... how important are they? I've got a set of manifolds with burst flexis (driver's side way worse than passenger side) and I was thinking about just cutting them out, bolting the manifolds to a scrap head, and welding in a straight bit of tube.

How much do they actually need to flex? If I use hex bolts to hold them on it's a damn sight easier to get to the one behind the steering column shaft.

In my case I had to lift the frontpart a bit to reach the 12 hexbolt of cyl. 6, otherwise the socket (½" drive) would not fit onto the bolt.
So I needed the flex capability.

I suppose it depends on why they are there. If they are there to deal with thermal expansion then if they can't expand, something else is going to go (or they'll just crack again). But if that is the case, how did the cast iron manifolds fitted to the same engine in a Classic survive and not crack? Although the cast iron manifolds did have huge holes for the bolts that could, as long as it wasn't bolted up solid with firegum on the gaskets, would allow a bit of movement. If they are there to allow a bit of flex to allow for manufacturing tolerances, then as long as the are bolted up to a head when welded, they should be fine.

Well this is just what you say, without flex harmonica the socket would fit.
But the straight part to be welded in must be exact to size, even with the flexpart in it I found it difficult to line up all flanges to the ports.

Early ones don't have the flexi bit. Later ones do. Must be a reason why they put the flexi bit in but you don't hear of earlier vehicles being significantly more prone to manifold problems than later ones. Maybe to do with improving gasket security. With no continuous flange between all the ports its possible that things could be shuffling round a bit to accommodate different thermal expansion between the alloy head and steel manifold causing gaskets to loosen off.

Probably an edge case thing. Some did, some didn't and putting the flexi in was intended to shift the edge so the some did percentage got lots smaller.

I'd go for it taking great care to have everything properly lined up with no stress before and after welding.

Clive

Re the bugger bolt, there's 2 ways do do it,

1. Remove inner arch, you now have full access with a 3/8 socket and Flexi.
2. Cut one of the bolts down, it then allows you to get the only extension piece you have to fit !
   I use both methods, depending on if I can get mudflaps off, which do I prefer, method 1.