[Mart]

I had one that was hard to turn over. It had the wrong main caps. luckily, with some other "wrong" caps fitted it was able to turn. You might have a cap on the wrong way round.
Pistons, rings, rope seals lifters, valve springs all create drag, but only a finite amount. If it is really heavy to turn, you need to look at the bearings.
When I was assembling one recently I checked it was still turning ok after every step.

Mart.