This is one area where the game is won or lost and an ongoing discussion point with tuners who continue down the route of small diameter inlet ports. No tuned OHV looking for real horsepower  (I refer mainly to the ‘bigger’’ engines 1196 & 1256) should be running with inlet ports smaller than 34mm at the head face…. And…. where this profile is run down to open-up the port throughout the run…

On the rather scarce pre ’66  993 or 1057 cylinder heads this a very straightforward task, requiring no reshaping.



This is the work done on the 993 Project cylinder head. On the 993 and 1057 engines a 32mm Inlet port is perfectly adequate for a powerful road spec engine and this work is easily accomplished ….simply open up evenly and blend radii. This port size will in fact be adequate for 85- 90 Bhp on these ‘small’ Little Blocks with the right goodies.

Firstly….The pic above showing the 34mm Ports on the 1273 road engine illustrate along with the Videos in Part One, that large inlet ports do not reduce engine response or low/medium speed Power.  The simple fact is that the clips show just how astonishing is the mid-range acceleration…along with the remarkable high rpm power….even at altitude…where it is vastly more difficult to generate Inlet Charge Velocity


On the race head one can imagine the resistance I had from colleagues to open the ports to that size. The trick however is not in the port alone. I will get to this work on race spec Cylinder head prep in the next series.

Every now and again you need to tolerate my direct comparisons with this engine and the BMC ‘A’ because there is general appreciation that a full-house ‘A’ at Vizard levels of prep, will consistently produce over 100Bhp/litre …Yet… This machine, which is a far more progressive design, is somehow weirdly regarded as having less potential. That is rubbish…. the only thing stopping the Little Block from matching an ‘A’ BMC and in my opinion having superior overall performance is a lack of focused development time and the boodle to get things done. Fact is, with the same level of $ and time the ‘A’ has had in the system over the years, I have absolutely no doubt this machine would emerge as technically superior.

AND HERE IS ONE OF THE REASONS THE LITTLE BLOCK FINDS ITSELF IN THIS SITUATION – THOSE SILLY ‘POST 1966’ INLET PORTS….as well as a GM being fast asleep at the tiller. Just imagine what we could have done if there were just a few engineers willing to risk their careers to produce a couple of special castings …..

As a result…on the 1078/1196 Opel and 1159/1256 VM, there is lots more work required by opening the ‘flattened” mid section to the inlets and whilst not ideal, can be opened to give effective flow. Work on that section nets far greater improvements than simply opening up the port dia at the head face. The main restriction is at a point around 30mm into the port…See Pics

A SOBERING FACT IS THAT THE STOCK 993cc 28mm INLET PORT WITH SMALL INLET VALVES (31mm) FLOWS ABOUT 10% MORE AIR THAN A 31mm 1196 INLET PORT WITH LARGER VALVES (32mm)  This is with both inlet valves fully open at 9.0mm Lift. Please note that this comment is for the 1962 to 1965 993 component, as the later model 993 heads used in the Corsa are identical to the ‘choked’ 1078/1196 items. At lower valve lift and part throttle the newer heads have better flow and swirl, so these do the job well on a standard engine….The point is that as you can see from the various in-car videos that the open port (993/1057) design works well enough at low rpm and light throttle…and…gives a massive advantage to response and power throughout the rpm band.

 Here’s the practical issue…. If you take a stock 993 cylinder head, open the inlet ports at the head face to the same size as the 1196 (31mm) and fit the 32x28mm 1196 valve config, you will have an immediate 6-8% increase in power compared to the 1078/1196 cylinder head. The same applies to the VM engines using the 1057 part.

 All is not lost however, because the later ports can be opened to flow reasonably well. It is important to ensure that work is done on that section which is most restricted, from about 30-35mm into the port. In opening this area the final shape will be different to the 993 port, however this makes a substantial improvement to flow.

The following pics perhaps demonstrate the difference more clearly

My frustration in getting this info to you is the fact that only by physically viewing the two different port configurations on the parts themselves can one see the true difference. The 993/1057 ports are beautifully contoured and easy to ‘open’ without reshaping being necessary. The race engine head shot above shows just how radical you can go on the Opel 993 ‘head. That Port size at the head face is 36.5×35.5mm and slightly oval due to the exit angle of the port.

