Strange issue with valve lash

[jpnl]

Hello all, new to the forum but a long-time lurker

I am normally able to figure issues out and this forum is such a good source of knowledge which is usually always a good help in getting stuff fixed.
But this issue.. I can't figure it out and really understand why this is happening.

I have a Kymco Super 8 4 stroke with a 52mm bbk, 24mm carb and intake.. NCY 52,4mm head with big valves and top racing vario with 6,9 gr roller weights.
Speed is about 70 km/h.. accelleration is good.. all in all runs good.
But..

I set the valve lash of the exhaust to .004 (0.1mm).
Intake to .002 (0.05mm).

I start the scoot and there is the usual valve ticking sound but very quietly. I let it idle till the rpm slows down to the normal idle speed.
Then I start riding.. the engine I guess heats up and after a 1-2 kilometers I get more valve ticking sound.
At first I thought that this could not be valve sound.. so i went looking in all other places where this sound could be coming from but it is really the valves. 
The exhaust valve in particular. 

I set the valve again to .004, to be sure.. went riding for a bit.. came back home and measured again with the engine pretty hot it measured .005 and was even able without a lot of force to push in the .006 stick. I let it cool off and it was back to .004.

Decided to try .003 on the exhaust and after riding it was .005ish when hot.
The sound at this setting is a lot better than starting it out with .004 so for now i left it at this but whats going on here..?

Before I had the 52mm bbk i had a 47mm, i did not have this issue, .004 and .002 was always fine and no valve ticking to the point it becomes annoying.

What should I do? leave it like this?
I have already replaced the oil pump to be sure that my head is not getting super hot due to not getting oil, tho i test the oil pump couple of times before replacing and it was spewing oil.. but i thought that if the engine gets hot that it might not work so well as the oil gets thinner.. so i replaced it anyway. I am using 10w40 btw.

Any ideas?

[pinkscoot]

You should always set valve lash on a dead cold engine, let it sit overnight. When the engine warms up the valves get looser that's why you got the higher reading. Are those the factory specs for the Super 8?

[jpnl]

Jun 8, 2019 14:52:57 GMT -5 pinkscoot said:

You should always set valve lash on a dead cold engine, let it sit overnight. When the engine warms up the valves get looser that's why you got the higher reading. Are those the factory specs for the Super 8?

Thanks!

I always thought it was supposed to get tighter as it warms up.
I understand it expands as temp increases but I always thought it is the valves that expand more than the head and valve train.
And yes it is the setting that all scooter dealers recommend here in the Netherlands.

[jpnl]

Actually now that I am thinking about it..
I have this head www.monsterscooterparts.com/product-brands/ncy/nyc-cylinders/racing-big-valve-cylinder-head-gy6-qmb139-engines-ncy.html

(hope its ok i link to a site like this)

I vaguely remember reading somewhere that the valves are of a different material than usual.
If that is true than these valves have a different heat expansion coefficient.
I am trying to find the source where i read this.

Edit : Can not find the source, and after researching a bit more I don't think that is it unless the valves are made of titanium.
Possible but not likely.