1985 YZ 125-N 58 mm Bore size

Yes I've had one or two YZ125 from that era, and when I bored the jug I used Cometic gasket specifically made with clearance for the larger piston.

I have two K's, almost identical motor to the N. One has a 56mm bore re-sleeved by Eric Gorr, and the other has a 58mm bore with Wiseco piston. I use the same gasket kit in both http://www.partzilla.com/parts/detail/aftermarket/XP-4142371.html and dont have any problems.
I had the cylinder, head and power valve on the 56mm cylinder all matched up by Eric Gorr. The 58mm cylinder came on one engine and to tell you the truth, i have never checked to see if the head was matched to it. However, i do know that when you bore these engines, you have to make sure you machine the power valve down to suit, otherwise it will catch the piston. I can tell you that i have been running both engines with the same kits and no problems. I fitted ProX rod kits to both engines.
See my engine rebuilds here.. http://yz125k.weebly.com/engine-and-transmission.html

So...Ks rip but the L and N slugs right? Can you make an 84-5 run like a strong 82-83?

Yes....take and put on a 83 cylinder and carb

Thanks for that update nicko. I have added your link to my ebay watchlist. Its interesting, as i use the Athena kit and with my 58mm bore i have no trouble. Perhaps i will rip the head off and take a look, but i have swapped cylinders between both my K's and all good so far.
Cheers,

Did your gasket come in a package that itemized the contents and describe the head gasket in a way that might be different that you notice ? Once I went online to get part numbers for Cometic and it was obvious that the oversized bore required a gasket that had been an option unlike a original yamaha oem head gasket. Maybe thats changed since then.