I have always used this thinking on the OHV. The Opel heads are good because the chambers are unaltered from the 993 through to the 1196. SO…all one has to do, is decide on your final bore size and open the chambers as shown in Part Two to match the selected bore. Then lop at least 2.5mm off the head and settle on a chamber volume that will give you the best static compression ratio*. That’s it…don’t lets get complicated. For the VM, the chambers are slightly bigger to start with but the same applies. If you take a look at the angle of entry the inlet port has to the chamber, there is not much to be gained by opening the spark plug side of the chamber. Now I know some tuners do so…but…I prefer to keep the chambers tight in this area and focus the “opening” of the chamber on the floor-to-wall radius laterally and opening the shallow end of the chamber.
The key comment above is the removal of material off the cylinder head face (AS NOTED IN Part 2)….and in combination with the open radius chambers, improves flow dramatically. All this does however…. is bring those valves closer to the pistons….so that brings us to a method of cutting the pockets I have used since day one of this exercise ….and that goes back to the early 70’s
But before we get there, here is something that comes as a ‘freebie’ for valve/piston clearance when doing a good valve/seat job.
On a stock cylinder head the height of the valve-head above the combustion chamber floor can be as much as 2.5mm. This is a combination of Valve Margin thickness, Valve Seat dia relative to the Valve dia (much smaller) and no Valve Seat Top Cut (30 deg). By reducing the valve Margin Thickness, Increasing the seat dia to within 0.2mm of the valve dia and a 30° top cut, this drops to just 1.2mm off the chamber floor….so we get a ‘free’ 1.3mm extra valve/piston clearance before we touch the cylinder head face. Now we need that because in order to get our all important squish measurement to 0.8-0.9mm we need to remove 0.5mm from the block deck…to lift the pistons above the deck face… that is with the use of a 1.3mm compressed cylinder head gasket…..and assumes we are using a basically stock Opel or VM sub assy…. So we’re still on the good side to the tune of 0.7mm
The following very basic layout of the parts in question gives an idea of the key issues in keeping flat top pistons
and the depth of valve pockets.