There have been a lot of posts in the last week or so about manifolds, and some were about cracked manifolds.
Not sure I am the master on this topic, but I'm close. Anyone feel free to correct anything I put in here.
Our manifolds are made from high temp rubber compounds with aluminum inserts on each head flange. They are slightly narrower than the space between the heads where they mount. When you tighten them into place, they stretch very slightly making it easy to mount them.
As with all rubber compounds, the elasticity of the rubber will deteriorate over time. Exposure to heat will accelerate this problem, as will exposure to electrical fields.
So if you own your bike for a while, eventually the manifold will deteriorate to point that it will develop cracks. The older your bike is, the more probability it will happen to you. If you are jetted lean, it will happen sooner due to the higher engine operating temps.
So eventually, if you keep your bike long enough, you are going to need to replace it. Its not an expensive part to replace, and will last a lot longer than a set of tires will. So consider it a replaceable in all the ways you do when you think of tires. It will just last a lot longer.
Some types of cracks are ok. In particular ones that develop in the intersection of the runners to each head and the flanges that mount to the head. Those are well covered by the aluminum inserts that are underneath them and almost never leak. If they concern you, buy some high temp RTV and fill them with it before reinstalling (be sure to let the RTV cure first). If you choose to keep a manifold with these types of cracks, be sure to wash it very well (dish soap in the sink is fine) and inspect the remainder of the manifold really well. If there are no cracks in the areas talked about below you should be fine.
Other types of cracks are a real problem. Any crack in the spigot going to the carb
is going to be fatal soon. Cracks that develop near the inner edges of the aluminum inserts should be treated the same, as should any crack that does not an insert under it. The inserts cover the area extending from the head flanges to just outside the center spigot tunnel. The center spigot tunnel (front to back) is only rubber with no aluminum insert inside.
This manifold has critical cracks in three areas that make it a throwaway (thanks for the pic Greyphart):
The cracks in the two runners to the heads are very near the edges of the metal inserts. In time they could start leaking and in are in an area of the manifold that flexes with every pulse of the engine. I wouldn't trust RTV on this at all. The one in the carb spigot is fatal, its just a matter of time (and not very much time).
Someone in another thread asked if porting a manifold made it more susceptible to cracking. I'm sure it does since it tends to decrease the amount of rubber inside the manifold, but would suggest that if the rubber is going bad, having that extra rubber just buys a little more time. Once the rubber looses its elasticity the manifold is going bad and it needs to be replaced. Considering that it will have be replaced in time (ported or not) I will keep porting mine. I'm willing to give up a little longevity for the extra performance. Others may feel differently.
Hope this helps. Again, anyone feel free to make any corrections by replying. I'm not proud