516 heads with double valve springs ?

You must machine that "step" against the guide flat with the larger diameter of the spring seat and you need to machine the guides to the proper diameter to get a seal in there. .630 is a popular size to machine the guide to. The 1.550 x .630 guide cutter will do the job. I had to machine the guides and mill the inner diameter of the spring seats down on my slant 6 head.

Having said all that, I do not recommend machining the main spring seat diameter DOWN into the head for any reason whatsoever. Some of the heads in that area are very thin as is and you may mill right through to the water jacket. If you need extra room for springs, the correct way is to go UP with the retainers with offset locks and or retainers or use longer valves.

I've had people argue me into oblivion about that, but that's how "I" do it. I will NOT mill the spring seat deeper into the head. NO NO NO.

Here is what my slant 6 head looked like after I machined the entire valve spring seats flat and enlarged them to 1.550 for 440 springs, but I did not machine DOWN into the head.

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