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The idea of a DOHC L-Series has captured the Nissan and Datsun enthusiast headspace since the release of platform. Only a few ideas have managed to leave the paper they were conceived upon. Hopefully Ghidorah is an exception to the norm.
The Rundown
The topic of designing or retrofitting a DOHC (Dual Overhead Cam) cylinder head to the L-Series platform has been broached by countless people over the years. Some have tried but production difficulties and the subsequent price tags have reduced a DOHC top-end to a pipe-dream for most. OS Giken has reigned supreme for the longest with their TC24-B1 and TC24B-1Z heads but have to share the limelight now with the new DatsunWorks KN20 platform. For the majority, these options are prohibitively expensive, leaving virtually no options left.
OS Giken
The OS Giken twin cam heads first premiered in the late 70's with the release of the TC24-B1. This head boasted a bespoke cast aluminium design, 24 valves, a refined thermostat design, and obviously, 2 cams.
These heads bolt straight onto the L-Series block but require modifications to the timing chain setup as the the placement of the intake cam is far from the natural chain curvature when running a single central cam shaft. There also appears to be another guide or tensioner bolted to the head. Photos can be viewed here: https://www.tomitaku.com/index_work.html
Some of the defining features of the TC24-B1 head include a single-piece feed-through cam retainers design much like a factory L-Series head as well as screw type valve adjusters.
According to OS Giken, only 9 of these heads were ever made but rumour has it that 11 may exist.
The OS Giken TC24-B1Z is a renewed and revised package when compared to its B1 predecessor .
The main differences include a departure from the single-piece cam retainers and screw type valve adjusters. The TC24-B1Z has a large single-piece lower cam girdle and a number of caps to retain the camshafts, like a more modern setup.
Also similar to modern offerings, the B1Z head now utilises a bucket and shim setup, much like an RB (but with less rocker arms.)
It's unclear if these heads will are part of a limited run like the B1 head as these heads are currently available to order with no apparent limit.
Goerz-Paeco
The Goerz-Paeco cylinder head is the earliest relic of DOHC efforts. This head was purchased at a swap-meet in California sometime in the early 2000's with all tooling required to complete the head and hand drawn schematics for the construction and finishing. The documents pertaining to the construction of this head date the head earlier than an the original OS Giken run of TC24-B1's; making this the very first DOHC Datsun L-Series head.
Despite this, no-one knows where this head came from or who made it. Naturally, one would assume that Goerz-Paeco designed and cast the head but Graf Albrecht von Goerz (yes, that Goerz) was contacted directly and claims no knowledge of this head what-so-ever.
The most notible feature is that the head is entirely symmetrical, depending on cam placement, the head would breath either left-to-right, or right-to-left; which sounds exceptionally helpful for engine bay clearance in Left Hand Drive markets where the exhaust and steering components coincide.
Whether the head was a result of clandestine late night sessions or died on the drafting table due to bureaucracy, its clear that considerable thought has been put into this piece and ultimately it's a shame it was never finished
The head was listed for sale on eBay in 2014 where the auction was pulled before finishing. The current whereabouts are unknown.
Joining the Heads
Three sections, bolted together, and sealed with an o-ring originally, at some point the o-rings were removed and replaced with a traditional gasket type material. Inspiration was taken from diesel or plant engines which have single cylinder, individually replaceable cylinder heads bolted together rather than a large single casting.
Brian had the heads dowelled to locate the segments together. Aluminium blocks were welded to the rearmost cylinder section and threaded rods run the whole length of the cylinder head to a custom timing cover. These threaded rods are used to tension the sections together, sandwiching the gasket material between the individual head parts.
Originally I assumed the sections were cut in 2 cylinder sections so the block to head oil feed located correctly, it was actually due to the thrust bearing location on the KA cylinder head.
Nissan put the thrust bearing in the middle of both the L-Series and the KA. Meaning that 2 KA24 sections have multiple bore center offsets due to having a thrust bearing in each cylinder head section. More on this below.
Oiling
Despite the block to head oil jet lining up an, external oil feed was still used. The KA only uses oil in the cam cap #1 area so a single feed is sufficent. The internally oiled cams deliver oil to the other cam lands and lifters. I believe Bryan used the oil gallery location for head dowels, meaning he couldnt then gundrill the galley for oil. Beyond this, having less internal oiling on a multi-section head reduces the chances of leaks/mixing fluids etc.
I plan to run an external oil feed as well... but... a hollow press fit dowel should technically be pressure tight and still allow the passage of oil. Food for thought.
Bore
The block gave me cause for concern as the KA24 is 89mm. The L28 is 86mm which doesn't sound like a big change; in the world of engine building L28s this is massive. The cylinder wall thickness is less than ideal for big bores which spells for fairly certain death. The 'accepted' figure is 1 in 10 blocks can survive a bore over 88mm, all others would die on the operating table. Even if I manage to bore over 88mm there's no certainty that the block won't immediately shit the bed when pushing power.
Naturally I started hording all the L28's I could find. Hoping that one of them would survive a big bore.
5 blocks later it dawned on me that the KA20 exists in Japan, with an 86mm bore. Same as the L28! I ended up lining up a couple cylinder heads from South Africa but noticed that they listed the KA20 and KA24 cylinder heads as the same thing. To be honest, I figured this was laziness but to be sure I called Nissan and had them double check the part numbers. Turns out the laziness was from Nissan as they just used the same head for 89 and 86mm bores.
To come full circle I was going through Bryan's YouTube channel when I came across this video: "Ll31dett block boring with torque plate datsun 240z 280z turbo nissan l28". Bryan notes how many 89mm blocks he's killed trying to support the DOHC, and now he's decided to go back to an 87mm bore. Facepalm.
