Mk 20 piston

[lexvil]

The “notch” may be intended to keep the tech from digging around to get a bite when removing the oring, just a guess.

My question on the high IP is; how many shims are in there?

 

[Perryed]

Only 2 washers. One at each end of the spring.

 

[Oldenboldiver]

That notch on the piston is entirely normal, all mk20 and mk25 (and Halcyon HP75 for that matter) pistons have it. I don't know why.

 

[lexvil]

Only 2 washers. One at each end of the spring.

Take one out and you should settle lower, probably close to 140

 

[rsingler]

The notch was deliberately added to speed sealing of the lower oring against the land. As you can see in the photo, the flat exposes the inner diameter of the oring just a hair, and helps lightly pressurize the cleaning oring above the seal.
Per René Dupré.

 

[Couv]

As you can see in the photo, the flat exposes the inner diameter of the oring just a hair, and helps lightly pressurize the cleaning oring above the seal.
Per René Dupré.

...learn something every day. Come to think of it, there has to be a way to equalize the pressure between the two seals or a relative vacuum would exist.

 

[buddhasummer]

Scroll down to my MK20 post:

Scubapro MK15 serivce

Added: offer taken up, it's gone.