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I bought a nice manual that includes 76 to 79 RD400 models. Unfortunately the only spec list I looked for at the end of the manual, only had RD400 F specs. 

Sharing a few resources from my Google Drive that might be useful:

• Yamaha RD400 Workshop Manual
• Yamaha RD400 Parts Catalog
• Mikuni Tuning Manual

I traced out the profile of the cylinder bases and compared them to the case and they lined up pretty much exactly.  Well done Yamaha engineers of 50 years ago

The YouTube algo took me down a rabbit hole of modifying crankcases to match the cylinders (ex https://www.youtube.com/watch?v=5nFcgDMzlZE).  Searched the forum and facebook groups and didn't really see much discussion on this for RDs.  Is there any benefit/need to doing this on RDs, or is it more of a small displacement thing?

I waited a full day after cleaning before applying the mop n glo, and it was indoors so they should have been dry.  Only other thing is that I sprayed the pipes with rust converter (phosphoric acid) after stripping (so strip >> rust converter >> sand >> dawn soap >> acetone >> MnG), so maybe that did something?  I'll re-sand/clean and try again.

Also considering treating the other pipe with Cerakote ceramic paint sealer that I have laying around and seeing how it holds up.

I'm switching things up and planned to go with the bare metal look for my pipes. I stripped them, sanded them, cleaned with dawn soap, wiped down with acetone, then used a sponge to spread mop n Glo over them. Let it sit for 20 mins, come back, and it's covered in rust.

What'd I do wrong?

Not sure if this should go here or the "Show Off" section, but wanted to show my crank that was rebuilt by Marc (aka 1976RD400C).  It was way out of spec and had a bent connecting rod - he put in some new connecting rods and bearings, trued it all up, and got all the work done in just a few days. It will be until Spring before I get it running, but so far it's looking good!

I've done lots of 400 cranks if anyone needs one done at a bargain price.

Where in NY are you located? Might have to take you up on that

Wait, so you're telling me it's not good luck to put a penny in the clutch housing???  (yes, was my poor man's method to loosen the primary gear nut)

I'll get some cheapy amazon special dial gauges to measure the crank's endplay.  But assuming it's not way out of tolerance, I have to think the seal just wasn't seated properly in the grooves in the crankcase and that's what caused it to get dislodged. Just want to make sure I avoid making the same mistake when I re-assemble.

Had my engine out of the frame while doing some body work and repainting, figured I'd take a look at how things are looking on the inside. Turns out the crank seal behind the primary gear is all chewed up. Well, good thing I found this before I did more damage.

(1) What could cause this other than incorrect installation? Could issues with the crank (out of balance, out of spec) cause it?

(2) Think I know the answer, but could I replace that seal without splitting cases? Even if I can, imagine I'll need to look to make sure there aren't bits of rubber floating around near the transmission.

They're good for carbs and carb parts (jets, gaskets, etc), spark plugs/wires/boots (they carry BR8ES), uni pods/filters and general purpose stuff for your bike (bar end mirrors, oils, grips, levers).  But you won't find anything specific to your bike that you'd find on Partzilla or Economy.

Came across this 1 hr 44 min long video of Stan Lipert re-assembling an RD400 motor with little explanatory prompts throughout, including him designing and fabbing his own chambers!  Super valuable resource!

FYI there's a 'how to' on testing the autolube pump on the forum:  http://www.2strokeworld.net/forum/index.php?topic=1119.0

Good to see Chuck in there!

The last few times I've pulled the oil line hose off the carb, the whole nozzle comes out as well, which worries me it might wiggle itself out while riding (and no oil --> bad day). I can likely mess with the brass press fitting piece on the nozzle to expand it a bit (or get new nozzles), but wondering if a more permanent solution might be to just drill/tap the injector hole and use a threaded brass barb fitting instead.  Can't think of a risk other than it leaking or vibrating out, but if done right with a suitable gasket/crush washer and loctited, seems it'd work just fine.  But if this was a better solution, imagine the wise engineers at Mikuni would've already done this. Any issues I'm not thinking of?

What happens when you throw a Hayabusa engine in a tiny Fiat 500?