Final note and here I do not wish to sound like that schoolmaster…but if the inlet port work is not done and the Ports opened significantly …especially on the newer heads….you are wasting time, energy and money on the rest…rather go and play darts or go fishing….that’s how strongly I need to make the point.



This is the method I use ….and the use of two stone types in opening the upper and mid-section of the ports

1- Stock port…note the closed upper and lower portions of the opening.

2- Start by opening the upper and sides of the port as viewed in the pic…target 34mm However open the port evenly around the radius to achieve an initial 33.5 diameter during porting, leaving 0.5 for final finishing.
Sides and upper portion as viewed must be opened vertically (straight down) into the port. This includes removing the ‘lump’on the upper section and running this smoothly down to the bifurcation. Do not polish the inlet ports, leave the rough grind marks from whatever stones/cutters you use …it is the size and shape of the port that is important….leave the surface finish unpolished

3- Lower section of the port as viewed in the pics…remove the ‘lump’ in the port and finish at the natural inlet angle of the casting.

4- This is what the completed head-face or ‘top’ portion of the port should look like at this point in the process….roughly 33.5mm dia with sides and upper section as viewed in the pic at vertical entry…. blending into the ’floor’ of the port at the natural cast angle


5 – Opening the throat happens at the angle depicted in pic #5. Using the long stone shown, continue opening this section progressively and evenly in a circular motion.

6 – Keep the grinding tool at this angle and progressively open the port until you see a ‘tunnel’ forming down the port. Continue opening evenly until you achieve a port diameter of around 26.5mm

7 – At this point switch to a ball shaped stone and blend the port entry sections into the ‘tunnel’ section

This port shape/size will double the area and please see the note below on further work…At this point the port is 33.5mm at the head face and a final clean is required to get to a perfectly round 34mm port. We will cover the shaping of the throat lower down near the valve in Part 4.

The pics above are basically what was done back in the day on Road Spec cylinder heads for 1159 and 1256 VM engines. I have not done any further work on the restricted port design to optimise flow for ultra high power and Race Spec engines as all my more recent work has been on the 1962 to ’65 993 castings. We will be doing that over the next few months to see whether we can match the flow of the 993 packages. There is a school of thought that with really clever shaping techniques one could achieve superior flow performance….we will see.
However the point is that if there are any experienced tuners out there who have done the hard yards on these heads in more recent times …I’m sure readers would love to know what works for ultimate horsepower….


We have covered the fact that valve sizes are less critical to performance than most of the other head work. If one gets the porting and combustion chambers ‘right’ then increases in valve sizes start to make sense. Here is a list  of recommended valve sizes relative to bore dia

72 – 75 mm           993 – 1057 – 1078cc                    33 x 28

78 – 81 mm         1159 -1196 -1256c c                     34.5 x 30

82 – 83 mm         1289 -1320cc                                 35.5 x 30.5

The stock 34.5 X 30mm valve sizes used in the larger VM engines are perfect for a good road engine….no need to throw $ at this by increasing valve sizes…..however….reprofile the valves  as best possible compared to specially made valves shown.  For the Opel you can use the VM bigger valves…for road car applications only ….because many of these valves have butt welded stems. However there are both Honda and newer OHC Opel engine valves that can be used. (I recut the cotter groove where necessary to enable the use of the stock valve spring retainer for Opel applications.). NOTE…this work, along with the blending of the port into the valve seat area, is critical to good air flow. No rough edges…

A note on the VM engine goes back to the 60’s where I could never understand why VM went to the 34.5×30 valve size with the restricted inlet ports on the 1159 and the 1256. On stock cylinder heads I never saw a measurable improvement over the smaller valved heads. Opel got this one right by keeping the smaller valve configuration on all engines and never suffered a power deficit in Bhp/litre terms compared to VM.


Don’t forget…just look at the torque/power curves in Part 2….it is hugely telling that the stock valve sizes can produce cylinder filling and a flat torque curve right up to 6500Rpm where the stock engine starts losing cylinder pressure at 4000Rpm. We will talk about the role that big valves play in Part 4.


This is a bit of a bugbear for tuners of the OHV and unlike the BMC and Kent crew where well tested special parts grow on trees (expensive trees but nonetheless there are a lot of them) The Little Block has very restricted special parts availability.  In the Piston department there is practically nothing except for a few pop-up forged pistons.

