Mercruiser 5.7 Manifold & Riser Question

[boater2018]

We have had our 2000 210 Sundeck for 4 years and have never been stranded, never had any temp problems. Because I purchased the boat needing work and don't know much about the history before I got it and the fact that the manifold on the port side didn't have a gasket at the block, I decided to pull the Starboard manifold to do a general inspection and install new gaskets, assuming it only had liquid gasket like the other. Also, there was some water leaking from one of the bolts on top of the elbow, so I wanted to be sure it wasn't leaking inside. I removed the riser elbow, then the manifold with riser attached to it, after removing the water hose from the bottom and the power steering fluid hose from the reservoir. I removed the old gaskets and sanded them smooth and it is ready to reinstall. The old gaskets were stuck really good and took 80 grit sandpaper to remove them. The inside of the riser and the manifold has no rust (relief) and is very clean, so I feel good about it's condition. Here's the main question:

The riser is attached to the manifold and I can't get it loose. I tried using a flat bar and attempting to separate the riser where I thought I might could get the flat bar to hold, but no luck. I also tried using a wood block and my 3lb sledge against the wood block and had no luck. It seems that it is bonded to the manifold and existing gasket. I do not have a rubber mallet, but can get one, it just hasn't seemed to budge at all so far.

So, do I leave it as is and put new gaskets at the elbow and the block since it isn't leaking? Or try something different to separate the riser? Freshwater only since I've had it. Don't know before that, but I suspect saltwater. It had 220 hours when I bought it and seemed neglected.

 

[Espos4]

That’s a spacer block. Looks like a 3” spacer.

maybe there’s corrosion in the holes that the studs pass through?

 

[Espos4]

I would double nut the studs and remove them to see if that allows them to separate.