This is how the water guide looks when the liner, water guide sleeves and head studs are in place. The AT25 uses an "O" ring between the water guide and the block while the AT27 does not use an "O" ring but has the water guide lapped to the block to ensure a water tight seal. The cylinder liners on both the AT25 and AT27 are lapped to the water guide and do not use any type of gasket between the liner and the water guide.