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@Jung
Got on with the heads. I made an arbor to hold them in a collet which made keeping everything square a bit easier. I lapped the head on a surface plate then dressed the plug hole on the mill. They’re cut for a bit less than 50% squish area. The target volume was 22cc, which is a bit of a cock to achieve.
There’s not enough meat in the head to cut a single radius dome, so you end up with a series of changing radius, spherical segments. I got relatively close with some calculations, but the last bit required a lot of stopping, and measuring the head volume with a syringe and bit of Perspex.
Not perfect, but I think they should work well…
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