The transfer port size is critical and it's best to leave it alone. Once you have modded it it is very difficult to put right again. Best leave this sort of thing to the likes of Gerald Cardew and the Venom team.
Just fitting the Venom kit will give a greatly reduced recoil on its own. Get the rifle Lazaglided rather than tinker with the 'port. If you do intend to go ahead with a port mod, read G.Cardew's The Airgun from Trigger to Target and his earlier one to The Airgun from Trigger to Muzzle first and get some advice from Venom or John Bowkett or David Welham at Airmasters.
In the 70s I remember people opening up the transfer ports on HW35's to 4mm in the hope of more power.... most times it increased the recoil, reduced the power and knackered the accuracy. Proceed with caution, especially with a good, scarce rifle like the '85.
Hope you like the sight!
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