...1.6" ?? I think You might have Your Numbers Confused. Did You mean 0.016" ? Or 0.160" ?

It would be Cheaper and Faster to Just Purchase the Correct Set of Springs. AND You could go with a 'Bee-Hive' Design, the Latest Technology.

You could also go with Longer Valves (Stems), But that would Drastically Change the 'Rocker Arm' Geometry! Perhaps THAT is Exactly what IS Needed! In the Photo, It looks as if You are already Set-Up to Check the Geometry.

Machinists will Add 'Spring-Seat' SHIMS, to INCREASE Spring Pressure, and to Protect Aluminum Seats, from Steel Springs. Milling the Seats, to Decrease Pressure, is Not Necessary and is NEVER Done, in My Opinion.

MJ

Peter I had springs with 120Lbs seated and 390Lbs/Inch and it was NOT enough to prevent valve float probably due to the heavy (lots of inertia) hydraulic roller lifters. Valve float occurred at 5200 rpm. I updated with 155Lbs seated and 427Lbs/Inch and revs to 6500+ rpm without floating now. I use a Lunati wodoo hydraulic roller cam fyi and full synthetic oil 5w-30 and standard flow/press blue printed oil pump.

The reason I chose the 120/390 springs from the beginning was my concern about the lifters collapsing but that proved to be insignificant in this case. And the tech support from Lunati Cams advised me that they recommended the 120Lbs but obviously that was a load of BS and caused me lots of headache in the troubleshooting.

So don't be afraid of going with 150lbs seated and of course use stiff push rods and valve geometri to suit.

On a note, had I known what I know now I would not have chosen hydraulic lifters but gone for solid rollers in stead and just done an annual valve play adjust ;-)

There is more to solid roller lifters than adjusting every year.  You need to replace them every 3-4000 miles.  The lash take up beats the needles rollers to death and then the roller wheel is no longer participating as the roller body digs into your cam.  You made the correct choice.

 

No problem milling your heads for the seat depth you describe.  I've done it multiple times.

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