A couple notes about the design of these parts... I have since done a lot of research on air-cooled engine fin design and heat sinks. Maybe I am looking into it too hard for some silly valve covers but its fun. Most of the hard math and physics/thermal dynamic stuff goes a bit over my head, but there is a lot of good 'garage theory' out there around the subject from racers, engine tuners and car guys in general - talking about inter-coolers, oil coolers, heat exchangers etc.
A couple of things I did right: The fin spacing and thickness is absolutely optimal for an engine that is to dissipate its heat in open air (rather than in controlled airflow with a fan and cover) at 1/4'' give or take - which was a fluke, but then again, I did just take the spacing from the existing head so you would expect the Yamaha engineers to be spot on. Plus the rough surface finish is about the best you can get for shedding heat.
A couple of things I will modify for the next prototype and eventual production run: Change the material spec from 6061 to 6063 - it has similar machining properties but has a 20% better thermal conductivity rate as long as the temper is left alone. Plus I think I will increase the internal cavities (volume) of both covers to catch more oil and hold it longer (to give it more of a chance to cool) before it drains back into the head and down to the sump again. I am hoping it will shed a couple degrees with its contact with the covers. The last contemplation is what finish could be best. As said, the raw blasted surface is optimal, but might corrode over time. It is advised to clear/silver anodize the parts to stop corrosion, but it does create a small thermal barrier. Then I also read that anodizing heat sinks black
might increase heat transfer by 3-8% but in some cases it can become a thermal barrier instead of radiator. Either way its nominal and if I do get some anodized in black, it would be more for looks than performance.
Lastly, just when I thought this was an obsession, I found this video of a TW200 running with its valve covers off, and just as suspected, there is an incredible amount of oil being spat out each tappet - even at idle. So imagine the volume at full tick. There is also a really cool video of a V8 hot rod with clear valve covers where you can see how much oil spits onto them and the chance it would get to cool
https://www.youtube.com/watch?v=x6NelDmVCOc
https://www.youtube.com/watch?v=svjytSjd6g0