I am currently in the process of building my .040" over 400 small block. I will be using Federal Mogul hypereutectic pistons with Hastings moly rings. My machinist told me that since these pistons run a little hotter than regular cast pistons, I should increase the top ring gap to .027". I have not seen this documented anywhere else, not even in the ring instructions. Unfortunately, my pistons did not come with instructions. Has anyone here heard of this? Everything that I have says that the gap should be .018-.020". What experience with this do you guys have?

Also, what should the piston skirt to cylinder wall clearance be with a hypereutectic piston?

That ring gap for your application sounds correct. I just assembled a .040 over 400 and used .028 for the top ring. Check this out: http://www.speedomotive.com/Building%20Tips.htm. Personally, I wouldn't buy any parts from these guys but I think their tech info is good. There's also some good info available here: http://www.federal-mogul.com/sealedpower/index.html.

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Scott
'69 400SB, Richmond 5-speed
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

If you look at the Keith Black web site, it has the explaination for setting such large ring gaps on the hyper- pistons