See also 86-90 Oil Filter Mount Parts, Specs and Dims in the Evo section of the Sportsterpedia.

The oil pressure regulator in the 86-90 pump can malfunction from time to time.
Having the regulator open prematurely is not optimal since you want full oiling capacity at all times (i.e. at hot engine idle). Reports of the valve leaking in the closed position and age on the regulator spring weakening it have prompted some to block the regulator off entirely.

Below are some mods to accomplish this if so desired.

The regulator valve is suppose to keep the oil pressure from getting too high. ¹⁾
See more on the Oil Pressure Regulator in the Evo section of the Sportsterpedia.

The oil pressure relief valve is factory set to open between 30-35 psi.
The stock spring can get weak and open too soon which will dump oil into the cam chest prematurely instead of where it is needed. ²⁾
And your low oil pressure light will never go off.
You can replace the stock plunger spring with a 5/8“x2” spring of heavier gauge from your local hardware store.
This will render the plunger inoperable as there will now be too much oil pressure needed to overcome the increased spring pressure.

³⁾

With this mod, you can render the affects of the pressure regulator inoperative while leaving the plunger and spring in the filter pad.
Normally, when the regulator opens to purge excess pressure from the filter pad, the oil is routed thru a hole in the pad and into the gearcase.

Oil pressure discharge hole 4)

If you drill and tap a plug or even a set screw in the discharge hole, the oil cannot pass through it.
This will shut (or plug) off the pressure regulating valve. This puts the oiling system in the same unregulated state as the 92-and-up sportsters.

Hole drilled, tapped and plugged with an 1/8“x27 NPT aluminum allen head plug 5)

This mod is reversible (as long as the plunger and spring are not thrown away).
They can both be left in place since without flow into the discharge port, it doesn't matter if the valve is opened or closed.
No oil will be diverted.
However, if desired, you can simply remove the plug and the regulator will be unrestricted as was before this mod.

This is a way to preserve the function of removing excess oil pressure from the filter pad and keep the gearcase safe from over oiling from the practice.
This mod has to be done in conjunction with blocking off the regulator discharge as in above.
Else the excess pressure will still be dumped into the gearcase.

The pressure bypass oil is diverted directly to the return hose near the oil pump with a “Y” coupling.
The clear hose in the picture below is only a temporary line to monitor the oil level in the sump during dyno testing. ⁶⁾

In addition, the plunger was seat was tapered. ⁷⁾