I need to buy some parts for a variety of engines and heads I'm working on. Running all over the internet and as usual there is a lot of confusing information caused by inconsistent cross referencing of STOCK NHT parts applications. While not 100% complete, below is the quick and dirty so far...It does not include all the part #'s. 1960 may be a change point on valves and springs, I have not researched yet. 500CC only uses short tip valves due to short barrels and only one length of short push rod series. There are however skinny steel and fatter SS aluminum pushrods. There are three 1.3" diameter valves: intake V26 short tip- Iron + early aluminum pre SS twins w/small bolt pattern exhaust V27 short tip- pre Commando twins up to 125871 exhaust V264 long tip- Commando, 750 & late 650 & Mercury post 125871 There are two 1.4" diameter valves: intake NM 17221 short tip- later aluminum small bolt pattern 88 99 650 includes SS semi down draft- intake NM 25499 long tip- Last 650SS & Mercury-post 125871 There are two 1.5" diameter valves: intake NM24330 short tip- 750 up to 125871 intake NM25501 long tip- post 125871 750 & 850 Commando Valve springs: There are early spring sets 06-7174 , for which I can see no need other than for early mellow cams. The mass differences for all the valves listed above do not seem to warrant stiffer springs. However, the 22729 SS cam is quite a bit hotter than the 3 earlier cams and would seem to be the primary reason for using the stiffer spring set 06-7070. Similar heads could have different springs depending on cam application. Any insights?