The problem is unless you pay an engineering facility to mill them out properly, its a bit of a job to get a good result with DIY equipment.
They are 100mm long as per standard Rover V8.
They replace the standard Rover 40mm steel stacks with 38mm bore with
these 44.4mm aluminum stacks with 41.4mm bore which equates to 16-18% increase in cross-sectional area.
I can adjust the height by varying their length or adjusting the base depth.
Once fitted just blend them into or fully port your manifold to match.
I tried using a pillar drill and hole cutter, but they are designed to cut holes with a center bore drill and not widen existing holes.
This is a work in progress and I will have to make a jig plate that bolts under the plenum base to give me center hole for the cutter. Then I will replace the drill center with a location bar so the drill does not widen the hole and loose my tolerance.
The nearest hole cutters only comes in 44mm, but they usually cut a slightly larger hole due to chuck oscillation on my budget pillar drill, so it should work out.
Due to the wider bore the flares overlap so the center 4 need need off-setting in height slightly.
Once the seat is cut the center has to be bored out to match. It could be filed, but I will probably flip over the jig plate and pillar drill it from the other side using a 41mm cutter, then finish it with a flap wheel.
The standard Rover plenum has varying clearances over the top of the velocity stacks ranging from 13 to 20mm. I decided to adjust the clearance to a mean clearance of 18 to 20mm. All the stacks are on different centerlines, so they need to be treated individually instead of in groups in the standard setup.
I will update this page as
this modification progresses.