Recommended Posts


Cast iron block and head, pushrod valve train, two fairly small valves per cylinder - hardly state of the art in these days of multivalve aluminium engines that develop 50% more power and weigh 2/3 as much. But there are still plenty of crossflows out there giving reliable service in Ford tintops and kit cars and built right they can go pretty well. The crossflow cylinder head was actually the first I ever did flow development work on back in the 1980s. After I designed my first flow bench, a colleague in the trade who was also a flow specialist came up with a couple of unmodified head castings which just happened to be crossflow ones and we had a bit of a competition to see who could get the most flow. Over a period of several weeks we tried just about every combination of port shape and size, valve seat width and angle and I think it's safe to say that we pretty much exhausted the potential of the crossflow head casting by the time we finished. I also used to get sent quite a few modified heads to flow test by a local rolling road operator several years ago. He was always being offered second hand ones for sale and he used to send them to me for a flowtest and based on that he'd decide whether to buy them or not and how much to charge when he sold them on. So I have a fair idea of how well heads by Burton, Vulcan and a few other firms actually flow. I don't see many crossflow engines in the workshop these days though although unleaded conversions, valve seat jobs and the like crop up every now and then. Copyright David Baker and Puma Race Engines

The crossflow was the successor to the pre-crossflow and was similarly a "modular" engine. The bore stayed the same size (81mm) right the way from the 940cc engine up to the 1600. All the capacity increases were achieved with progressively longer stroke cranks. This made the small engines very oversquare - almost like bike engines really. Given that you could use exactly the same big valve head on any engine size you could build a real screamer out of the smaller versions and engines revving to 10,000 weren't uncommon. This guide will be confined to the 1600 though because if you're going to fit a crossflow engine you might as well start with the biggest capacity you can get. Copyright David Baker and Puma Race Engines


Compared to more modern "thinwall" cast blocks, the crossflow can be bored out a long way to gain capacity. 83.5mm gives 1700cc and 85mm gives 1762cc on the standard 77.6mm stroke crank. 85mm is pushing the limits though at 4mm oversize and bore porosity is a possibility if your particular block is a bit on the thin side. Pressure testing after the block has been bored or x-raying might be advisable if a really big bore is being considered just to check that nothing is about to break through into a waterway. Capacity does relatively little for peak power though (see other engine technical articles on this site) although of course torque does increase and the rpm at which peak power is generated drops in proportion; which may mean you can avoid expensive steel bits in the bottom end to cope with the higher revs which a smaller capacity engine would need to run for the same power output. The standard piston is available in oversizes up to 0.090" which gives 83.25mm. The 83.5mm BDA piston (AE part number 20372R) is supplied with an unfinished flat crown and can be machined for a bowl volume as desired which is ideal for compression ratio increases in race engines. It is only a cast piston but is the Powermax design - very sturdy and pretty much unbreakable.The 85mm oversize piston (AE part number 18954KR) has a finished crown and is a forged piston. It is still available but horrendously expensive. No doubt there are other options available and I don't profess an encyclopedic knowledge of the crossflow parts bin compared to engines I build more of. Copyright David Baker and Puma Race Engines

One problem with the Heron design is that the pistons are very heavy. Because most of the combustion chamber volume is in the piston dish, the piston crown is enormously thick and the dish is over 1/2" deep. Also the valves open into deep piston cutouts (which form part of the chamber volume too of course) and so flow is shrouded to an extent. On big budget race engines it is possible to machine a shallow chamber into the head itself and use more conventionally shaped, and lighter, pistons. The debate has raged for years as to which is the best way to go. My feeling is that if the debate has raged so strongly there can't be that much in it or a decisive winner would have emerged more easily. It's certainly a lot more work to do all this of course.

The other problem with the Heron design is that the combustion chamber shape that results doesn't burn very well and a consequence of this is the need to run a lot of ignition timing. As a technical aside here, the less ignition timing an engine needs to run the more power it generates. Ideally you want to ignite the mixture just as the piston reaches TDC so all the energy from the burn is pushing the piston down on the power stroke. Ignition advance is required because the mixture doesn't burn instantaneously on any engine so the spark has to occur before TDC in order to get the mixture well alight by TDC and some of the burn is wasted in trying to push the rising piston back down the bore. However, we're only talking a few percent difference in power between good and bad chamber shapes so it isn't worth getting worked up about when there are more important things, like head flow, to concentrate on.

