Mike,
I had the same problem with the manifold/catalytic converter gasket
disintegrating every few months on my 1977 MGB. The fixes I found had to be
used in unison:
1) Make sure the mating surfaces are perfectly flat. I bought a new
catalytic converter b/c I didn't want to mess with the old one and trying to
fly through hoops tuning the car with the old (read: hollow) one in place.
With a new cat, the car is very easy to pass if everything else is in good
tune. I also took the manifold to a machinist friend, and he planed the
matting surface. I was surprised how warped the surface was.
2) Use new studs, washers, brass nuts, and even lock nuts. The studs and
brass nuts get tempered from the heat, and the threads get worn. Chase all
threads before installing the new hardware.
3) Make sure all supporting hardware for the exhaust system is in good shape.
The rubber mountings can wear out and sag, putting a lot of stress on the
manifold joint. 4) Check the support strap from the transmission bell
housing to the exhaust down pipe. The strap connects the engine-steady rod
to a clamp on the exhaust pipe, and is very important as it relieves
horizontal stress on the joint.
5) Use the right gasket. The correct gasket (ordered from Moss) is about
1/8" thick. I tried using store-bought gasket material, cut into shape with
an exacto knife once...it let loose before the engine even warmed up.
6) Check the motor mounts. If you have a bad motor mount, the shaking engine
can put a lot of stress on the manifold joint. Also, a sagging motor mount
can allow the engine to rotate enough to stress the joint during hard
acceleration.
7) Check the intake/exhaust manifold gasket and mounting. The manifold studs
(that screw into the cylinder head) can allow quite a bit of variation in how
you mount the manifold to the cylinder head. The manifold should be mounted
level, and midway between sitting on the studs, and pressing the studs upward.
If you have checked all of these tips, you should have a long-lasting
catalytic converter gasket. I am currently running dual SU HIF-4's on my
car, but I have to switch back to the single ZS setup every two years to smog
my car, so I've been around the block with this system.
Good luck, and let me know how it works out!
Steve Sanchez
1977 MGB Roadster
Bakersfield, CA
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