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I think I might have the answer for you... https://www.belmetric.com/stainless-castle-nuts-c-3_558_1376/cn8ss-castle-nut-stainless-steel-p-8342.html?zenid=3j9n5fjlbd4p3hntj7jub44nl5 Looks to be a lot shallower than what you bought, although maybe not quite as shallow as the Nissan OE item. I've bought from Bel-Metric before. Good vendor, good products, great catalogue. BTW, if all else fails you should maybe pay your local Nissan dealer a visit. This might be a PN that's still available.

@S30Driver good news! Neoprene plug material came in sooner than I thought. I can send it out first thing Monday. For anyone else: They can be made fairly easily, but if you don’t care to toil with cutting and drilling a plate or using RTV I’ve got 10 plug kits available. Works very well on my 76 280z for the last 2 years, as well as the 1980 280zx throttle body pictured below. If there’s any doubt send me a picture of the throttle body and I’ll be able to tell. The kit has: a CNC Machined Blocker Plate, 1” Neoprene Crush Plug, and 3mm Button Head Screws (3), seal material (not pictured). Initially I re used the seal at the throttle body and it worked well but fashioned a new one later on because I wasn’t comfortable with how brittle it was. When I get my CNC router/laser back up and running I’ll cut some finished gaskets for the next batches. Anyways just message me or follow the link on my page to my eBay shop. If you’d like to make one yourself and don’t know how to go about it, I can help you there too. So feel free to ask for help. In the mean time I’m still working on rebuild parts and fabricating diaphragms for the BCDD... mainly because I like a challenge, but I think I can restore true stock functionality of it... which is not entirely necessary ?‍♂️ but may be some people’s flavor.

Picked my car up from body shop prison. Just did touch ups on rocker panels and rear valence. STOKED !!!!

Yes, that too! My buddy in San Diego said they’re pretty hard to come by in working condition, but needs em to pass. Pricey when he found one too. Hopfully I can make a functional kit and have another option for people aside from blocking it off!

I've not had my noise verified, but it's very acceptable to me. I use both a resonator in the tunnel and a muffler at the rear. 2 1/2" pipe all the way, both in and out. I think installation is the key to ground clearance as I have no issues with steep driveways or gutters. Resonator - https://www.magnaflow.com/products/10426-performance-muffler-magnaflow-4-round-centercenter-straight-through-performance-muffler Muffler - https://www.magnaflow.com/products/13216-performance-muffler-magnaflow-4-x-9-oval-centeroffset-xl-multi-chamber-performance-muffler Pictures;

Thanks for the additional clarification and I'm really sorry, but I'm still not sure I'm getting it. Is this right? You had the engine rebuilt 600 miles ago and everything seemed perfect including the oil pressure. Then 100 miles ago, all of a sudden, your oil pressure (as indicated on the dash gauge) shot up. But other than the oil pressure, nothing else seemed wrong. Then just a day or two ago, while investigating the high oil pressure, it started smoking badly and that's when you pulled the head. Did I get it that time? I feel so dense. If I don't get it this time, I'm going to just keep my mouth shut and stay completely out of it!

Switching to carbs is an interesting proposition that I’ve considered. Good and bad I guess. At least the car will drive post EMP attack of you have carbs. Sent from my iPhone using Tapatalk

@Av8ferg You're wanting a 240. If you haven't already spent a bunch on your efi buy these carbs. You'll have more comfortable 280 with 240 performance. I'll find rossiz'z thread on doing just that. It's a fantastic read with good results as planned. It's on my laptop, sorry but I'm using my phone atm. In my opinion that's the draw to 240s. Those SUs, when dialed in, are awesome. Totally different than efi to me.

You describe the black soot in the cylinders as oily and yet I don't see the oil sheen in the pics. The pics look like sooty cylinders from running too rich. Stick your finger in the tailpipe and take a sample. Is it reasonably dry black soot or is it wet, oily black soot? I'm leaning toward no oil pressure problem (new engine) and it's running extremely rich. Also the fact that it started running lousy in short order rather than gradually getting worse, like fouling plugs do, seems like this is a mixture problem. That said, the oil pressure is something I'd keep an eye on.

Some more photos of it when it was listed on ebay last fall. Odd mirrors on the fenders. Did it come with the original seats or the later style shown in the ad photos? Was the exterior side molding trim just stuck on with adhesive or did it it leave holes in the sheetmetal?

Hopefully this isn't a completely dead thread but I'll put it out there anyways. Regarding this topic, I've had BCDD issues on both of my Z's 1976 280Z and 1980 280ZX. Both times due to tears in diaphragms. I just ended up machining a blocker plate, RTVing the bypass on the AFM side, and plugging the manifold side. I still have some blocker plates and rubber plugs laying around if anyone still has this issue On another note, I'm going to mold some new diaphragms and do a write up on how to rebuild the BCDD... if successful of course. After disassembly and reverse engineering some parts it seems feasible to return the function of the BCDD rather than blocking it off. Hope you guys got it figured out since then though. It gave me headaches when I first ran into it.