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What do you recommend to stop oil leak on intake manifold in GMC 350 gasoline truck? Have used red forma gasket?

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I'll assume you are talking about the front of the intake (or could be back) where the intake meets the "China wall" ... If so, there's a couple of things which may work for you.

Here is some guidance:

  • When you pull the intake, ensure all mating surfaces are completely clean of anything. Ensure you've scraped off all of the old silicone gasket material.
  • After it's cleaned up, ensure there's no dings, nicks, scratches in the intake. Also make sure it isn't warped. You might be able to eyeball it, but using a straightedge and feeler gauge on the mating surfaces would probably do you better.
  • Use a thick, continuous, even bead of sealant from the corner where the head meets the block from side to side.
  • When you go to put the intake back onto the engine, you need to ensure you go straight down and into place. If you jiggle it or move it around after placement, you'll usually end up with a leak.
  • If I find a typically hard to seal problem, I'll use Permatex "The Right Stuff" sealant. Ensure you follow the directions carefully. If you do, you should get it sealed without further issue.
  • Torque the intake manifold using the correct torque and following the correct toque sequence. If the stock intake and heads, I believe you can follow this guide:

SBC intake torque sequence

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