There is absolutely no reason you can not shut the engine down if its getting too hot and then refire and continue break in once cooled/sorted. I have done this many times especially when anxiety is high or there are leaks. I have seen air pockets in the cylinder heads many times and I've seen the impeller come loose on brand new Milodon big block pumps more than once. This is where the dyno has a distinct advantage. Also the metal clad Viton valve stem seals are the only way to fly these days. J.Rob
