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97 civic HX K24a
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What is the max vtc degree for this skunk2 ultra 4 camshaft in 12.5.1 piston?
That combination sounds like it needs to be clayed for piston to valve clearance @ different vtc points to determine the max. pistons can come with a 12.5:1 ratio but have different valve reliefs /dome heights and design. It will take a few hours and cost a little to get the stuff you need from tractuff but it will save you a headache later if you over advance. You could “probably” use a stock RBB 25 gear and be fine.
 

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Arouse the DAMPFHAMMER!
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What is the max vtc degree for this skunk2 ultra 4 camshaft in 12.5.1 piston?
Depends on the valve relief depth and valve size As well as application (torque curve shape related ex cam phase). It is definitely limited, where, depends on above mentioned.
 

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Mahle K20 pistons can have their intake valve pockets deepened by 1mm.
p2v is also different for K20 and K24 due to different kinematics.
 

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97 civic HX K24a
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Sorry, I misread. The same would apply , I would feel fine running a 25 gear without checking anything but you could be missing out on valuable vtc range and lose power by limiting it. I would take the time to clay the engine and check your clearances. It very well could be fine up to 40 degrees or so and that would be night and day for the midrange.
 

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Arouse the DAMPFHAMMER!
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Estimated compression Around 13:1 ? Those pistons have quite the dome. Would the k24a head increase CR further ?
Yes, it's 12.8:1 with an 5 ccm dome, which would give you 11.0:1 in an 86x86 engine. The head volume is almost the same for all castings, around 50-52 ccm if they are fresh, clean and un-cutted. Honda uses different dome volumes and quench heights to adapt the different combustion approaches, from the eco to the performance applications.

One issue with the stock piston is it not well known to have an deep valve relief height. About 1.5 mm less then a Supertech 12.5:1 piston. With such huge cams it is very likely that the VTC won't be optimal, which would be already the case with extra deep valve relief depths, but here just even more retard will be seen. Finally the scavenging will suck in the midrange and power comes up when the redline was already passed. The issue is, those pistons can't handle the heat adequate, what will be needed when spinning to the optimal breathing speed range. They will crack or size the liners a bit bigger (depending on piston to wall clearance). Those cams are for really big displacements (102-108 mm stroke and bores well over 88 mm), to reduce the optimal engine speed range down where the engine is capable of. I would see an S2 U2 much better for this setup.
 
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