diagnosing mystery rattle/ flex plate clearance

[spedigo]

i’ve had this 79 for a couple of months and I’m working my way through all its problems.

It’s had a pretty good intermittent rattle when under decent acceleration that I have assumed might be broken blades in the torque converter. at least that is where the sound seems to be coming from. I Finally got around to looking at it yesterday and unbolted the converter from the flex plate to see if I could isolate if it was coming from either the flex plate or the torque converter.

Flex plate is not cracked, but it is definitely warped and I will be replacing that. but I noticed that there is less than 1/16 of an inch clearance between the flex plate and the bell housing.
what is normal for this?

The wobble on the flex plate with that small amount of clearance could give some grinding I guess. There’s a bit of paint missing on the housing, but it doesn’t look horrible and the teeth look OK. Can anyone tell me what that clearance should be?

or another possibility is the wobbling flex plate is making contact with the starter at higher RPM. Picture of that attached.

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Also, I spun the torque converter and all I really hear is some fluid slosh but it doesn’t sound like there are any loose/broken blades. I’ll try and include a short video for the sound. (Note, I did spin it at a higher RPM using a drill and didn’t hear anything out of the ordinary.)

i’ve been assuming an issue with the torque converter existed because as soon as it goes into gear, it’s got a rough loping idle that I haven’t been able to tune away. It Idles fine in park and in neutral.

just wanna be sure that I work my way logically through all the various parts to see if I can find the offending sound but also to be sure that I replace the questionable components with the right size stuff. The previous owner put together a bit of a Frankenstein engine and transmission.


flex plate:

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TC:

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[Buck69]

If you know the flexplate is warped, questioning clearance to t/c or anything else, would be moot for me.
What ignition system does it have? I ask because some will adjust timing to try to smoothen out the idle. For those, you could lock them in base timing mode to eliminate it from contributing to the loping idle. A timing light would also help identify that if it is an issue.

 

[Ricko1966]

79 what? I'm assuming truck but don't want to assume 2 or 4 wd

 

[squaredeal91]

To do videos you have to upload to you tube first then attach on here

 

[spedigo]

If you know the flexplate is warped, questioning clearance to t/c or anything else, would be moot for me.
What ignition system does it have? I ask because some will adjust timing to try to smoothen out the idle. For those, you could lock them in base timing mode to eliminate it from contributing to the loping idle. A timing light would also help identify that if it is an issue.

it’s an HEI. i set timing to 8° BtDC and about 34 total timing. it idles like a champ. only When in gear at a stoplight does it get rough.

 

[spedigo]

79 what? I'm assuming truck but don't want to assume 2 or 4 wd

sorry i thought that was included
79 C10 2wd 350/350

Numbers indicate transmission is likely a rebuilt 79 350. The engine, at least the short block, is an 88 to 91. 305 TBI heads. Like I said, a bit of mish mash when I got it.

 

[spedigo]

i added links to the videos of flexpkate and tc to my original post above.

 

[Ricko1966]

sorry i thought that was included
79 C10 2wd 350/350

Numbers indicate transmission is likely a rebuilt 79 350. The engine, at least the short block, is an 88 to 91. 305 TBI heads. Like I said, a bit of mish mash when I got it.

Okay 2 wd r&r flex plate,the cheater version buy 2 more long starter bolts un screw a transmission to engine mounting bolt 1 on each side replace with the long starter bolts. Unbolt the torque converter,unbolt and drop the rear of the drive shaft wrap the ujoint in tape so it doesn't come apart. Pull and plug your cooler lines,pull the shift linkage unbolt the cross member at the frame. Support the transmission on a piece of wood an a floor jack support the oil pan on a block of wood and a scissor jack undue the remaining perimeter bolts. The transmission will slide back far enough for you to unbolt and remove the flex plate then you can see what's wrong. Was it put in backwards,put in bent etc. Etc. but now it's easy enough to slip in a new flex plate slip the transmission forward put in 2 or 3 perimeter bolts. Common sense will tell you what to put back in bare minimum for a test start to see if the problems gone. If you aren't sure what bare minimum is,ask . Your lope at idle could be vacuum advance make sure it pulls in fully at idle in neutral then pinch the line shut with hose pliers or vise grips. Put it in gear let me know if the problem is gone.

 

[Old Guy Bill]

Cracks in the flex plate occur between the bolt holes and flange, very hard to see without removing it…..

 

[spedigo]

Cracks in the flex plate occur between the bolt holes and flange, very hard to see without removing it…..

I’ve had the flex plate off to replace the rear seal and there weren’t any cracks. But i’m replacing the flex plate anyway
The thing i would really like to know is if that less than a 1/16” gap with the bell housing is typical?

 

[Buck69]

Does the gap change with the engine barred over in different positions? That could indicate there is a runout problem with the crankshaft.
I don't believe the overall diameter of the flexplates really changed if that is your concern. There was differences with them, with 1 and 2 piece rear seal models along with tooth counts.
Are you going to install new mounting screws with the new flexplate?
Looks like you still have a pretty good leak back there.

 

[spedigo]

Does the gap change with the engine barred over in different positions? That could indicate there is a runout problem with the crankshaft.
I don't believe the overall diameter of the flexplates really changed if that is your concern. There was differences with them, with 1 and 2 piece rear seal models along with tooth counts.
Are you going to install new mounting screws with the new flexplate?
Looks like you still have a pretty good leak back there.

yeah, it’s still leaking. It’s better than it was when i got it by far but I’m gonna have to address it again.
I’m getting very different suggestions from Summit, rock auto and O’Reilly on the flexplate size. based on the block information and the transmission, summit is saying a 153 tooth and everywhere else a 168 tooth. 168 tooth is what is on there now.
Probably won’t get to look at it until next weekend, but I’ll turn it over and see what it looks like

 

[Buck69]

yeah, it’s still leaking. It’s better than it was when i got it by far but I’m gonna have to address it again.
I’m getting very different suggestions from Summit, rock auto and O’Reilly on the flexplate size. based on the block information and the transmission, summit is saying a 153 tooth and everywhere else a 168 tooth. 168 tooth is what is on there now.
Probably won’t get to look at it until next weekend, but I’ll turn it over and see what it looks like

Someone will chime in and correct me if I am wrong here, but it is my understanding that it is your starter that dictates which tooth count you go with. The diameters are the same with both, which effects the clearance you are concerned with.
If the starter has a staggered bolt pattern you want the 168. If inline, go with the 153.

 

[spedigo]

Someone will chime in and correct me if I am wrong here, but it is my understanding that it is your starter that dictates which tooth count you go with. The diameters are the same with both, which effects the clearance you are concerned with.
If the starter has a staggered bolt pattern you want the 168. If inline, go with the 153.

sounds right. mine is staggered that’s an answer then. thanks.

 

[Ricko1966]

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.