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January 12, 2007
Two Years Ago Today

Manifold Destiny

Al Blake down at the machine shop called me the other day and asked me to come down and have a look at my exhaust manifolds. He's cleaned and magnafluxed them and found a crack in each one in the exact same spot: the pad where the intake manifold sits on top of the exhaust flapper valve. As clever as I think the engineers in 1941 were, they didn't seem to know much about castings. A radius would have eliminated any future problems, but it probably would have added some expense to the machining process. 

Machined pad on the manifold is prone to cracking (highlighted area).

Close-up shows crack highlighted by magnaflux. I wonder if there's
any benefit in grinding down all that casting flash?

I'm facing the decision whether I should invest some substantial cash in having the manifolds Jet-Hot coated or if I should just paint them in case they do crack in the future. I'm sort of leaning towards having them coated anyway for two reasons: 1) they've probably been cracked for decades without any ill effects, and 2) it doesn't seem to me that the crack will fail in a major way; these are some pretty thick castings. Jet-Hot coating may even help fill it. With the gentle use this car will likely get and with the improvements I'm making to control temperature, I don't think they'll get any worse. And if they do fail, I'm guessing replacement  manifolds will cost a lot more than the coating. Anyone have any thoughts on the subject? E-mail me.

We also talked about permanently closing the flapper valves. Al usually pins them closed, which seems like a better idea than tack welding them like I was planning. It also avoids putting any additional heat in the area to encourage the cracks. 

No other cracks in the manifolds, which I guess is a relief. Note the heat riser
tube running through the exhaust manifold. This connects to the carburetor
choke spring to speed warm-up. Interestingly, the tube simply passes through
the manifold, heating air as it goes into the choke.

The good news is that my intake manifold is just fine. The crank has been sent out to be turned 0.010" undersize and should be back next week. I'm still looking for some insert bearing rods for the engine and have heard from several sources that Terrill Machine in Texas offers rods already machined for insert bearings on an exchange basis for $65. I'll be speaking with them soon.

Healthy intake manifold.

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Last modified on 01/02/2008

Thanks, Fidget!