You don't ride on warped wheels so don't reinstall a warped cylinder head.
It's an oft forgotten check when replacing top ends, bottom ends and just about any metal to metal surface contact on your dirt bike that requires a gasket. Heat, vibration, crashes and time eventually converge to warp the once super clean surface holding two metal parts together.
Every gasket mating surface should be checked for unevenness or warping prior to putting together again. Even slightly off-caliber, two uneven surface heads replaced wear out faster and can cause significant damage to internal parts. What's more, you're not getting appropriate torque on the various screws and bolts because of the inexact fit. So, you're simply ruining those from the get-go and in many respects trying to force a round peg into a square hole.
A cylinder head in need of resurfacing
Resurfacing a cylinder head isn't a hard process though it does take some time and the correct tools. It shouldn't be confused with machining or milling though all three processes accomplish the same thing. Milling or machining is performed using a mill or machine process. Of course it's much faster, but it's an expensive piece of equipment and not something you'd keep handy in the garage. If you send your cylinder head to a local mechanic for machining or milling it's likely sent to another machine shop that owns the correct equipment.
So if you can resurface a cylinder head on your own you'll save a lot of time and even more money.
How to Resurface a Cylinder Head
First, get a smooth flat surface. Not mostly flat or mostly smooth. Your wood workbench with dried glue, practice staples and spilled paint doesn't work. A slab of granite is ideal, a thick sheet of flat glass works, as does a stainless steel countertop, maybe a concrete block (though it tends to chip) - anything hard that's perfectly flat without nicks, abrasions or any other imperfections that might hide warping on the cylinder.
Next you'll need several sheets of heavy duty sandpaper with a superfine grit like 320. Grab a lubricant like WD-40, silicone spray, or even Chain lube.
This power valve cover can be resurfaced
With even pressure, drag the cylinder head around the lubricated sandpaper in a Figure 8 pattern. Depending on the warp severity you'll have to drag it around for several minutes or much longer. Expect to burn through a number of sandpaper sheets. Lubricate constantly. Check your results by laying the cylinder head on your chosen smooth flat surface sans the sandpaper.
Any metal-to-metal surface on your dirt bike that uses a gasket can and should be resurfaced anytime you remove the parts. Like anything that's married, once that bond is broken you need to start fresh. That includes replacing the gasket too.
Resurface the following parts if taken apart:
- Crank case
- Crank case cover
- Cylinder head
- Power valve cover
A metal face featuring a recessed O-ring set-up should not be resurfaced but perform the above recommendation if the attaching side is flat.