Great point about the cam bearings. I disassembled the engine and dropped it off at the machine shop.  They will hot tank it and check the bearings.

Dogscarf

if the front cam bearing is installed the wrong way it would flood the rocker cover with oil

Has this engine been apart before for cam bearing work? or rebuild work...
  Thoughts that the cam bearings were installed wrong blocking passages  creating pressure.

 any other history of rebuild to change something to create excess pressure?

Engine is out now, but as mentioned, have you tred a different gauge?

 

I have tried two different plungers and currently have a ball in there.  I have also tried two different springs.  I have also tried the adjustable unit.  I still had high pressure at 100+psi.  I had to remove the engine anyway to fix the leaks and install a new syncro.  The second gear syncro was of dubious quality and didn't last long at all.  More info when I when I the beast all apart.......

Dogscarf

OK, I stand corrected but the plunger type can become lodged and damaged on the outside diameter and seating surface if debris has circulated in the engine. If it sticks open, too much oil can go to the sump and rob the engine of needed pressure.

Buy this https://minisport.com.au/mini-oil-pressure-relief-ball-cooper-s-type and https://minisport.com.au/mini-oil-pressure-spring-original-cooper-s

or buy this kit for Cooper S's up to 71. https://www.minimania.com/part/AEA536KIT/Oil-Pressure-Release-Ball-And-Spring---Cooper-S

Sorry.. but the ball is NOT an original feature... And I’ve never (sic) had an issue with the factory plunger/spring setup.. 

And, there’s no need to go to any real trouble with adjustable nuts.  A few copper sump drain plug washers under the standard cap (between the cap and the block) will drop the pressure if its too high ... still too high? add a couple more. (If its too low you have another problem).

Cheers, Ian

Keep the ball. It is original to the Cooper S motor. Make sure the spring is AEA536. No one uses the plunger, get a bit of sh1t caught between it and the bore and it will jam open.

EDIT: high oil pressure has nothing to do with oil leaks.

In the search window above type in adjustable oil pressure and look at the results.  I believe it was Keith Calver in MiniMagazine that did a tech article on how to build one.  

Agree with Kermy and Alex.  Ditch the ball bearing, replace it with the plunger and correct length spring.  Some recommend lapping the plunger to match the seat.  Then try shortening the spring and/or try a combo of washers under the domed nut (which effectively shortens the spring). For what its worth, I tried using the adjustable oil pressure valve, didn't like it-couldn't stop it from leaking around the threads.

Some of the new chains are junk.
The IWIS single chain is supposed to be of good quality or convert/replace the front cover with a later one that has a timing chain tensioner.
I would have tried getting to the bottom of the oil pressure problem by maybe installing an adjustable oil pressure part before pulling the engine.

The pressure relief valve originally had a bullet shaped plunger not a "ball". All things being equal, putting in a ball bearing results in high oil pressure. You need to need to shorten the spring or put some washers under the plug.

Makes me think of that Traveller with S engine that was on BaT last week.
Driving along with a warm engine the pressure showed 100+.  I asked why so high, "shows how healthy the engine is"

Did you try a different and calibrated gauge before taking the engine out & apart ?

Fellow Minioids,  I pulled the engine of my 1968 Cooper S Mk2 this weekend.  The oil pressure was too high at 100-120psi.  Prior to this, I tried a new spring and ball in the front of the engine to no avail.  The engine leaked more oil than the Exxon Valdez.  Anyway, whilst dismantling the engine I noticed that the brand new duplex timing chain had 3/4 an inch of travel to either side.  WTF????  Time for a better quality timing gear set.  Anyway, what could be causing too high of oil pressure?  Some swarf in the oil galleys, the oil pump not lining up right, the oil pump gasket being blocked?  Thoughts?

Dogscarf

1968 Morris Cooper S
1965 Austin Cooper
1967 Morris Cooper S Aussie