In this question of the week, John Reid talks about how to make replacement gaskets for your engine-to-muffler connection using automotive gasket material. It’s easier than you may think!
John, One additional tip when using the RTV Silicon. One thing that can occur with RTV is that almost all of the material can be squeezed out of the joint when the bolts are tightened. A few heat cycles can break loose what little material remains and the leak reappears. I have found a way around this that I have even used with success in industrial boilers. Begin by thoroughly cleaning both metal faces with Brake Cleaner. After the cleaner has evaporated, smear a very light (almost see through thin) even coat of RTV on both parts and let it dry for a few hours. After the the RTV is thoroughly cured, smear an additional thin,even coat of RTV on both parts and immediately assemble. The first, cured coating of RTV will not squeeze out and the second coating will fill any voids. This method also seems to allow repeated heat cycles without failure.
I use the same technique for gas engine mufflers. However, the statement that gasoline does not affect silicone is wrong and misleading. I would never use silicone on an intake manifold, crankcase, etc. of a gas engine as gas will turn silicone to a useless jelly. There is not enough exposure to gas at the muffler/engine interface to worry about so I do use it there and it works quite well.
I have used this high temp’ silicone for many years with very good results. To add to John’s good advice, it can be used to good effect on the threads of four stroke mufflers and headers to prevent leaks. Also, if you coat the outside of a muffler with it – thick as you like – it does an incredible job of taking the metallic sound off the exhaust. To do an even better job, put a layer on the muffler, wrap some medium glass cloth around and then re-coat the cloth. I used this for years on the header pipe from my van (people carrying) due to the noisy metallic rattle and it completely eliminated the noise.