Marc,
An earlier post indicated that all twin carb MGB exhaust manifolds are
interchangeable, well there is a slight difference you need to be aware
of.
The 18V exhaust manifolds (and intake manifolds also) have a thicker
mounting flange at the cylinder head, this flange is thicker than
earlier manifolds by about 1/4 inch.
If you try to install a mismatched set, the thick load washers under the
manifold bolts will cock to one side as they try to bridge the
mismatched flanges. This will cause the studs to bend and/or break off.
The trick is to buy some cheap washers at the hardware store, the same
diameter as the thick load washers. Hacksaw these washers in half, and
use them as spacers on top of the low side of the manifold flanges. Then
everything will be the same height. An alternative is to use the
1972-74 intake manifold along with your exhaust manifold, so both pieces
have the thick flange. You may have to block off some smog ports in the
later intake manifold, though.
Hope this helps, PK
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