I'm going to put my silent thunder back on for this year, I'm currently running external flappers and no platform. My internals are gone, I can see the rods they were on but that's all that remains. I'm assuming I can order new ones from a formula dealer, along with all my gaskets to remount the silent thunder box, but how do I install them? They appear to be on the inside of the tips inside the engine compartment. I'm guessing I'll either have to pull the tips out or remove the rubber hoses coming from the manifolds onto the tips. By the looks of the rubber it's going to be a serious job trying to bend them back or pry them off to get to where I can slide in the new rods and install the flappers. Any advice, or is there a DIY thread on this? I'm thinking at this point the easiest is probably to remove the screws and hose clamps and pull the tips out, install the flappers and throw them back in. But that makes me wonder if they are epoxied too, this boat is an '89 and doesn't appear to have ever been apart so I'm sure it's going to be fighting me!

Here's a tip: Remove the tops of the manifold and leave the rubber on them. There should be 4 bolts on top of the Exhaust manifold, loosen the clamps on the transom side, remove the 4 bolts...and you will be good. As far as the replacement of the internal flappers, you can get them from Teague Custom Marine, or you can seach around and find the original replacements. TAG will guide you to the proper company and part number. Last I knew, these are silver soldered in place, so make sure you can get someone to help you that is experienced i doing so. You will not be able to use propane to solder...Any HVAC "guy" can help!

I just did this last year on my buddies boat. Let me know if you need other info or help!

Dave

I went thru the same problem last year. I have an '88 357, after pulling both engines to rebuild, I noticed all 4 flappers were gone. They were riveted in the tip, so I called Formula for replacements and after several calls I gave up trying to find them. I really didn't feel like pulling the platform off to change the tips, so I took out the extra parts and put it back together without them.
Do it now while it's easy. Buy new tips and do it right. Good luck. BC

I'm not 100% sure because of the age of your boat, but the internal flappers I have replaced were not that difficult accessing them from the manifold and rubber exhaust boots. Once you get enough of it loose from the manifold there is enough play to get the job done. A little time consuming, but I think I would try that route rather than removing the exhaust tips. The flappers I had replaced, although in a later model boat, mounted from the inside and had a rim that seated on the oblong exhaust pipe. For more definite answers, you should try the DIY forum on OSO here. There are lots of knowledgeable folks surfing that forum.

GOOD LUCK!

Just did mine on an 88. The origionals are riveted. Remove the exhaust risers from the manifolds, with the hoses. Chisel the rivets off, and replace the flappers and brackets with short bolts and lock nuts. Not a big deal. Brackets and flappers are available from any mercruiser dealer. Don't forget new riser gaskets, and to drain some water from the manifolds before you pull the bolts

You wont get the hoses off without taking risers off. A leaking riser gasket will do big damage, so it won't hurt to R&R the risers and have a look anyway. Mine was showing some signs of leaking, so it was new manifold time.

Not sure what year and model that we are talking about here but I have a '97 382 FasTech w/HP500's. I took the Silent Thunder box off and found my internal flappers gone also. I called Formula and Mercruiser dealers, looked on different parts websites and couldn't find anything that would work? So I installed external rubber flappers on the tips (left the ST box off) and didn't replace the internal flappers. Ran all year like that with no problem but would rather have the internal ones as a backup. Do you have the Part #'s? I have 4 " exhausts.....

This mb a bit late, but the part#'s are 12647 for the flappers. And 12646 for the support brackets. 1 flapper for each pipe, ands 2 brackets for each pipe. Some #6 or #8 1/2 in screws with lock nuts, and you are all set. .

I've thought about this also, why not just clamp on some Salsbury-type external flappers to the tips inside the platform? You say your platform is off now, so, clamp on the external flappers and put the platform back on. As long as the external flappers do not make contact with the inside of the platform when the open, there should be no problem. The original clam-shell type flappers that fit inside the tips are junk. They're way too wimpy and thats why everyones (including mine) are gone. Plus, they rattle.

I have large 4" cut-outs in the bottom of my platform with deck plate fittings to close them off. I've looked up in there through the cut-outs and there appears to be enough room to install external flappers without interference with the internals of the platform.