If you're not familiar with the WPC process, I'll describe it based on what I've learned from their website:
WPC treatment is not a coating that can wear off, the process results in a micro-dimple surface that reduces friction and strengthens the product. They "fire" an extremely small media at parts extremely fast, to compact the surface at a microscopic level. These dimples then serve as oil reservoirs that hold oil in place when the equipment is not in use. This process is similar to shot peening, but instead of using beads, the media resembles a fine powder. This results in a surface treatment that increases strength through thermal discharge, sealing up minor surface cracks and tool marks from machining...without a resulting dimension change. The surface becomes condensed with improved density, and this overcomes the issue of brittleness associated with "hardened" parts.
One of the things I was really surprised by was the packaging, individually sealed and branded parts bags! Now I had seen some pictures of this packaging on the internet before, but only for a new set of treated gears. I had no idea they gave every product this special treatment, however, it's completely necessary to protect the gears after treatment from corrosion since they're now bare metal. Make sure you package your gears well when you send them to WPC because they send the finished product back to you within the same materials.
Once you open the packaging, the first thing you'll notice is superior cleanliness. WPC does mention that you need to thoroughly clean the components yourself to remove any possibility of leftover media. After cleaning the parts you also need to apply a layer of lubricant so that no surface corrosion can form. This is really no different than handling untreated parts, any bare metal part will corrode if left unprotected.
I did a lot of research on what commonly fails in the C60 6-speed transmission before finishing up the Cusco LSD install. The last thing I wanted was to put a new differential and bearings in this box, then have to worry about launching or adding power and having a failure. After posting my first few C60 videos the comments started rolling in, and they all had a similar context. Google, YouTube, Facebook....they all yielded the answers I was looking for; 3rd and 4th gear. Most people break third gear in drag racing situations, apparently wheel-hop causes these kinds of breakages. I measured all of the gears in my WPC prep video and found that the third and fourth gears were thinner than all of the other gears. The 5th gear set was 15mm whereas 3rd and 4th were 13mm, so I skipped treating 5th since 1st and 2nd are also 15mm yet never have an issue.
When I was searching the far corners of the internet for every possible C60 failure, I came across a few images of broken output shafts. All of these instances again involved drag racing. I believe the issue is due to the physical size of the pinion teeth since the C60 has the most aggressive final drive ratio of all the C-transmissions. The easiest way to distinguish a 3.9FD from a 4.5 is to simply look at the teeth, the tooth profile in the above image may look identical but did you notice the 4.5 on the right has the bearing race removed? The 3.9 shafts have 8mm of tooth vs the 4.5 shafts only having 5mm(refer back to the first image also).
I chose to only have the known weaknesses of my transmission treated since I'm not wealthy, but you can have all of your components done. To get the 3rd and 4th gearset, and output shaft serviced, with shipping, tax, and insurance it cost me $182. To me, that was a steal for all of the benefits you will gain. WPC advised me beforehand that it would be "about" $500 for all of the gears, both shafts, and the ring gear. Keep in mind though that shipping will be more due to the increased weight.
I don't know anyone personally that has had any gears treated by WPC, nor do I know anyone that has broken a C transmission. Like I mentioned, I wanted to feel at ease that I've done everything possible to ensure this gearbox survives abuse. I was also excited to see the WPC results myself and run my own test. Overall I am extremely happy with the whole experience, customer service, attention to detail, and an awesome end product. Huge thanks to Izumi Ogawa for streamlining the process. You can see my video below: