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I am thinking some were 1/16 -27 NPT. Thats pretty small as far as pipe taps go and not on a lot of charts. Had an 85 hp from the early 70's but I don't remember for sure. If you need any info on correct tap drills let me know. If memory serves me correctly you can even get by with a 1/4-28 straight tap in an emergency for a 1/16-27 NPT. Don't quote me on that I would have to look it up or try it on scrap to be sure. Let me know if you need any info and I can probably find it for you regarding taps etc. Best of Luck.
There are definitely 1/16-27 pipe taps. I have one and I am looking at it right now. 1/4-20 tap would be for a common 1/4 inch screw. 1/4-20 NC (or National Course) to be precise which is one of the most common screws in the USA. A 1/8-27 NPT (National Pipe Taper) as 14ftgruman correctly states is very standard. I have seen many 1/8-27 fittings with a 1/16-27 zerk tapped into it at a right angle to make a right angle fitting.
Check some charts. If they are 1/4-20 then they are NC not NPT which means they are straight. If it is a 1/4 NPT then it is a 1/4-18 NPT. You may very well have some grease fittings that are 1/4-20 but as I stated they will be a straight thread not an NPT or taper thread. iwombat states in the original post they appear to be NPT.
