PRD Fireball Shop Manual/Correcting Poor Head Seal

Introduction

Don't attempt to reinstall the head if you have any doubt that it may not be flat, or after having overheated the engine (mild or severe). Instead, verify the flatness of the head surface. To do so, you'll need a "known flat" surface - ideally a ground piece of marble, or a machined metal plate will also work so long as the tolerance is acceptable (up to 0.001").

If the head has deposits from the coolant passages, you have no choice but to lightly sand those down before checking for flatness; use the following procedure to remove the deposits, don't attempt to sand them by hand.

Know the Rules Before You Proceed

Before altering original dimensions of any part that may be scrutinized, know the applicable dimensional minimums and maximums. For the 2012 PRD 125cc Controlled TAG engine, the minimum cylinder height from machined surface to machined surface is 86.80 mm.

To measure cylinder height as described above, place the jug upside down on a "known flat" surface. Take a caliper, and insert the depth measuring implement into one of the bored holes for the head studs (the long studs that run through the entire jug and the head). Carefully hold the gauge as normal to the jug base as possible, and stop when the caliper just touches your "known flat" surface. Your reading is likely to be a around 87 mm.

Flattening the Head and/or Jug

Have a flat surface, a fat (chisel tip) Sharpie, and an assortment of wet-sanding paper in 200, 400, 800.

1. Using a chisel tip Sharpie, mark the entire (flat) surface.
2. With 800 or higher grit paper laid upright on your flat surface, run water over the surface, over the paper, and place the paper on your surface, ensuring all are clean and free of debris.
3. Gently sand and the surface on the paper, applying even force by grasping it centrally. Sand in a circular motion for a set number of revolutions. Repeat the same number of revolutions in the opposite direction. Rotate the part 180 degrees, and repeat the process.
4. Observe the Sharpie marks. If the marks are fading reasonably evenly (without any visible pattern), your surface is quite flat. If the marks are clearly removed in a region of the head, but are fairly bold in other regions, your surface isn't flat. Follow the next step if your surface isn't flat. If none of the Sharpie was left, you sanded too firmly or too long. Try again.
5. Use at least 400, but preferably 200 if you need to remove at least 0.001". Using the same patterned sanding technique described above, sand until you can observe the cross hatching from the sanding process across the entire surface. Sand more firmly, but ensure the sandpaper remains flat against the "known flat" surface. You shouldn't need to hold the paper down to the surface if you have enough water involved; the surface tension of the water will keep the paper firmly in position. If the paper is moving, you have debris on your surface(s), or your "known flat" surface isn't very much so.
6. When you're satisfied that the surface is flat, step your sandpaper up one level i.e. 200 to 400, and sand for long enough to smooth out the rougher grooves from the coarser paper.
7. Repeat the process in the previous step until you've achieved a surface that is polished to your liking - you can literally do this forever, so don't go overboard unless you're more interested in sanding then racing. :)

Note: This procedure is an alternative to machining or "skimming" to achieve a flat surface. Not only is this procedure easier and requires no machinery, but it is also illegal (in most cases) to machine your head. Using a mill or lathe in an effort to remove as little material as possible likely requires added tooling and time consuming fixturing effort; sanding removes only the material required to get a working seal of the head.

Perfectly flattened jug.

Super close up of a perfectly flattened jug.