And!!! If you are lacking in the compressor department, get yourself a hank of 1/4 or 3/8 inch clothesline or braided nylon rope, remove all of the sparkplugs, rotate the engine until one piston is coming up, but don't bring it all the way to the top; insert the end of the rope into the sparkplug hole and feed as much of it as you can into the hole without losing the other end, then roll the engine on up until the piston presses the rope up against the valves. That will hold them so you can use an "over the top" valve spring compressing tool to remove and replace them. But, the air pressure thing is better.