I know which bearing i need for my headset but the only measurement i cant find out is thickness of cartridge. As 6.5 seems to be the smallest should i just go for that and will it really matter if it should have been 7 or 8mm?

Generally, yes.

So its vital i find out the thickness first. Its taken me long enough to find out what size and chamfer angle that i think i might have created a new record for the amount of time it takes to replace 2 cartridge bearings. Some days building a bike is a right pain in the posterior.

Title might be a bit misleading but its the thickness of the cartridge that bothers me not a bearing.

Whats the headset, most decent manufacturers will list them

Its a Ritchey WCS but thats all i have to go on, its a press fit (semi integrated),it could be a logic zero stack but certainly cant find this particular model for sale which is confirmed by Westbrook cycles. Its only the top cartridge that needs replacing and i am trying to avoid a whole new headset.

Quite often I find a new headset is cheaper and less hassle, even before 3 failed attempts at getting the right bearing.

I know which bearing i need for my headset but the only measurement i cant find out is thickness of cartridge

This generally changes along with the angles of the bearing faces, 36 deg, 45 deg etc, so it needs to be right.
The easiest way (I find) is to accurately measure the outside diameter of the bearing and go from there. Bear in mind that there’s (at least one) 52mm diameter and a 51.8mm diameter so you’ll need callipers to measure. This is a lot easier when you have a selection of new bearings in front of you, I’ll admit…. 🙂

I went through something similar trying to replace a Specialized integrated headset. Does your bearing have a part number on it? My bearing ended H8, meaning a height (thickness) of 8mm.

I managed to tap out the existing bearing using the square bar of my set of clamps! It was a real struggle as there wasnt much bearing to get at and it was rusted in good and proper. Cleaned it up but there no markings at all so i have measured all the critical stuff and its 7mm so i am sorted now, what a pain.