The initial plan was to change as little as possible in the head and reduce the amount of work required. Then, while disassembling the engine, the cause of the low compression in one of the cylinders was found to be a valve not seating properly. So the valves had to be reseated and this was all done by my local engine shop.
After thinking about it hard I also decided to change cams so I would only need to remap the engine once and not go back once I changed them.
The challenge with the head has been caused by the new parts and work carried out. The reseated valves sit higher into the head which means that smaller shims will be required, and additionally the clearance for the new Tomei cams needs to be higher as well (from 0.025mm on OE to 0.15mm, according to Tomei). This has meant that I need to run quite small shims, much smaller than is available by Nissan. Ideally the seats should have been replaced on a high mileage engine such as this one.
The first challenge was actually measuring the shim height required when an undersized shim of that size doesn’t exist to measure the clearance. So I decided to assemble the rockers onto the valves with no shims at all, but the rockers had to actually be modified to clear the spring retainer, as shown:
I could then assemble all the valvetrain and measure the clearances with a feeler gauge. An additional problem was the hydraulic lifters; I had purged them all of air before installing so they were rock hard, but when I installed the cams and rotated them to check the clearance between the various lobes, the high lift of the cams seems to have pressed some oil out and they went soft, making an accurate clearance measurement more difficult. I am not sure if they are supposed to behave like but I rebled them when I put the new shims in.
Modifying the shims was made easy with the use of the lathe. At work they had some great kit which allowed me to grab onto a very small edge of the shim and machine it down to as thin as 0.69mm.
The additional problem was the lowered preload of the valves; I checked the spring stiffness limits and compared this to the reduced preload and found that I am cutting it fine. If my springs were midway between the standard limits then my reduced preload will still yield a force higher than the lower limit, but I haven’t measured the stiffness of the springs so that is a bit of an unknown. Many lessons learnt.
With the new shims in the clearances are pretty close, maximum of 0.30mm with a target of 0.20mm. Hopefully after a bit of running the reseated valves will settle and close that up a little.
Also installed the Rocker Arm Stoppers from Apex Performance.