At first I was annoyed with myself chasing phantoms but now that I think about it, a bad guy in a mask is much better than an actual phantom. I can't get the time back but I did save myself a lot of pain down the line. The game's the game.
Timing Chain
No clue, the original forum is broken as hell and there are no surviving photos of the timing chain setup that I can find. One of Bryan's posts makes mention of a fixed tensioner that is was described as 'harsh'. I believe the angles are less than ideal. All I know is that the bottom gear was modified from something with the right tooth count, the tensioner is fixed, and the chain is simplex lower and duplex upper. *shrug*
Camshafts
Unlike the timing chain, I actually have info here. They're RB26 cams, straight from the horses mouth. Info courtesy of Majors, the RB26 cams were used as the bore spacing is the same and both engines use solid lifters. The cams have to be gundrilled for internal oiling and cross drilled for lobe oiling. The front cam journal has to be milled narrower to fit the 26 cam retainer. This needs to be done for the actual cap AND cylinder head.
I will not be doing this.
Project Ghidorah
Joining the Heads
I'm aware that there's precedent with the diesel industry having multiple heads either running separated or joined with a gasket, and despite seeing Bryans engine running successfully I never wanted to join the head in the same manner as he did. The issue is that it's stupid and ugly. Having 4 gasket surfaces and gasket seals INSIDE the head seems ridiculous. Pinning the head sections solves the location issue and welding threaded bosses to the heads allows you to compress said gaskets but again, it's just dumb (in my opinion.)
I plan to weld the exterior of the head sections, mill the head studs out to accept a sleeve, and then weld the top and bottom of the sleeves into the head. If water were to "leak" into the un-welded portion of the head cross-section it could move its way around the head stud sleeve, but never actually make its way up the head bolt to the oil, or down to the block. McAdam raised the issue of welding and distortion on the head, despite welding bosses onto Bryans head for the compression rods, and that chamber repair and reconstruction is done on cylinder heads every day. If I have to heat treat and anneal the head so that it's a homogenous piece once completed, so be it. The cylinder head will have 0 chance of leaking due to perishable degradation and it will look like a factory casting.
Access to the top of the head bolt landing is limited and may require either brazing, either by hand or in a furnance. I'd still like to have it welded first and foremost but if it has to be brazed then so be it. The braze melting point is higher than the running temp of the engine, if I'm running the engine hot enough to degrade the brazing then the engine has been destroyed.
Oiling
The following three paragraphs were written before speaking to McAdam Majors (provided guidance and insight to Bryan Blake). I've decided to leave the paragraphs as they provide insight into some of the KA details despite being wrong in their assumption and conclusion.
"For the longest time I could never understand why Bryan Blake opted to join 3 KA head sections rather than just two. As above, Bryan has the front and rear thirds of one KA head on the L6 block with the remaining middle third coming from the middle of a second KA head; resulting in 2 joins between 3 sections. After beginning my own KA DOHC project I believe I've understood why he opted to construct the head in this manner.
The L6 block has two oiling jets, one at the very front of the block, and a second in between cylinders 3 & 4 on the drivers side of the engine. The KA24DE head also has a middle block jet on the drivers side meaning that it is a bolt on affair when using the L4 block (the front oil jet is not used). Bryan has sectioned his head so that the middle L6 oil jet (in the block) corresponds to the middle gallery in the KA head, the only way to achieve this is to use the middle of the KA head (cylinders 2 & 3) in the middle of the L6 block (cylinders 3 & 4); this leaves him with 2 cylinders to populate at the front and rear of the block respectively, hence the 3 sections.
In my construction, I've chosen to have a singular join between 2 three-cylinder sections. While my oil jets don't line up like Bryan's head, I believe the work involved in joining 3 sections compared to 2 far outweighs the simple solution for no internal block-to-head oil passages. The KA head has very clear embossment on the outside of the head where the oil galleries have been gun-drilled during construction. An external oil line feeding from the block to one of these bosses essentially solves the oiling issue without the need for an additional head section. There are two locations of the side of the KA head that can be fed oil as both of these are exposed to the outside of the head and feed directly into the main gallery. The oil supply can be jetted using the fitting internal diameters, much like an internal oil feed jet operates. "
The head sectioning is due to the thrust bearing position (idiot!)
I spoke to McAdam Majors and he mentioned the fact that the spacing between cylinders 3&4 (the middle two cylinders on the L-Series block) is 98mm while the rest of the spacing is only 96mm. The KA platform is the same with the thrust bearing being in the middle, being a 4 cylinder engine means that larger space is between cylinders 2&3, not 3&4 like a 6 cylinder L-Series. Therefore, sectioning the heads in 2 cylinder chunks allows the KA thrust bearing spacing to align with the L-Series thrust bearing spacing; ultimately, no bore offsets.
The finished head will be stacked in the following order from front to back: Section 1 (cylinder 1&2 ), Section 2 (cylinder 2&3), and Section 3 (Cylinder 3&4)
Bore
The chambers on the KA24DE head are 89mm to match the original platform bore. The L6 L28 bore is 86mm from factory which means a total bore of at least 2.5mm is required for the head to be an appropriate application. Having a lip on the top of the cylinder walls will cause issues for the engine due to being a carbon trap, ultimately leading to detonation. Having a reduced bore compared to the head also poses an issue with valve clearance.
An interesting alternative to the KA24DE head is the KA20DE head. The KA20DE has an 86mm bore meaning no boring would be required for use on an L6 L28 block. Obtaining these heads is difficult as they were only sold in the Japanese domestic market.
Update: 25/05/24
This guys^^ makes a lot of dumb assumptions without doing sufficient homework. Turns out that this
Timing Chain
a
Camshafts
a
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