Motor is 3 years old, rigt around 100 hours. Doing the annual maintenance decided to replace the water pump impeller. Dropped the lower unit that came out with some resistance, rebulit the pump greased the shaft and tried re-installing, no joy. The drive shaft reaches the power head and will not enter the spline no matter how much I manuever it. Suggestions!
Your shift shaft might be 180 degree out of alignment my guess 
The Yamaha F70 gearcase shifts in reverse compared to most standard lower units. The “dogleg” (the offset portion) at the top of the shift shaft needs to face the port (left) side of the engine when correctly installed.
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If the shifter and everything is aligned, then the spline can be aligned by rotating the flywheel a few degrees until it slips in. Some engines have a water pressure tube in front also.
Thanks, will try both suggestions in the morning.
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Good advice above. One tip - if you are going to be doing your own maintenance, search for the service manual for your model. You can find PDFs if you look hard enough, or can buy one on ebay for a fraction of the cost. Some repairs require special tools and the manual spells out what is needed.
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I learned in the school of hard knocks to put the binnacle/ shifter in forward gear on any twist shift lower units before dropping the lower unit so when you go back in with it you can turn the prop shaft to turn the drive shaft to line the splines up in the crankshaft. The shifter splines also have to line up and be in the correct position to maintain sync. You can confirm the lower unit is in forward by turning the prop shaft counter clockwise which should make the drive shaft turn clockwise. Once you get it stabbed have the bolt that goes in under the anode ready to run in. Once that bolt is in you can confirm shifting sync by testing forward, neutral and reverse before installing the remaining bolts. I just dropped and installed the lower unit on an F70 yesterday and about a hundred times in the last few years.
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Spot on. My older brother taught me that years ago and he learned it as a teenager working in our dads boat shop from the lead mechanic.
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Thanks again to all that replied. It would be easier with a second person, but I managed to get it to slip in.
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Reminder to grease all the splines and bolts