STEP 1 Take it off the car and to a machine shop
When you first take a head off the car, it’s going to be pretty cruddy. Take it to a shop and pay to have professionally cleaned. Most shops are "green" and don’t chemically strip parts anymore, they bead-blast which produces a cleaner look.

P79 fresh shaved .080"

• Look in the yellow pages for a machine shop that is familiar with “race” or import work. Stay away from places that “specialize” in truck heads or fleet work as they usually won't be interested in your performance project. I never walk into a shop in khakis and a polo shirt. If you want to be taken seriously and get a reasonable price, show up in jeans and a t-shirt. It helps to familiarize yourself with head “lingo. Tell them you want the head beadblasted, surfaced, and a valve job done. It should run about $200.

All P79/90/90a heads cam with hard steel valve seats from the factory, so it's doubtful you will need a one replaced. Most valve guides are usually ok as well , but trust the shop if they tell you something is needed. All Zcar heads come with "3-angle" intake seats stock but its worthwhile to ask them for a 5-way valve job on the intakes if you like...a good shop can give you as many angles as you want. The Nascar crowd is into "radiusing" which creates a smooth intake seat rather than an angled one. Radiusing creates a very thin line for the valve to contact and is said to produce a finicky seal though. Exhaust seats generally get by with a single edge seat.

Good shot down the intake runners

Once you get the bead-blasted head it home from the machine shop, inspect the valve bowls, you will see rough casting marks and ridges from the factory. Using medium emery paper or a dremel tool, and cutting oil (WD40), spend an evening smoothing out all these imperfections. Most importantly, smooth the transition from the intake port to the underside of the valve seat. Usually the shop won't have done that.
Don't go haywire grinding the exhaust runners larger, but by feeling with your finger tips you can sense ridges and imperfections that need to be smoothed out. Hardcore racers and the V8 crowd really ream out the ports, but that kills low end response. So just a basic cleanup. Be careful not to nick a valve seat.

HERE'S THE FUN PART: Matching the intake manifold. On carbureted engines bolt the manifold to the head finger-tight. Then with the carbs off look down the carb throats of the manifold with a strong light. You should be able to shift the manifold around slightly to find the best position. Once you find it note the “overlap” between manifold and port edges.

• - Take it off, and using a dremel tool with a sanding band, grind away the manifold edge where it overlaps the manifold port. Be careful not to take too much off, but with care you should be able to produce a perfect transition between the two. I find the stock and Fel-Pro intake/exhaust gaskets difficult to match because of the metal ring insert around each gasket port.
  
  - Here’s a tip to improve the durability of the head's manifold bolt holes: Install 8mm x 1.25 x 40mm double-ended studs in all intake and exhaust bolt holes. This lets you tighten away to your heart’s content. Very recommended.
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STEP 3 INSTALLING THE VALVES

• For the ultimate in sealing, even on new valves, "lapping" produces a perfect mesh between the valve and seats. If you’re unsure how to do this watch some videos online, it’s easy. Always install new valve seals when you have the valves out. Use this time to check the keeper grooves on the valve tip to make sure there are no burrs.

CHAMBER DESIGNS

The filthy chamber below on a the left is a N42/N47 head. These “open” chambers were designed for dished pistons/emissions and provide little quench with 8.3:1 compression. The cleaned circumference below the valves is recessed about 2mm. The entire chamber is circular. The E88 is similar in design.

The one on the right is a P79/P90/P90a chamber. It's actually larger/deeper in volume than the N42/47 at 53.5cc. Compared to an open chamber design you can see how the flattop piston "quenches" the mixture into the sparkplug as it reaches the top of it's stroke. This is a pretty standard shape for most modern heads. The photo is after shaving the head it .080", I measured it at 46cc in volume. Note the extra water hole on the bottom right vs the N47.

I didnt want to spend a small fortune to replace the pistons with higher compression domed versions. And I'm not sure if competition pistons were designed to fit the P79/90 combustion chamber. This left me with a dilemma, what to do? The easy route was to shave the head.

Some say that OHC heads are not supposed to be shaved because it adds slack to the timing chain and throws off the timing. On the L-series the chain tensioner can remove some of the slack, but it doesn't restore the correct link-distance on the driver side of the timing chain which is more important.

The solution is to shim up cam towers after shaving the head to remove chain slack. But this then means the lash pads l need to be replaced with thicker ones. This is fine if the rebuilder does it, but having to go through the hassle of measuring and buying new lash pads yourself is a big pain.

Well, I had asked my friend the late "Z Doctor" in Roanoke, Va. who had his own Z mod shop . He had been doing serious work on Z's since the late 1970's and knows (and has tried) more technically on these cars than anyone I've ever met. My favorite projects of his were a 2000 Roadster converted to fuel injection, a bullet-proof Rotomaster turbo 240Z running 26psi of boost, and a blinding 240Z with a turbo RX7 motor I’ve seen cornering on 2 wheels during an autocross(!)

His mod discussed below below raises compression a bit above 10:1, maintains valvetrain geometry, and keeps the cam chain tension accurate. It also lets you use the stock springs, lash pads and cam. It's a bit involved, and it looks pretty complicated, well... it is. But with care, a place to work, and some preparation you can pull it off.

FAQS

SO WHAT ARE YOU ACTUALLY DOING IN THE MOD?

