Let’s build the ideal L-shaped tuned head with the latest modern technology. At the beginning of 2021, such a project suddenly started.
An L-type 6-cylinder engine that has been loved both in Japan and overseas for about 50 years. In its long history to this day, there are many wonderful tuning menus that can be said to be the crystals of wisdom left by our predecessors.
The old-fashioned normal L6 with turn flow & 2 valves is a powerless engine that is not suitable for sports cars, but once a tuning scalpel is inserted, it transforms into a different engine that hits 100 to 120 PS or more per liter. It has potential. However, some hurdles had to be overcome in order to carry out the ultimate L-shaped mecha tune.
Among them, the most problematic ones are the part of shaping after applying argon welding to the combustion chamber and the port polishing with the capacity expanded to the limit. They have risks such as thermal distortion of the head body due to argon welding to the combustion chamber, penetration trouble to the water jacket at the time of port polishing expansion processing, and there are few tuners that can complete the processing, and it takes years to complete.
There was also the problem that it took a long time in. In addition, the number of N42 heads in excellent condition has decreased year by year, making them valuable, and it has become difficult to secure a base head, which is one of the reasons for raising the hurdle to tuning.
If so, we would like to optimize the water jacket for cooling by providing a sufficient port wall margin while realizing the ideal port diameter and shape with the ideal combustion chamber shape and volume from the beginning. If such a head could be newly designed and manufactured from scratch, wouldn’t it be free from the above-mentioned depression?
The delusion that could be called such a dream story was nothing more than a rice cake drawn in a picture before the gathering of L head project members. Here, I would like to briefly introduce the basics from the start of the project to the completion of the head, and I hope that everyone will warmly watch over this magnificent delusion.


Accuracy and precision with detailed engineering for the classic L6 engineering.

First Shakedown for JMC L6 head at Twin Ring Motegi on November 21st, 2021.
5axsismachining Simulation


  • To find the ideal combustion chamber shape and volume while setting the head height to 108 mm, which is the same as the genuine one, the thickness of the upper surface of the head has been increased by 1.2mm while 1.2 mm has been removed off combustion chamber bringing valve closer to the piston side. It has been designed with the common troubles in mind caused by surface grinding, such as ones related to cam chains that often occur due to the usage conditions of various commercially available camshafts.

  • After analyzing the material of the normal head, we adopted AC4CH for higher strength and durability, and then applied T6 heat treatment.

  • While forming a big port, the margin thickness to the water jacket is set to 3.5 mm for IN and 3.0 mm for EX (* approximately 2.15 mm only under the IN spring seat), to improve safety against troubles (port wall holes/cracks into water jacket).

  • The combustion chamber is formed by the latest 5-axis CNC machining, and the shape and combustion chamber volume between the cylinders are the same.

  • The port core used to form the port during sand casting has been created by 3D printer to achieve a precise and consistent casting surface and high dimensional accuracy.

  • The diameter of EX port 3&4 has been adjusted to the same size as other cylinders with the aim of making the exhaust flow velocity consistent.

  • The design of the protrusion, that is caused by the escape part of the head bolt, inside of the port has been changed to a smooth shape during the sand-casting process, and that improves the flow rate.

  • Engine hanger mounting bolt hole has been created on the manifold side.

  • Direct ignition coil mount base bolt hole has been created.

  • A compact combustion chamber is achieved while keeping the standard head height at 108 mm same as the normal N42, which makes it possible to build an engine with a high compression ratio specification without the surface grinding being performed. In addition, the risk of gasket troubles caused by the decrease in head surface thickness is significantly reduced.

  • By eliminating the unevenness on the top surface of the head as much as possible, the shape is designed so that unnecessary oil pools are unlikely to occur.

  • After analyzing the water jacket of the genuine head by the detailed CT scanning being performed, a newly designed water jacket takes into consideration efficient and even cooling. The jacket capacity has also been increased by approximately 1.5l, and water channels have been provided in areas where they have not originally existed, enabling even and consistent cooling between cylinders.