After buying lots of hose(s) that isn't the right stuff, and researching the 2 different types of hose that Honda used in the 750, and cutting up a bunch of old ones to figure out why some last(ed) 50 years while others lasted 15 years or less, I've figured this much out:
1. In the early 750 the hoses were unlined. They were just AN10-size rubber (fabric-covered) hose over AN6-size pipe fittings, with custom-made ends to fit the bike. I have even seen some very early sandcasts that didn't have the fabric cover, and they were SAID to be original hoses.
The FLOW SIZE is set for AN6 hose ID, so anything that size or larger will meet the engine's flow needs. We've seen this proven out in choppers and various racebikes over the years: many of the oil coolers use AN6 (some Lockharts used even smaller ID) hoses with no ill results.
2. In the 750K5 (and probably during the late 750K4) the hoses became lined-type hoses, ID size being otherwise the same: this is when they started cracking (after 10 years) right where the line turns a little bit on the rear (inner) hose where it exits the tank. This hose gets bent upward slightly, just ahead of the tank's connector, after it is bolted into the engine: the (stiffened from age & heat) liner doesn't appreciate that very much, stress cracking after a time - these are the most common leaks we hear about. This is the oil supply line to the engine, and air leaks here can cause low oil PSI when the oil is cold. I have only seen one bike that outright refused to prime the engine's oil pump, even though the engine had run in my test stand first (different oil tank), when this inner hose would weep when hot - so I know it can be an issue.
3. In the later K2 (and likely until the K4-5 change) there are SOME hoses with smooth liners inside and some with just the normal rubber finish inside. I suspect, from knowing the other 'experiments' that came out of the New Factory (2nd factory) for this bike, that the lined hoses appeared in the New Factory bikes, but not in the Old Factory bikes. This means that most of the bikes made before 4/1972 production did not have the lined hoses, because the 'New Factory' opened somewhere around that time. This corresponds to my notes, which showed that the K4 and later bikes seemed to be the ones that had the weepy hoses the most often, with those last log entries being circa 2001. Today it can be hard to tell if a given bike has original hoses or not, but the lined hoses are the ones in my collection that leak.
So...I'm going to find some unlined AN10 hose with no steel braid in it, nor over it, which should fit the collars after crimping a standard AN10 ring onto the AN10-size, non-stainless-overbraided, non-inner-lined, smooth rubber ID hose over the Honda fittings. I have a set of fittings from a 750K3, which should fit my K2 fine. They are different (return hose length, angles) from the 750F0 forward: it might be a while before I can make a set of those, as I have no F0+ bikes here to test with.
I'm hoping to beat the $200+ cost of buying a set from Japan. Mine are bleached to almost white now, and their fabric jackets are frayed from all the out-in trips (3) my engine has experienced . The chrome-braided hoses are always too thick (especially for the F0 and later bikes), using up every available mm when installing them, which frequently causes their early failure for lack of wiggle room with temperature. I have seen more of those chrome-covered ones leaking than any others, and have installed 2 sets, swearing that kind of work off forever after the last set from Crapy on a custom cafe' I built some 10 years ago. I ended up shaving off the side of that one's engine (right side) with a half-round file and cut off the chrome mesh behind the frame member the hose was bent around in order to get a straight-enough path for the supply line on that 750K5.
