Top end rebuilt 1k ago. engine bored 70 over. Since rebuild about every 300 miles push rods tighten themselves up on back jug and have to be readjusted. solid rods. Any ideas what would cause this? Adjustments done according to proper procedure each time.

Personally, I would have just gone 1,000th's over on each cylinder bore and matched the 1,200 c.c. Harleys off the lot. Their have been T.V. biker build off's, that have even bored a 886, to a 1,200 c.c., now that's crazy! Just think 1,200 c.c.'s , with two high performance pistons and a couple of 15 horse power a piece four way plugs and who needs a blower. :)

You may still have gaskets trying to compress on the engine and if you had the valves and seats ground, they may be still seating in at that mileage. As the valves and seats wear, the valve stem raises up reducing the lash. All Ironhead Sportsters have solid tappets and should be adjusted to zero backlash. It's always best to set the tappets on these engine a bit loose rather than a bit tight. Always adjust the valves (tappets) with the engine absolutely cold. Keep in mind that the valve lash is set at zero with the engine cold. As the engine heats up, the cylinders and heads "grow" upwards introducing lash into the valve train. When it cools and contracts, it again should go back to zero lash but not hold the valves open. Make sure you get the locknut tightened properly.

Good Luck
Steve