The standard crank and rods are fine for road use, as with most engines, but the parts are obviously getting old nowadays and will have seen a lot of miles. Crack testing isn't a bad idea for such old cranks although I have my doubts about the process to be honest. Most cast iron parts have some minor cracks in them and seem to run fine regardless as cast iron isn't very "notch sensitive" so the cracks don't tend to propagate. I think you can consign a lot of cranks to the scrap bin after crack testing which would have carried on working for many years to come. You pays your money and takes your choice though. If you hold a crank up by one end and tap it with a hammer you can tell by the sound if there is a major crack to worry about. A good crank will ring like a bell and a bad one will sound flat and dead. No help if you're tone deaf of course :)

If you have the budget, and of course the engine builder, to be able to achieve 100 bhp per litre or more then you'll need to be thinking about 8,000 rpm or so and steel crank and rods and maybe even steel mains caps too to stand the strain. That's also the way to go to get longer than standard strokes and even more capacity but it isn't really cost effective in my opinion other than for all out race engines.


The port design is actually pretty good - round and fairly straight and with a reasonable downdraft angle so the flow per square inch of valve area can be brought up to pretty high levels. Being a Heron type head there's no chamber shrouding to worry about either. For the engine size though (in 1600 trim at least) the valves are small. The 1300 and standard 1600 had 38.2mm inlet valves and 31.5mm exhausts. The 1600GT engine had 39.2mm inlets and 33.7mm exhausts. As a very minimum go straight to these sizes for a modified head because the increase in valve area is well worth it in power terms. Most modern engines have much bigger valves to start with. The 40mm inlets of the VW Golf , 41.6mm in the Peugeot 205 and the 42mm ones of the CVH engine for example. The first common oversize for the crossflow is a 41.3mm inlet valve which is available off the shelf in OE material with a chromed stem for very little money. It fits nicely with the 33.7mm exhaust in any head and makes the most cost effective solution for any decently modified engine. The problem with going much bigger than this is the spacing between the inlet and exhaust valve. The specification figure here is 1.54" (39.1mm) with a few thou variation depending on how well the tooling at the factory was set up when the head was drilled. This measurement limits how big the valves can be before they touch. Copyright David Baker and Puma Race Engines

Add the radius of the inlet valve to that of the exhaust and compare to this datum and you can see what the nominal spacing between the valves is. For example with the 41.3mm/33.7mm setup the combined radii is 37.5mm which leaves 1.6mm between the two valves. Go much thinner than this and the "bridge" of cast iron between the valves tends to crack in severe use. In fact even at 1.6mm it's a bit thin for comfort. You can certainly squeeze in a 42mm inlet valve or even a tad bigger before the two valves are in danger of actually touching but how long the head will last is anyone's guess. There is a solution though for more radical stages of tune. The valves run directly in the material of the head itself in 5/16" drillings - there are no separate valve guides as standard. When the guide holes wear out it is a standard machining operation to drill and ream the holes oversize and fit guide inserts - either cast iron or bronze. The normal guide insert used is 7/16" O/D (11.1mm). When this operation is being done it is possible to offset the new holes and space the valves further apart. Just what we need for an all out race engine. It means some additional work to space the rockers out to compensate of course and to machine platforms for the valve springs to sit on properly.

With offset guides you can fit 43mm or 44mm inlet valves in the hole but then you run into the next problem - getting the port and valve throat machined out big enough to suit the big valve without breaking through the casting into a waterway. To an extent you are in the lap of the gods as to how thick the head was cast and whether there was any core shift or not. I've seen other specialists charge £2,000 for an ultimate big valve crossflow head because of the work involved and the chance that you'll scrap a head or two before you get one that's cast thick enough to do the job. There's certainly no point trying to get a very big valve in the head if the ports aren't opened up to adequately supply that valve with airflow or it simply becomes a matter of the port itself becoming the restriction to flow rather than the valve.


