Zed Head

That exposure is normal, but the wear on it is not. It's the pole of the magnet, the piece that the coil is wound around, I think. Don't forget that the grinding of the wheel on that piece will cause heat too. But, overall, if you got no continuity, you got no sine wave, and the ignition module won't produce no spark. It ain't gonna work. Might as well take the distributor out and disassemble it to see where the wear is. In a prior thread that I can't find somebody had a worn shaft and we talked about Redi-Sleeves and other ways to fix it. You might not be able to find a rebuilt unit but there are places that offer rebuild services. You send yours in and they fix it and send it back. Look at RockAuto. OReilly says that they have them https://www.oreillyauto.com/shop/b/ignition---tune-up-16776/distributor-12503/8c7c95368caf/1978/nissan/280z?q=distributor

I'd start with the Google machine. Seriously. I know that they get clogged if they see a lot of fuel over time, but don't really know if the materials inside change over time. Do you know what your regions emissions tests entail? Some states have different rules for different regions. In Oregon all I needed to pass was idle emissions. But even just that was difficult.

The place to start is to tune the engine to Nissan FSM specs. Set timing, valve lash, etc. exactly as Nissan says. If you have a catalytic converter make sure it's in good shape. With the factory stock EFI system there's not much that you can do except get it as close to what the factory sent out the door as possible. The 280Z's were designed to pass emissions. Everything about the tuneup procedure is to get them emissions testing ready.

And, on the Porsche synchros, there are several discussions about steel baulk rings in the early 5 speed Nissan transmissions. Another side thread. JMortensen said he used to work for a shop that used Swepco 201 for those in the Porsche transmissions, and a blend of Swepco and ATF for the "brass" Nissan rings. I used the blend on an old 5 speed ZX transmission that had problems with Redline and it fixed it. It's spendy but good stuff. Most Porsche racing shops will have a 50 gallon drum of it in the shop. That's where I got mine. https://www.classiczcars.com/forums/topic/19361-gearbox-oils/ https://www.classiczcars.com/forums/topic/19261-early-5-speed/

Apparently Pennzoil developed the "synchromesh" formulation for GM. So if you can find Pennzoil Synchromesh, it's the same formulation, as the internet seems to show. But, for whatever reason, maybe because a copyright wasn't acquired or not enforced or because GM let the formulation data out, there is a "synchromesh" for several other brands of oil. And a "Synchromax". https://www.amsoil.com/search/?text=synchromesh&timezoneOffset=420 https://www.valvoline.com/our-products/grease-gear-oil/synchromesh-mtf https://www.pennzoil.com/en_us/products/other-oils-fluids-fuel/manual-transmission-fluids-axle-oils/synchromesh-manual-transmission-fluid.html https://www.royalpurple.com/product/synchromax-manual-transmission-fluid/

Where's the head? Is it a stock camshaft? Regrind, aftermarket? Has it had work done?

ZX distributors. 123 ignition. Someone probably has a used one. If it's the bushing in the body that's worn you might just get a body and swap your parts in to it.

If you're not getting continuity then a/the wire(s) in the pickup coil probably broke from the vibration of the reluctor wheel beating on it at high RPM. There's more than just a magnet in there. I don't know the construction of the Nissan unit but here's a typical diagram from the interwebs. Not my forte...

Your air gap is uncontrollable.

That looks worn out. I don't think I've seen one that bad before. I think that some people have devised ways to repair that bushing surface but it takes some work and proper tools.

Could be. Supposed to be continuity and a certain resistance. It's in Engine Electrical and the "Bible". The breaker plate moves the wires to the pickup coil and sometimes they fatigue and break or short out.

Should say "Problem disappeared.". They do that sometimes. Your solution didn't match the symptoms. That's where the extra commentary is coming from. You might be back. Good luck.

Here's an old thread about problems with the ignition switch. Test at the ballast and coil first like SteveJ says, if there's no power there, work your way back.

Are the battery terminal clamps tight? Sure sounds like a connection opening up due to heat from high current across a loose and or dirty connection. The ignition relays on the 280Z's were put in a spot that gets wet if there's a leak. Same reason, different location. Current heat.

Might just be OEM Datsun pistons. Not uncommon for old cylinder walls to show little wear. Apparently the blocks are made from pretty tough metal. Are there any markings on the bottom of the piston tops? https://maseraticompound.com/products/datsun-240z-pistons-rods-bearings-and-rings http://datnissparts.com/x-1973-240z-l24-oe-oversize-cast-pistons-1-0mm-040-84-0mm-12010-h2716-set-of-6-also-fits-l26-l16/

Clutch issues show up more in reverse because it has no synchronizer. So, if reverse is fine, then worn clutch is less likely. You can have one worn synchro and fluid will make the difference. But, your problem seems odd in that you say it happens when downshifting. I assume it's fine while upshifting? And why are you being aggressive in downshifting? You do mean the shift from 3rd down to 2nd, right? 3rd gear is usually the first synchro to wear out also. If the fluid change doesn't work and you decide to take the transmission out, save the fluid, It won't be ruined from just a few miles in a worn transmission. It will be fine to use after a rebuild.

The right fluid can add thousands of miles to a worn transmission. Redline, Synchromesh, etc. Plain old Valvoline 80-90W isn't good enough.

Turning...

There's a rheostat in there somewhere. This is from the 78 FSM,

Can you get a picture of the shoulder, just for fun? I assume that it is visible.

Check for voltage at the plugs if you're not sure (I love my multimeter). Check the ground circuit from the bulb mount metal to body ground. Can't remember if those bulbs use a single electrode and a mounting ground or if they are two electrode, with a ground wire. Grounds are the hidden circuit killer in the old Z's. And, if you replaced the bulbs recently, some of the new bulbs from the big aftermarket guys are actually too big to fit. They'll go in but then work loose over time. On the dash cap, if you have some Mylar sheet, like for making transparencies, you can slide it between the gauge edge and the cap edge to make a ramp to slide the gauge out. Add a little lubricant of some kind to help. Armor All, for example.

How different are they? Could somebody enlarge or move the holes on a newer one to get it to work correctly in the older application? If you have both maybe you could take some measurements and add to your modification program.

My point about checking continuity back to the ECU was about the fact that the one that connects to pin 13 would be the coolant sensor circuit. Since there's only two in the question, the other must be the thermotime switch. Look, I made a circle. Those new Nissan parts sure are shiny. They look good. I seem to recall something about pieces of colored tape on the individual pairs. Maybe that's the simple answer if you have the original parts on your car, and the tape is still there.

Hope they don't have kegs at the rallies. https://www.theguardian.com/us-news/2020/aug/29/theyre-not-social-distancing-us-college-towns-close-bars-as-infections-surge

For anyone getting lulled in to believing that the situation in the USA is normal. These curves should be very similar, at least. You can't fake deaths. "Fake deaths" isn't a thing, although it might be tried, no surprise. Over three times as many people are dying of COVID-19 in the USA as in our"comparables". Modern developed countries. We used to be one.