loud knocking noise coming from under motor

[ulises milanez]

my 1987 gmc high sierra just started making a loud knocking noise sounds to be where engine mates to tranny i unbolted torque converter and noise goes away any idea what that may be

[88Jimmy4x4]

were the TQ converter bolts tight?

[ulises milanez]

yes they were tight

[Tx Firefighter]

Likely a cracked flexplate. They're cheap and available at any parts store.

[f.monroe]

I bought an 87 that had a bad knock. It was cheap and I was thinking it needed a new engine. I looked close and the flex plate was cracked at the mounting bolt area where it connects to the crankshaft. Look at that closely. You can see it with a good flashlight.
Frank
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[ulises milanez]

i will have to check that out as this flexplate is maybe 8 months old thanks for the info ill look to see if its cracked

[INSIDIOUS '86]

A lot of newer flex plates seem to be cracking...
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[Chevy Fleetside]

I vote flexplate also. I thought I had a rod knock one also and the flexplate was acually broken but it still drove. I drove it 60 miles like that to the shop.

[88Jimmy4x4]

Originally Posted by Tx Firefighter

Likely a cracked flexplate. They're cheap and available at any parts store.

Aside from swapping it out, isnt there a problem since the flexplate is balanced with the other rotating mass?...I know mine was when I built my engine

[KQQL IT]

Usually the flex plate/flywheel is left alone. For that reason.
And 350's are nuetral balance 400's have a weight.
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[hatzie]

Originally Posted by KQQL IT

Usually the flex plate/flywheel is left alone. For that reason.
And 350's are nuetral balance 400's have a weight.
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I think the single piece rear main seal engines have a weight on the flex plate too.

[f.monroe]

Originally Posted by ulises milanez

i will have to check that out as this flexplate is maybe 8 months old thanks for the info ill look to see if its cracked

Be sure to check the bell housing bolts . I have seen them loosen up and cause the flex plate to be in a bind. Since yours is only about 8 months old I would look at it closely.
Frank
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[KQQL IT]

Originally Posted by hatzie

I think the single piece rear main seal engines have a weight on the flex plate too.

I was thinking that as well. But wasnt sure.
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[88Jimmy4x4]

Originally Posted by hatzie

I think the single piece rear main seal engines have a weight on the flex plate too.

exactly, and his being an 1987 would be a one piece seal correct?

my machine shop required flexplate to balance on my one-piece rear seal block 5.7L 350 (USA racing engines....wop shop in Houston)

let me be clear lol....I'm asking you guys for him and my own future reference if this would cause a problem with just bolting on another right out of box?

[hatzie]

Originally Posted by 88Jimmy4x4

exactly, and his being an 1987 would be a one piece seal correct?

my machine shop required flexplate to balance on my one-piece rear seal block 5.7L 350 (USA racing engines....wop shop in Houston)

let me be clear lol....I'm asking you guys for him and my own future reference if this would cause a problem with just bolting on another right out of box?

Yup. If it's the original iron it should be one piece.

I bolted one up to the targetmaster 350 in my 76 along with a yard fresh 700R4 & NP208 and rear driveshaft. Then I drove it from Arizona to Cincinnati with whipped ball joints... because I had more balls than brains. It ran just fine the whole trip. Pulled that combo in favor of the diesel that it's running now.