[Darklen]

Machining the cylinder head pin openings is quite tricky. If you open them up too much, the head can twist in the cylinder. Too much twisting can bind or even break the tappet plate. Ideally, they should be drill open just a hair larger than the gearbox pins so they just fit with minimal torsional (twisting) movement. Front to back movement isn't so much of an issue as the tappet plate can only move so far forward (it hits the gearbox shell) and so far back (movement governed by the tappet nub on the sector gear).