Ebor Bikes
https://eborbikes.com/ have sold these all over the world since 2013. We used to supply them in 0.7mm thickness but now we use 0.9mm copper sheet. They are accurate, laser cut, and annealed in high vacuum so they are as soft as copper can be got. See the website for more details. They can be reused if re-annealed.
I like Julian's suggestion about asembling the rocker box on the head and the using a torch to look for any gaps.
Like others, I have also concluded that the head steady on the S/A models may cause the rocker box to move slightly, destroying gaskets. I have a RGS on which I put a head steady with a hard rubber bush interposed, this has a copper rocker box gasket which is oil tight (last time I looked).
On engines I work on, I often recut the rocker box joint face by about a thou or two by
1 Mounting the rocker box, right way up on a Brideport mill, and milling the 4 seatings of the long rocker box bolts to the same height. I use the end of a 20mm carbide end mill.
2 Mounting the rocker box upside down on the bed of the mill, with spacers accurately of the same thickness underneath the machined seatings, holding it down without distorting it (use clock gauge on corners and check that when tightening down, you're not distorting it (this is easier said than done, as the rocker box is certainly not rigid when being treated like this!).
3 Use the end mill used in step 1 to take a skim off the joint face such that the tool has just touched the rocker box joint faces all over. This shows that the sides of the exhaust part of the joint face were often distorted very slightly.
I do have a rocker box where I used a CNC mill to cut a groove right round the joint faces, 1.1mm deep and 2.1 mm wide, to accept 1.5mm O-rings or O-ring cord, but my health has now declined to an extent where I have't yet built it into an engine.
Hope that helps,
Jon
