Did you or whoever assembled the engine spin the engine after each piston/connecting rod was installed and torqued to check for binding? How does the engine spin by hand now? Put a socket on the crankshaft bolt and try to turn it by hand with the spark plugs removed. If it spins freely then you may have an issue with an electrical ground, low/dead battery. Do you have an automatic or a stick? Maybe there's binding somewhere between the torque converter/clutch and the transmission or the engine?
If there's an internal problem with the engine then it would have to come apart to fix the problem.
I just built a new motor with a blower and it takes 55 ft lbs of torque to turn the motor with no plugs in it. It is in the car and the tras is bolted to it. So I am turning the motor the blower and the converter and trans pump, with 55 ft lbs of torque. This shound give you some idea of what it would take to turn the motor over.
Way out of left field here, but if this is a new Motor are the plug wires routed correctly? Did you advance the Cam at all? Are you certain the Harmonic Balancer is good (degreeing tape?). Did you have the Starter checked? Is this a new complete Build (new Car?)-if it is, what gauge of Battery Cables did you use? Did you have your Battery Load Tested?
Back in the 70's I rebuilt an engine and thinking i'd be smart I used STP oil treatment as an assembly lube. Needless to say it was very stiff to turn over. About like what your experiencing. We tried using 2 fully charged batteries, that was better but still very slow. Out come the starting fluid (ether I think it is. Commonly used to start diesels in cold weather). Still nothing. :sweat: I was about to give up and pull the motor out and wipe all the STP out of there but decided to give it one more try. :sweat: Hit the key, one crank and varooom. Startled the hell out of me. For the next 2 weeks it wouldn't turn fast enough to start when it was hot but started fine when cold. after that all was well.
1) As mentioned, check gauge of starter cables. That could be a problem.
2) There should be a ground cable from the block to the chassis. Make sure this is good and big enough. You might want to connect a cable between one of the starter bolts and chassis. If you're using engine plates that directly bolt to the frame and engine you can skip this step!
3) If the timing is to far advanced the engine will be hard to turn. Try cutting back the timing a little with the distrubutor and see if it turns easier -- you can do this while it's cranking. It won't hurt to break the engine in with the timing retarded a bit.
4) It's more likely that the bearings are tight than the rings. Did you check bearing clearances w/plastigage? I didn't once, and it turned out that the bearings were tight.
5) Did you "prime" the carb? Pour about 1/4 cup of gas down the carb then start. Make sure the air filter is on and don't stand over it. An open carb and a back-fire could torch the hood insulation, that's why I say put the air filter back on after doing this. Any time I've started a car it's had a little gas trickeld down the carb first, even if it was just run out of gas. Works every time! If it fires but doesn't start, do it again. Might take 2-3 times pouring a little gas down the carb to get it started. I'd do that rather than use starting fluid.
I hard a hard time gettng a flat head six to start a long time ago. Luckily it was a stick and I lived on a farm at the time. Dragging it down the road for 200-300 feet in second gear with the tractor did the trick! Make sure you have a 30' or longer tow strap/chain, don't want to hit the tractor when it starts!