After re-building my 150 (08) scootcoupe engine with a 170cc "big bore kit" as well as a new rod-crank assembly (these are not kymco parts, but made to fit the gy6 engine) a hand turn on the crank shows the piston making contact with the head. tried two base gaskets, but still not enough clearence We suspect the rod length or crank throw. any suggestions? maybe a genuine kymco crank assembly?

To put it in layman's terms it means tat the big end bearings are shot and it will be new bearings and crank shaft grind which leads to engine rebuild which leads to lots of dollars. I do not consider it a stupid question at all as it is a common statement by "shade tree mechanics" looking to make a fast buck at your expense.. It is best if you have a series of tests done to determine the condition of the engine first. Find yourself an accredited ,reputable mechanic and have the following tests done.1--- have a compression test (dry then wet) done to determine the condition of the rings and bores. IT will also find if there are cracks and head gasket problems( minimum psi should be around 120psi). 2--- have the oil pressure tested with a mechanical gauge to find the working oil pressure ( minimum idle pressure around 20psi) reduce the engine idle to as slow as possible and listen to any knocks in the motor. Pull the plug lead of each cylinder in turn and if the knock subsides it can be worn gudgeon (wrist pins) pins in the pistons. When you have the results from all these tests you will have a good guide as to rebuild, replace with a reconditioned engine or get a good unit from the wreckers.

It's rare that a piston goes bad. So, you need to determine why it failed.
Most times, the cylinder wall is damaged during a piston failure making it impossible to just replace the piston. Check the wall for score marks and out of round condition as well as actual size VS original bore size. (correcting that requires machine work) The new piston must match the cylinder bore as it is now, NOT what it was when manufactured. (same applies to rings).
The old rod can be re-used if it is not bent and providing that the big end (crank end) is still round. Again, the rod needs to be assembled by a machine shop as the wrist-pin is a press-in part.
Most times, you will find that the amount of actual work necessary to replace one piston exceeds what would be necessary to replace the engine in both time, $ and reliability.
If you want to try a replacement though, see if you can get a piston/rod assembly that is still in good condition from a blown junkyard engine, Plasti-gage the bearing and replace if necessary, clean out the ring grooves or replace the rings. Use a hand held cylinder hone and break the glaze on the cylinder walls. If you are careful and everything goes well, it MIGHT work.
Personally I would never replace pistons in an engine without doing all the necessary machine work though.

Common problem. The water admitance valve is stuck in the bore. The valve lets in more water when pressure drops and when wand is closed the pressure builds and the incoming water is blocked. Remove the valve spool and clean the bore with crocus cloth on a dowel rod. Replace the O Rings and lube with plumber's grease and re-assemble. Often you can buy an O Ring kit for the unit. Deposits build up and stick the valve spool at the high pressure end. The valve is usually crossways to the shaft and right where the water input is connected.

Verify that the valve tappet clearances are set correctly. Insufficient clearance will definately cause valve leakage. Also make sure the carburetor is correctly fitted to the cylinder head with no leakage in between.
If neither of these correct the problem, then you will need to go back and make sure the cam timing was set properly when the engine was reassembled - and/or make sure the valve weren't damaged when the engine was overheated.