REPLACING THE BROKEN EXHAUST MANIFOLD
The manifold to downpipe flange should have four Bsf-thread studs in to which the downpipe is secured. But in the past, those original studs must have corroded or stripped and someone had simply cut the studs off and drilled out the holes, fixing the exhaust on using regular nuts and bolts. That’s annnoying, but in the past when the car was worth £500, it was just what people did. To save disturbing the manifold now, James tried using some replacement nuts and bolts, but you guessed it – when torquing them up, the drilled out (therefore weakened) manifold cracked.
The problem with exhaust fixings is the continual heat they’re exposed to. In time, the studs and fixings become very tight and seized. Try to undo a manifold to block fixing and you can be almost certain the stud into the block will simply shear. If that happens on one stud, the job can go from bad to worse, because the others are likely to do the same and then you’ll have to deal with the engine block. That’s the reason mechanics don’t like to disturb exhaust manifolds unless they really have to.
The trick is to try some heat on the bolt using a gas torch. The heat can expand and loosen the thread and also break up corrosion in the process. Less corrosion means less force is needed to turn the nut, which means the force shouldn’t get high enough to shear the stud. James shows us how, with care, the bolts can be coaxed off without damaging anything. 1 The manifold flange holes had been drilled oversize to take bolts, weakening the flange. When this corner broke off, a new manifold was needed. 2 The 60-year-old manifold fixings to the engine block are likely to shear if force is applied. The trick is to put heat into them with a gas torch first. 3 With care, all our fixings came undone and the manifold is removed. Note the towel to keep crud from falling into the exposed valve gear. 4 On removing it, we see clear evidence that one manifold gasket had failed. In reality, it would have had to come off to sort this anyway.
5 The engine mounting face is then cleaned up, and new exhaust gaskets are fitted. Easy to check exhaust tappet clearances now, too.
6 The beautiful new Craddocks manifold goes on, using new BSF nuts on the engine studs, and correct brass nuts on the exhaust downpipe.