• - shaving the head .080"
• - installing 1975-78 L28 valves which have .080" longer stems
• - shimming up the cam towers .080"
• - shimming up the spring bases by .080"

WHAT DOES THIS RAISE THE COMPRESSION TO?
I had the machine shop CC it when it was shaved and they got exactly 46cc. That equates to 10.06:1 on a L28 with flattop pistons. At 500 miles my new motor showed 210-215 psi on each cylinder(!) But remember... this P79/P90 head mod is meant for flattop piston L28 motors only. If you use a dished L28, the compression will only be in the low 9s which won't be worth the trouble.

HOW MUCH HORSEPOWER DOES THIS PRODUCE?
Well, racing lore says you get about 10hp for every point increase over stock compression. So using that, this alone should add 15-20hp. I have a hotter cam, porting, free-flow exhaust, SM needles and a high output ignition so I'm going to rate my increase at approximately 25-30 extra horsepower. Not exactly small-block Chevy territory, but street performace is significant, see here for my Dyno results.

N42s, and N47s ON FLATTOP PISTONS WILL INCREASE COMPRESSION TO WELL BEYOND 10, WHY NOT USE THESE?
I won't pretend to know everything about those heads, but I posed that question to the Z Doc who I trust. He said those 2 heads are widely used, but can ping more easily on flattop motors as their low quench chambers are designed for dished pistons. This shape lets the flame kernel spread away from the spark plug and ignite. A good friend of mine uses a N42 on his F54 block with good results.

SPRINGS

See the extra washers under the spring?

Because .080" longer valves are now in the head the springs must be shimmed up to from the bottom to retain the same spring tension. The washers are hardened steel and are there to prevent galling of the aluminum head...don't get them from the hardware store.The stock head came with only one shim under each per spring is exactly .040" in thickness. So just add 2 more under each outer spring (total fo three) and you can achieve the desired increase of .080". Washers used to be available at MSA and were inexpensive.

The single stock washer under the inner spring is also .040" thick. But you can only add a total of 2 washers under the valve seal instead of 3, as it won't let the valve seal lock-ring seat properly. The inner spring is more of a "helper" spring and won't be adversely affected, I've had no problems using only 2 washers here and I rev my engine to 7,300.

SPRING TIP: "Valve float" is when a spring can't control itself at high revs and causes the valve to flutter and bounce off the seat. Decades ago V8 motors came from the factory with very poor springs that would float at low revs, so racers in the 1960s started using heavy rate springs. But Datsun springs were made properly from the beginning and didn't suffer from float. The same springs were used on 1970-1983 Zcar engines, and the 240Z could rev to 7,000 with no problems. Under race conditions anything is possible, but I've never heard of or experienced valve float on a Zcar motor and I rev up to 7,300 with stock springs.

TOWER SHIMS
Cam tower shims used to be available at Carquest auto stores. The shims were sold in boxes of 5 shims at .020" thickness, and the part# was 216-6148. But they're not carried any more unfortunately. A company called Goodson used to make shims for the L-series head, but in .015" only. Not sure if anyone makes them anymore, good luck in your searches.

REPLACING TOWER SHIMS
While replacing the cam towers is not difficult, it requires some concentration and a quiet place to work such as on a bench. Note that the towers are stamped with a number before you remove them.

- Make sure the shims are free of grit and oil (oil thickness is actually measureable). Put the shims on each tower base, and tap the towers down over the studs (it helps to have them loosely around the cam).

- Make sure the cam journals are lubed with motor oil or install lube. What you're doing is slowly tightening, and rapping the towers so they all end up in line. They can't move side to side, only up and down. The straight cam forces them to line up, so with manipulation and spinnning the cam by hand, they will eventually match.

- Spend a few minutes moving from tower to tower, when the cam finally spins freely, STOP. Resist the temptation to keep on tightening to "make it better". The bolts only torque to 10-12lb, you will strip one out if you're not careful.

- This tip from the Nissan rebuild bible is the secret: "If the cam binds when torquing the bolts on a particular tower, simply loosen the 2 bolts and rap the top of the tower with a hammer and retighten."

- Before you begin use blue Loctite on the bolts, you don't want to remove them once you get it right!

It took me about 20 minutes of tapping and retightening to get the shimmed towers to line up, but it worked just fine. Be careful not to overtighten the bolts and ream out the boltholes. The cam should spin fairly freely with just two fingers.

ROCKERS and LASH PADS

I couldn’t believe it, but luckily every one of my lash pads’ wipe patterns lined up fine. Even after shimming up .080 with the .080 longer valves, the wipe patterns were fine. Some were biased slightly towards the valve end (more lift), but they were still positioned on the rocker OK.

Keep in mind that anytime you change valve lengths you will need new rockers arms. Even the slightest change in wipe pattern from this mod will cause damage if you reuse the old rocker arms.

In June of 2003 I rebuilt my shaved P90 a second time and added a new cam, rocker arms and lash pads. The engine went from standard mild Z clicking to nearly silent now. I suspect the worn lash pads were causing a bit of noise. I now feel lash pads should be replaced on any new head rebuild. They may look ok, but they wear like anything else, and can add slop into the valvetrain and increase noise.


Go to Motorsport Auto or Black Dragon Auto and buy "Modifying Nissan OHC engines", by Frank Honsowetz. Review the section in it about measuring and adjusting your lash pads. While I was lucky, having all this machine work done can leave the valve height a little weird. And if the wipe pattern is off you can destroy your cam! Don't underestimate the lash pads!

CONCLUSION

I have modded three heads using the shave/shimming method with great results. It added a nice snap to the engine by increasing compression and improved acceleration and power. In the late 1990s heads and parts were plentiful but in 2020 shims and replacement parts are pretty scarce. So this page is more of an archive of a head mod when it was easy and fun to accomplish.