EXHAUST RISER ON 2004 MKII with M35B

[canuck]

We bought the boat brand new (#1678) in 2004 and since day one I have noticed that the riser could be moved slightly at right angles to the exhaust flange. ie moved slightly to port. I thought it may be the s/s clamp and replaced it along with a new gasket but it still has a slight amount of movement. Engine hours are 210. There is no black residue under the riser or on top of the HX indicating an exhaust blowby. There is no smell of exhaust in the engine compartment or aft cabin while underway. Is this just the nature of the beast? I am thinking of putting some Permatex Hi Temp Copper sealant which is made for exhaust manifolds.
Also, (a) I use a 50/50 engine coolant and was wondering how often this should be changed, (b) replacement interval for the impeller?
Thanks for your input.

[Ron Hill]

Can : From the questions that are coming up on the M35B engine exhaust riser, I'm beginning to wonder if it's screwed into a flange that is bolted to the engine??   
I'm baffled about your replacement of a stainless clamp and a "gasket".??  The only gasket I'm aware of is between the flange and the engine??  The only stainless clamp on the riser that I know about may be on the riser insulation.  Guess I'll have to look at a MKII to know what you're talking about!?

As far as the antifreeze 50/50 internal mixture, it should be changed just like your auto - about every 4 years.