RE: H-9 Suhkoi & DA 85
The new SU-26mm is designed for the DA-85 to bolt directly onto the firewall. The distance from spinner to fwall (or length of DA-85) is 152.4mm. The 33% SU-31 distance from fwall to spinner bp is 187.9mm. So you'd need 35.5mm standoffs behind the DA-85 to mount it in the old SU-31.