I'm running 10mm of lift in my Gen3 which fit with no modfication to the head.
The head castings are definately different and I didn't think you'd be able to run that much lift in the vast majority of the 185 heads due to the cam hitting the outside of the head casting on the oil gallery. There were some changes to make more room with the later 185 heads and with the 205 heads. I also wouldn't want to run that much lift on shim overs as they are likely to dig into the edge of the cam, the 11mm cams barely fit onto the shim under buckets.
I'm not too sure if the shim over keepers have a different profile on the bottom allowing more clearance to the seal and guide.
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