new one with the old in a pot of water on the stove.

You only need to drain the coolant below the thermostat.

You don't even need to unclamp the upper hose. just leave it in place when you remove the top housing.

I don't know what the click sound is, but it probably isn't the thermostat.

Getting enough water out without draining the whole system will be the trick ;)

As for the click, it really sounds like a piece of metal being forced against another, and finally giving way and being able to "slip" past it, like it's caught up until forced.

Really no idea what else it could be, it consistently happens at the opening time during warm up and it does seem to come from the thermostat area.

A pot of water on the stove will tell all.
Start it off with hot tap water.

About 4 litres of water will do it when draining.

The sound may also be coming from a failing clutch fan _ maybe _ something to look into if it turns out not be the thermostat.

Hmm, funny you should mention the fan, i was unsure if it was operating correctly the last couple of days (it was when i last checked)

I will have to pay more attention to it, last thing i thought it did, was to engage after a minute of running, i'll need to confirm that though.

It's something that would make a metallic click noise, more so then a thermostat.
If you can see it (maybe with a mirror) look at the coiled heat spring in the centre of it, see if it's broken or cracked.
Also crab the fan and move it from side to side, there should be no play and it should be very stiff when cold.

It should be very loud on start up and then quiet down after a few minutes.

my stock fan clutch made no more or less noise when engaged or not. my heavy duty clutch sounds like a jet when i first start

And here we are back to current day, and i was just thinking (as i investigate my fan clutch) if the click noise i heard was actually that instead.

My fan is 'stiff' when cold but it no longer quietens down after a minute or two, no more than the engine revs dying down would cause it to.