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Discussion Starter · #1 ·
I have a '70 that I put a 383 into, new flex plate and TC. Removed my old TH350, bought a freshly rebuilt one, and put it in this weekend. I now have a loud, consistent knocking coming from the upper section of the bellhousing that wasn't there before. Sounds loudest right where the engine meets the trans topside. TC bolts are the same as before and all tight. Housing bolts all seem tight. Flexplate doesn't appear to have any cracks (shouldn't since its brand new).
Anyone have any advice on where to go next?
 

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I have a '70 that I put a 383 into, new flex plate and TC. Removed my old TH350, bought a freshly rebuilt one, and put it in this weekend. I now have a loud, consistent knocking coming from the upper section of the bellhousing that wasn't there before. Sounds loudest right where the engine meets the trans topside. TC bolts are the same as before and all tight. Housing bolts all seem tight. Flexplate doesn't appear to have any cracks (shouldn't since its brand new).
Anyone have any advice on where to go next?
Pull the trans back out and look for scoring on the bellhousing and bolts first. Check the TC on the input side of the shell, look over your starter nose and really give everything a thorough once-over before jumping into the trans having something wrong. I had the trans dipstick tube tab sandwiched between the block and bellhousing once as a teenager and totally missed it. Started the car and damn. Bap bap bap bap... Thought it was the trans... Pulled it back out and realized what I had done (Or not done correctly)... Hahaha... Ugh! Hope it's something that easy
 

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Discussion Starter · #4 ·
Thanks for the quick response and good options. Not a lockup converter, no. Almost sounds like the flexplate is banging against the top of the bellhousing. I'll pull it and see what's what. If that is the case, what would cause that and how would I remedy it?
 

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Thanks for the quick response and good options. Not a lockup converter, no. Almost sounds like the flexplate is banging against the top of the bellhousing. I'll pull it and see what's what. If that is the case, what would cause that and how would I remedy it?
Well, gotta see what and where first. Also, make sure your converter bolts to the flexplate aren't bottoming out on the tc shell before they completely tighten. That's another typical noisemaker for sure.
 

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You can also whack your flexplate with a mallet prior to taking the trans out to see if there are loose converter bolts. Just turn the crank to a bolt, whack the flexplate gently towards the engine and listen. Then, turn to the next and repeat. May save the trouble if yanking it out... Just a thought.
 

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Discussion Starter · #9 · (Edited)
Got a little further today. Disconnected the TC and backed it off the flexplate just to isolate the problem. Still knocking without the TC engaged at all. Triple checked my housing bolts and they seem tight. Even jacked up the trans just a bit to see if I could create daylight between the block and the trans but nothing.
If I look at the flexplate now disconnected from TC with the engine idling it looks like it has a visible wobble (just a couple mm's). I'm not sure how much, if any, it should have it being "flex" and all. I'm thinking I could have bent it pulling the trans in and out. It's a tight fit so that is totally possible. I can't see any visible scoring from my limited view underneath on the bellhousing or on the flex plate.
Any parting advice before I yank this thing out again? Really appreciate the help so far.
 

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Discussion Starter · #10 ·
Got a little further today. Disconnected the TC and backed it off the flexplate just to isolate the problem. Still knocking without the TC engaged at all. Triple checked my housing bolts and they seem tight. Even jacked up the trans just a bit to see if I could create daylight between the block and the trans but nothing.
If I look at the flexplate now disconnected from TC with the engine idling it looks like it has a visible wobble (just a couple mm's). I'm not sure how much, if any, it should have it being "flex" and all. I'm thinking I could have bent it pulling the trans in and out. It's a tight fight so that is totally possible. I can't see any visible scoring from my limited view underneath on the bellhousing or on the flex plate.
Any parting advice before I yank this thing out again? Really appreciate the help so far.
And also checked to make sure the starter wasn't getting in the way. No banging happening there either.
 

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Mike - 74 Nova Baltimore, MD
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Is this a new 383 build?
If so, my guess is one (or more) of the connecting rods is hitting the oil pan while the engine is running.
You may need to drop the oil pan and look for any contact marks on the pan (especially in the area where the pan is bolted to the block)... and if any are found, just slightly bang the area of the pan out with hammer.
 

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Its a good point. I'm not thinking its the transmission any more. Something with the way it was connected. Definitely something I caused not my trans mechanic.
Sounds like you need to pull it again, sadly. Hoping it's just the flexplate wobble hitting something up top... Fingers crossed!!
 

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Discussion Starter · #17 ·
d to drop the oil pan and look for any contact marks on the pan (especially in the area where the pan is bolted to the block)... and if any are found, just slig
Its a new crate engine
The knocking wasn't there before with my old trans and its loudest from the top of the bellhousing. I'm not hearing it from the oil pan so right now I'm not thinking it's my engine. Lot of new stuff going into this build so it's been difficult diagnosing for sure.
 

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Discussion Starter · #19 ·
is your crate motor built with a roller-cam? can you time the knock with the rpms of the motor? Or is it random? Wondering if it may be the distributor or something in the valve train... something to look at...
Engine does have a roller-cam. The knocking is with the RPMs, it isn't random. Doesn't sound like something loose bouncing around more like a bang along with the RPMs.
I'm going to pull the flex plate and try it again. That should answer my bent plate theory. If it isn't that then it's gotta be engine but I'd be surprised (not my first time surprised though).
 

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Engine does have a roller-cam. The knocking is with the RPMs, it isn't random. Doesn't sound like something loose bouncing around more like a bang along with the RPMs.
I'm going to pull the flex plate and try it again. That should answer my bent plate theory. If it isn't that then it's gotta be engine but I'd be surprised (not my first time surprised though).
Just don't rev it without the flexplate attached. Just start it, listen then shut it off. Sounds like a good plan. Hoping it's not a roller in there on #7/8 making all that noise...
 
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