Motorcycle cylinder head conversions


Hi guys, it was brought to my attention that there is a kit available to convert vw polo engines to a hyabusa cylinder head, such motors are often seen in hillclimb cars here in Germany. There are 2 companies that produce the kits, however one has closed down, further research found similar kits available to convert mini engines to a BMW K1000 head. This got me thinking, maybe there is a possibility of fitting a motorcycle head to a daihatsu engine. The GSXR 750 has a similar bore size and pitch as the K3-ve, and when photos of the corresponding head gaskets are overlayed the bores, head bolt pattern, and coolant galleries appear to line up unbelievably well. A GSXR750 head in original form should easily flow 150hp, not to mention the vast array of aftermarket cams, intakes, headers and so on. Second hand heads are available on ebay for as little as 80euros. After investing some thought into the subject I am thinking the k3 timing cover could be easily modified, timing sprockets would be fairly simple to adapt with some lathe work, an older style carby intake could be used leaving the factory daihatsu ecu in place to handle ignition, eliminating the need for an aftermarket ecu. The biggest hurdles would be the larger coolant galleries on the front right corner of the k3 block, and ofcourse oil supply (possibly and external line could be used) and oil return/drainage.
Its definitely a cool idea, but what I dont know is, which direction does a motorcycle engine turn? Does the firing order match or would custom camshafts be needed? And what galleries are for oil supply on the k3 block and gsxr cylinder head?

Hopefully someone could help fill in the missing pieces, and if it looks like it could work I will look at making up a template from the head gasket and checking it against my old motor.



Here’s a quick ramble. What would be the benefit of the bike head? Much of that extra power is from the huge revs, but they don’t have a great deal of torque. What are the combustion chamber volumes? Which flow better? BMW K1000 head on an A series BMC works because they are three port exhaust and two port inlet - very old tech. I don’t know much about the Polo, my Girlfriend’s daughter has a 2006 and it is woeful to drive and pulling the big plastic cover off the engine did not inspire confidence that what was underneath was worth playing with. The gap between the Gixer and Dai designs are prob not as great as those in your examples. I’d want to look at a cross section of both the K3 and the GSX. The water galleries lining up is not a big deal. Many can be plugged and there is benefit reversing the flow on older engines so water goes through the head cooling it and then going into the block. Some engines do not have enough balanced cooing flow and can benefit from forcing coolant more to the rear of the block before coming into the head (complex stuff when you do this and run lots of temp sensors). And some open deck blocks (Honda or Subaru) are closed up a heap to stop bores cracking under huge boost without to cooling issues (I guess good oil cooling helps the block equally) to the block, the head needs the cooling focus. I can’t see the oil feed hole for sure (one of those holes on the LHS in the image?). But you could run an external line/s from block to head. Oil return would be more of an issue and I would say those top right of the image are the main return. Getting oil back to the sump quickly is a big deal. Firing order on a Gixer cylinders are numbered 1-2-3-4 from left to right sitting on the bike, firing order is 1-4 and 2-3, what’s a K3? Engine in the chain driven bike spins the same direction as the wheels turn, so looking at the flywheel it is clockwise. With so much effort to do the swap, if the firing order was wrong, rather than new cams to match the order you’d just go with an after market ecu.

A lot of work. I’d put the effort into porting a K3 head and making an ITB manifold and some headers.


The main reasoning behind the idea is availability of aftermarket parts and the ITB’s, if it was to fit without major work then it may be worth while. As for the design, I dont have any photos of a k3 head cutaway but I have this photo from the gsxr

The intake ports look much better IMO, much more vertical, whereas the k3 intake ports are much closer to the base of the head and I imagine the curve would be much sharper coming from almost horizontal at the intake to almost vertical at the valve. But head/port design is by no means my area of expertise, it could very well be they are not so far apart flow wise, I really dont know.
With the oil drainage, were you talking about the top right of the photo of the block?

I think that is actually for coolant, because it is directly above where the water pump mounts.


Coolant, gotcha. I guess oil returns via the front. If the measurements work you are the guy here would get this going it will be you. Do not consider me a naysayer, but rather just chucking out problems solving thoughts. It’s way outside anything I’ve done or considered though.


Your input is always welcome, im just toying with the idea at the moment. It would be great if it was do-able, but there is a bit of uncertainty as ive never had the head off a k3 and so I have no idea where the oil feeds ect. are located, but I think its something that could be worth looking into. I need reasons why it wont work, otherwise I wont be able to get the idea out of my head and I will have to do it :sweat_smile: