Vibration issue


New Member
Apr 3, 2024
Hello, please read the whole thread before posting because you will save your own and my time doing so.

347 / 302 with a T5 Tremec is vibrating very bad. 67 mustang
It vibrates when standing still and also in and out of gear, clutch in or out - SO ITS NOT THE DRIVESHAFT - Thanks!
I made 100% sure its not the headers and i have cut a lot into the TCI bellhousing to make room for them.

I have spent countless of hours trying to diagnose this and this is my last resort.

Its vibrating worst around 2200 rpm but in general more rpm = more vibration.
I have changed the engine because i had a 347 laying around and i know that thing did not vibrate. Also bought a brand new flywheel just because i had no idea if the old one was 28 or 50oz. I know my 347 is a 28oz engine because its freshly built - Also the harmonic balancer is new.
I removed the transmission again and started up the engine with the bellhousing and flywheel etc. on. - no vibration. I then go install the transmission again and the vibration comes back - then i was thinking its the transmission mount. Bought OEM style rubber trans mount and also rubber engine mounts - didnt help.
I removed the transmission again and installed everything without clutch pressure plate and clutch disc. still vibrating. I can visually see the input shaft is not even spinning but still vibrating. Then i am thinking now - it simply cant be the engine because i KNOW this engine was in my other mustang with a 28oz flexplate and no issue - but then again transmission not even spinning how would that be the cause? I then remove the transmission again and install new pilot bearing just in case the pilot bearing was "banging" on the input shaft when rotating and causing the vibration - but no luck for me.. I then removed the transmission again and i find some play in the input shaft. asked several transmission guys they say that the input shaft play is OK. I can feel the output shaft also moving the i move the input shaft. This is not the cause?

If anyone can please help me its highly appreciated.
Sorry for the bad english
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I know it sounds very elementary, and actually sounds like BS, but my dad had the same exact issue with my sisters 65 fastback with t5 conversion. He went through everything you have, rebuilt the trans twice, replaced the flywheel and clutch assembly, still it vibrated . Like you with the trans out and just the engine running, no vibration at all. Come to find out, after much internet surfing, a fellow told him that he had the same problem, and it turned out to be a bump, more of a welded on reinforcement, in the trans tunnel that the transmission was rubbing on. He told dad to pull the trans and he should see the shiny spot where it was rubbing and causing a vibration. He pulled the t5 (5th time) and sure enough, there was a shiny place on the trans. He looked up, and found the corresponding shiny place on that hump in the trans tunnel. A cutoff wheel and a couple whacks with a 5 pound hammer gave it the proper clearance, no more vibration. Hope that works for you, I know it about drove dad crazy.
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