The one i have here is 3mm depth....the port is 5mm so 8mm total, closer to the 7mm of the 77 than the 80 really as the 80 is 11mm or so.

Its pretty obvious that a TP around 10mm or less is what we need, the 80's is still pretty damn efficient, i feel its starting to be pushed on the 95/98 at 15mm.

Barry...you really need to shoot a 25mm sleeved down 52 in .22 with 70mm stroke...way way different to a normal 52. Im doing a 75mm stroke .177 next
Also you need the full fac spring in the sleeved down guns to get the power, consider if I removed the sleeve, reinstalled the 28mm piston I would be pushing 24fpe thru this D52.

105mm stroke 28mm piston is around 65CC
70mm stroke 25mm piston is 34CC

The D52 has near double the amount of swept volume needed to make 11fpe in .22 in standard trim, this should tell you how lazy the spring needs to be with the 28mm piston....not good