Now at this point Dr Clark will get to being a little insistent….and at the risk of upsetting the pop-up piston crew lets have a look at the Little Block’s big brother…The Small Block Chevy.. If we look at progressive development of the Small Block (now in its 67th year) the availability of aftermarket cylinder heads has yielded a simple but powerful message…..SHALLOW COMPACT COMBUSTION CHAMBERS WITH HEALTHY SQUISH AREA WORK BETTER THAN THE OLDER LARGE CHAMBER DESIGNS REQUIRING POP-UPS …..and these able to achieve good compression ratio with the use flat-top pistons. Piston suppliers have responded and provided a comprehensive range of valve-pocketed performance parts to do the job.

Now there are variations on the above with lateral slots  across the piston as opposed to pockets …as well as mildly raised flat sections in the shape of the parent combustion chamber….all very satisfactory….we will look at just how similar the two engines are in this area when we move to more powerful road spec engines.


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.

Firstly,this is a worst case scenario running big valves (35.5) and maximum removal of material from the cylinder head. In earlier comment I noted my preference in running a milder camshaft for road applications and that is because the trade-off resulting from best Squish Clearance, Open Chamber radii / 2.5mm off the cylinder head and smaller valves is vastly better than any benefit from a ‘wild’ cam profile along with the biggest possible valve sizes. Being moderately conservative allows the use of stock pistons which in turn allow valve pockets to be cut to a depth of 3.5mm giving the workable compression ratios of up to 11:1 and avoid the use of pop-ups.

In Part four we will look at how we decide on the right static compression ratio for the job….and each and every engine build requires a simple calculation to establish the Dynamic Compression Ratio of the completed assembly in order to establish the static value…..that is if you want an engine that responds like crazy to commands from your right foot.
I know I mention this at every opportunity but the random selection of cams, head mods, manifolding and stock ignition advance curves make for engines that are way off the optimised pace.

I recently had a chat with a very well informed engine man who was stunned at the fact that I was able to run an 11.6:1 compression ratio on the 1273 road engine….and here’s the point… doing this on 95 (RON) octane fuel and a static ignition timing of 18° BTDC through to 30° @ 3800Rpm…With No Knock.

The key to this…..? Decide on a camshaft first…then build the engine around that. 

The Cutter is made from a valve 3mm bigger in dia than the installed valve. This gives the required clearance and minimises the amount of material removed. I use dedicated sizes for inlet and exhaust for that reason.

So why do I use this method….a number of reasons besides the cost saving!…Firstly in using flat tops there is always the necessity to achieve good compression ratios. So this method allows the exact placement of the pocket on a particular engine. That in turn allows the minimum dia for the cutter which in turn removes the least amount of material to get the clearance required. Believe me in the early days searching for good compression ratios on a 993cc race engine with flat tops one learnt every trick in the book!

Here is what was needed for the 993 Project engine…a light pocket cut for the inlet valve only


Most folk do not realise how critical valve throat and seat work is to getting good low valve lift air flow. It’s difficult enough to flow good air when things are wide open but attention to detail is the only way to get that critical air flow at low lift …this piece of the puzzle is crucial in helping to keep bottom end engine response in place.

A basic three angle valve seat /throat works for inlet and exhaust on a road spec engine. The drawing below gives an idea of how it should be done.
On race spec engines this is more of a 5 angle job with an irregular shape to the throat… but we’ll get to that….

30° very light Top Cut Approx 1.0mm
45° Seat (Inlet 1.2mm, Exhaust 1.5mm seat width)
60° throat cut …

Use the full dia of the valve. The outer edge of each seat to be just 0.2 mm
smaller than the valve dia.

Throat diameters to be between 85 and 87% of valve size.


Once complete, Take a strip of 400Grit sandpaper and using your forefinger, rub the seat area in the direction of airflow , crossing the three angles. Do this radially around each throat. This breaks the sharp edges created by machining and is the very simplest instant low valve lift air flow improvement you will find in tuning an engine anywhere. Modern valve seat cutters do a good job by doing the cut in one pass ….but the sandpaper trick is still worth doing. Ideally one would want the area to be like the ‘bell’ on a trumpet with the valve seat area just wide enough to seal on the valve and transfer heat effectively.


We will talk more about Valve Throats, Exhaust porting as well as Camshafts next.