Just having a quick glance at a crossflow head can be incredibly deceptive. Anyone who has a bare casting to hand go and have a look at it. Which are bigger, the inlet or exhaust ports? It looks like the exhausts are tiny and the inlets are much bigger right? Well, wrong I'm afraid. The inlets have a big chamfer at the manifold end which can deceive the inexperienced eye until you actually measure properly. The exhausts are round and about 27mm in diameter but look small. The inlets are slightly oval and a tad deeper than they are wide and look big - but they are only about 27mm wide and 28mm deep - almost the same size as the exhausts. A 2p coin at about 26mm diameter is a handy guide to the port size and you can see that it fits in both inlet and exhaust with about the same clearance all round. Copyright David Baker and Puma Race Engines

Now go and read the first article on cylinder heads and you'll see that if an inlet port is not to restrict its valve it needs to be up to 75% of the valve diameter for road engines and 80% for race engines. It depends on port efficiency, intended cam lift and other factors but it's a good guide. The crossflow inlet port is therefore small even for its standard 38mm or 39mm valves and to supply a big valve properly like the 41.3mm one it looks like we might potentially need to open up the ports to nearly 31mm (75%) for road tune and 33mm (80%) for race tune. That's not even considering the ultra big valves mentioned above. Whichever way you slice it that's a lot of cast iron to grind out. If the head were aluminium it wouldn't be so much of a problem because high speed carbide porting tools go through aluminium like a hot knife through butter but cast iron is a different animal. Grinding it out is hard, dusty work and it wears the tooling out pretty fast because it's a very abrasive material - and carbide tooling isn't cheap. Anyone got a clue where this is leading yet?

Yes - you've got it in one. It's easier to stick a flapwheel up a crossflow port and make it nice and shiny than to actually stand there for hours laboriously opening the ports up to the right size and shape to work properly. More so than almost any other type of head you buy off the shelf you find that crossflow ones are rarely modified to anything like their full potential. So very few engines make anything like the power that they could make, but we'll come back to this further on.

To get a big valve to work properly we need to open up the ports by at least 3mm or 4mm in diameter for a top spec head. That's more metal removal than is required on almost any other big valve head you care to mention and it being cast iron just adds to the workload. Compare for instance to the Pinto which has 36mm ports to start with which are actually too big for most valve sizes and hardly require touching. It takes an awful lot longer then to do a crossflow head properly than a Pinto one so you might be forgiven for thinking that crossflow heads cost a lot more money. The fact that they don't mean that either there are very nice people out there doing all that extra work for free - or that extra work isn't being done. Copyright David Baker and Puma Race Engines

But port walls are only cast a few mm thick anyway so it isn't just as simple as grinding out all that metal even if you do want to do a crossflow head properly. Even using the 41.3mm valve means you run pretty close to finding water if you start pushing the porting "envelope". Start thinking about even bigger valves and offset guides and you can maybe start to see why a state of the art head costs so much to do.

Unleaded Heads

It's well worth thinking about having the head converted to suit unleaded fuel while any mods are being done. To achieve this all that's necessary is to fit steel seat inserts on the exhaust side. The standard valves are fine and don't get badgered into changing them for "higher" spec ones at more expense because it is totally unnecessary. The way the insert is fitted is pretty critical though. Too tight and eventually the head will start to crack, too loose and of course they can drop out in service. The seat and port throat should also be machined so that the inserts blend properly into the original casting and decent 3 angle seats are a must. In fact when an insert is fitted properly it's hard to tell it's there apart from a minor change in the colour of the metal. Just about every engine machine shop in the country has jumped onto the unleaded conversion bandwagon but not all of them get it right so that it becomes a reliable long term solution. It takes time and very accurate machining to do the job properly and I've seen expensive ported heads turned to scrap by bodged unleaded conversions. Don't go for the cheapest quote - get it done by someone who understands exactly what's required and that can vary from head to head. Copyright David Baker and Puma Race Engines


Cam in block, pushrods and 1.5 to 1 ratio rockers up at the top - all pretty conventional. The rocker ratio is a good bit higher than on the even older Leyland A and B series engines and so generates pretty decent valve lift from the usual cam profiles. Higher ratio rockers are available but not worth the expense other than for race use. There's certainly no shortage of cam profiles to choose from but most people are familiar with the old xx4 series that have been around for ages. The 234 is a nice road cam, especially with DCOEs, but is still fairly long duration and not really what you'd want in heavy traffic. The 244 is too hot for road use but people are often too greedy with cam choice and end up regretting it later. Because the cams all cost the same, people think they might as well go for the hottest one they think they can get away with. Bad move - when in doubt always pick the next cam down in the range. The extra tractability will mean more to you in the long run than the few bhp right at the top of the rpm band that you hardly ever use. The 244 doesn't really do much below 3,000 rpm, especially with a big valve head. Anything higher than the 244 is strictly for competition use.

