I built this splitter in 2004 and I haven't noticed any issue with having the pump vertical.

Nice log splitter. I just cut my beam for my first homemade log splitter. Thanks for the inspiration! Question: What did you use for the cutting edge? Did you need special arc rod? Also: I have a 22gmp pump, but it looks like they recomend horizontal shaft only. I see yours is a vertical. Any problems? I never understood why they say this? I need to upgrade my engine to more hp, and there are a ton of great vertical engines for sale for real cheap. Thanks again for sharing:)