Whatever you don't don't let the shop cut the head and/or bore, there isn't any extra meat there. You'll need to Lapp the them to assure you have a proper seal.
Here's my write-up on the subject
Okay this goes for the Ninja 500, but I'm sure the Ninja 250 is the same, if not please let me know
The intake valves are case hardened to a thin level of hardness and much in the way of metal removal with destroy them.
Why do you think you need to lap them? if they are worn you need to re cut the seats and fit new valves ,no lapping required. If they are worn badly enough to need anything at all they develop a groove that offself is the detriment to flow. Lapping will make that worse.
Head Gasket repair
Lapping the head
A common failure of the EX engine is, a blown head gasket. The usual cause of this besides the obvious low water or frozen coolant reasons is, warpage of the mating surface between the head and the cylinders. The string of events leading to overheating is:
High pressure gas leaks from the combustion chamber into the coolant system, thereby over pressurizing it and blowing coolant overboard, then the coolant shortage causes overheating which causes even greater warp age, add on infanatum...
As well as the above, any engine that has had the head removed for any reason need to be re lapped. This engine is a very flexible unit because of the cam chain gallery down the middle. The only thing holding the two cylinders together is the very thin wall of the front and back sides of the chain gallery. 1/8” if you’re lucky. This means that the engine works like two single cylinder engines running from a single crank held together by scotch tape. As long as they remain together they are usually fine, but once disassembled they need to be re flattened
The repair:
Unfortunately it is not a simple as replacing the head gasket. Because the “gasket” is not compressible, it is not able to absorb even slight distortion or warp age that may not even be detectable with the usual straight edge type of inspection. The up side of this is that you can re use the gasket.
Tools needed:
A lapping plate; usually a steel flat plate or a pain of flat plate glass. I’ve used my table saw table successfully (the cast iron type)
Lapping compound: Valve lapping compound available at a Auto parts store
The means to wash the parts clean after lapping. A bucket of Kerosene is fine.
The Idea is to use the abrasive to grind down the high spots of the head and the cylinders till they are perfectly flat. See the pictures below.
The process move the part over the surface of the Lapp in a figure 8 motion
Clean and inspect your progress often. Shown is a partially cleaned surface
You are now ready for reassembly. Clean the old head gasket with lacquer thinner and Scotch brute or steel wool. Then repaint it with 1 coat of spray enamel on both sides (1 coat)
After assembly increase the Head Bolt torque to 40 lbs. ft.
Now’s a good time to degree you cams.
Freshly lapped set
Shows the process and the lapp plate