With the higher lift cams you start to run out of clearance in the valve spring area. Springs can go coilbound and the underside of the spring cap can hit the stem seal or the top of the guide. There's also a peculiarity of the crossflow head casting to watch out for. The valve spring seats are often machined such that they gradually sit higher at one end of the head than the other - an error in the tooling setup on the production line. Take a ruler and measure from the rocker cover face to the spring seat. At the thermostat end you might just find that the spring seat is 1mm closer to the rocker face than at the other end of the head. That means you have 1mm less clearance before the valve spring goes coilbound and of course the fitted length of the springs and therefore the spring loads are all different on every valve. If this isn't corrected when the head is machined you can run into real problems such as wiping out cam lobes and breaking springs. If you've ever had an engine that just kept wiping off cam lobes at one end of the cam now you know why. On a properly modified head you need to machine the gasket face, the valve seats and the valve spring seats true to the rocker cover face. This ensures that every valve and spring has the same geometry and clearances. With high lift cams the spring seats and the guide tops need machining deeper anyway so this error hopefully gets corrected as a matter of course. If separate guide inserts are being used then the depth they get fitted to will vary with cam lift to ensure no contact with the valve spring cap.

If you have to use double valve springs because of the cam lift then make sure the spring caps are properly machined to take the inner springs. If you don't do this then the inner spring just seats on the tapered nose of the spring cap and eventually the top coil gets forced apart and bursts. If it happens at high rpm and you end up bending the valves when the spring breaks you'll curse the person who built the engine and saved this particular ha'porth of tar. Copyright David Baker and Puma Race Engines


I wonder each time I write one of these guides how much point there is in listing potential power outputs because I know you're all going to go and compare them with what everyone else claims (but rarely produces) and end up thinking they are easy to achieve. You hear about plenty of 200 bhp crossflow engines but you never see any tested. There was an all steel full race one in CCC magazine recently with that much claimed power (recently being spring 2001) which ended up making 162 "flywheel" bhp on the rolling road if I remember correctly - and it might not really have had that much but only an accurate engine dyno would have told. I trust the supposed flywheel figures from most rollers about as far as I can swing a cat that's hanging on to a grand piano for grim death but that's another story.

As for road engines it seems people think all you need to do to get 140 bhp is fit a 234 cam, DCOEs and "modify" the head a bit. Hmmm. It's easy enough to buy a particular cam, carbs and anything else that comes with a part number and is identical wherever you bought it. The power at the end of the day is largely down to the head flow and that can vary from little more than standard to 25% more depending on how well it was modified. An average engine in the above spec makes about 110/120 bhp if you're lucky but on a really good 41.3/33.7 head that's been modified by an expert you can see close to the 140 sure enough - mid to high 130s anyway. Maybe another 8 bhp or so with a 244 but you'll get fed up of the lack of low down power on the road. Any tractable road engine with more than 130 bhp is pretty good actually compared to most of what's out there. It isn't really worth talking about the ultra big valve heads because hardly anyone has the money to afford one and anyone sensible would start with a better engine if they wanted that much power. Give me a CVH any day to be honest - as a road engine, properly modified, they eat crossflows for breakfast and cost much less per bhp to build and weigh less too. It takes a good big valve crossflow to make the same power as a good standard valve CVH which given the size of the standard CVH valves shouldn't come as much of a surprise. As the CVH was really the spiritual successor to the crossflow engine it might be instructive to do a point by point comparison of their relative strengths and weaknesses and if I get time I'll write one. Copyright David Baker and Puma Race Engines

Share this post

Link to post
Share on other sites