diagnosing mystery rattle/ flex plate clearance

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Bennyt

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Just an FYI as long as you are buying a new one. Pioneer sells an HD version flexplate. It used to be only $5-10 more at O'Reillys with an account but it is considerably thicker and better quality. It used to be SFI rated but I don't think they put the sticker on any more and unless you are racing and need to pass tech, the sticker isn't worth the extra $75.
 

Ricko1966

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Just occurred to me look at the warp,see if it looks like it's bowed from not having the convertor seated when they pulled the engine to transmission bolts tight. You may need a transmission front pump.
 

Buck69

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I just went through this a couple months ago with a '97 Vortec in front of a T400 in my '86 CUCV. It was relatively fresh in my head.
I went with a 168 tooth TCI (HD) flexplate and ARP mounting screws.
 

spedigo

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Yes flexplate is base on starter mounting as Buck said,153 is straight,168 is staggered. Many rear seals have been replaced more than once,due to oil pressure sender leaks. About your lope I told you before to get full vacuum advance and trap it,see if the problems gone. I call it dithering,don't know if it is correct term. But what it is at idle in neutral you have may have enough vacuum signal to hold the vacuum advance steady and fully,now you put in gear,and no longer have the vacuum to hold the advance steady,so the vacuum advance cycles in and out causing the lope. The fix would probably be a different advance can. But only after you've done everything to pull maximum vacuum at idle.
well, it held a vacuum when I did as you suggested. Holds pretty steady around 18 or 19 Hg.
here’s the clip:
https://youtu.be/41skLEZjyJU?si=TOrIFDKL8NS0TTYn
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But whenever it goes to drive, it’s still a bit lopey. not as bad as it was when I first got it I don’t think but it’s definitely dropping about 200 RPM as soon as it goes into gear. I’ve got it idling around 850 right now. It’ll drop a couple hundred off of that when it hits gear.

clip of dropping into gear typical:

xc_hide_links_from_guests_guests_error_hide_media
 

Ricko1966

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I don't think you are following what I'm trying to tell you. With the vacuum advance fully deployed at idle in neutral. Now pinch the line going to the distributor shut. Now the vacuum advance will get no more,no less vacuum it will have what it has,constant. Now do you still have a lope when you put it in drive ,with the vacuum line pinched closed?
 

spedigo

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I don't think you are following what I'm trying to tell you. With the vacuum advance fully deployed at idle in neutral. Now pinch the line going to the distributor shut. Now the vacuum advance will get no more,no less vacuum it will have what it has,constant. Now do you still have a lope when you put it in drive ,with the vacuum line pinched closed?
yes that’s what i did. vacuum holds after line is clamped and it’s in drive. but now maybe it’s less of a lope and more of a stumble due to the big rpm drop
 

trucktony

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I had the same problem with the mystery rattle
when under load
pulling off from a stoplight with more then half throttle I had this sound that was the same as detonation
A lot of turning on the HEI and day's later I found out that the flexplate (which was a bit bent like yours)hit the high torque starter nose/gear.
My thinking is : when under load ,the flexplate gets pulled a bit by the force on the transmission OR the axial movement of the crankshaft is put to the back (by which ever force??) and just touches the starter gear
I shimmed out the starter with the shims that were delivered when new (to put the starter more backwards) and the rattle is gone
to be honest : there was less then 0.040 room between starter nose/gear and flexplate when in off position
I must say that my apache truck had the same and they both got it when I changed to a 700r4 (but I don't know if thats part of the cause)
Good Luck and let us know if you found anything
Tony
 

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