TH350 spun a bearing! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

TugOwar · 02-20-2005, 11:32 PM

You guys ever heard a th350 "knock"?

Developed a knock on the way home from work saturday night. Today I went out to try to find it. Cranked it up and slid under it and it sounded like it was coming from right between the motor and tranny.

I removed the torque converter bolts and slid the torque converter back away from the flexplate and cranked it up..... I can barely hear a low end "bump" in the motor. Its a tired old 305 V-8 so I know the mains have wear on them. But it was nothing like what I was hearing with the tranny hooked up.

Put TC bolts back in and the knock is back and worse than before.

What is inside the front end of a th350 that could cause a knock? Maybe something inside the torque converter?

Hotrod70C10 · 02-20-2005, 11:38 PM

My buddy has a 84' 1/2 ton and it was doing the same thing, He drove it like that for a good six months. He just put a new tranny in it this week. I'm not sure what's up with it though.

TugOwar · 02-20-2005, 11:48 PM

hmmm did he change the TC as well?

dons454s · 02-21-2005, 01:15 AM

sounds like the front pump bushing and possibly the pump is out of it.

cableguy0 · 02-21-2005, 01:23 AM

a blown torque convertor will make almost the exact same sound as a spun rod bearing. theres also two sprags in that thing that will make a loud racket when ya blow em apart. the only way to find out for sure. is to yank the tranny and tear it down and look

**COBALT** · 02-21-2005, 04:09 AM

Torque converters have moving parts in them. A sprag clutch can lock up preventing the center from spinning, but any other time both halves are moving against each other. If it begins to fail by either the sprag clutch or some damage to the impellers the halves can start to strike each other causing particles and other bits of material to end up in the fluid. A good indicator that a torque converter is failing is the transmission fluid comming out of the torque converter looks like silver paint.

I wouldn't keep driving it until the transmission fails. That fluid works its way into pan via the clutches and the valve body. The more damage the torque converter receives the more particles end up in the fluid.

Replacing the torque converter is a cheap fix compared to the need to rebuild the transmission.

dwaite72lnghrn · 02-21-2005, 07:00 AM

I would look at the engine again. You still heard the noise after the TC was disconnected. Did you pull the dip stick and see if it had any bearings on it. With no load the knock would get lighter. Rod knocks are when you let of the rpms you can hear or feel them. If I remember right wrist pin noise is all of the time. Don't write off the engine as a noise maker.

modifiedGMC · 02-21-2005, 07:19 AM

You could tke a length of rubber hose and with the engine running pull the engine dipstick out and stick the hose over the dipstick tube. Then put the other end to your ear. You should hear a noise something like a sewing machine running but if a rod knocks you'll hear it. Just take your RPM's up a little and back off while using the hose and it will make the noise more noticeable.

GMC AMI · 02-21-2005, 07:45 AM

A mechanics stethoscope works well for this. You can isolate the noise better.

TugOwar · 02-21-2005, 11:05 AM

Great information guys thanks very much.

I wondered about the decreased load idea. When I had the TC unbolted I ran the motor up to about 3500 rpm and there was no knocking. I will try the hose and see what I can hear.

Here is another possible clue. Sitting in the truck, holding the brake, engine at idle, when I pull it into reverse the knocking stops until I speed up the motor a bit.

**COBALT** · 02-22-2005, 07:09 PM

I believe (someone correct me if I'm wrong) the torque converter is locked up when spinning in the opposite direction. If that's the case then I bet it might be that front pump bushing has opened up, and you're hearing the squealing of the converter kneck on that brass bushing.

Is the torque converter loose in the transmission when it's completely seated in the front pump? Does it wiggle around at all?

**VetteVet** · 02-22-2005, 07:43 PM

Another way of checking for a rod knock is to pull the spark plug wire on the suspected cylinder and if the noise goes away then that rod is bad.
VV

JBstor · 02-22-2005, 09:01 PM

If the engine stops making noise with the TC unbolted, get a trouble light and take a good hard look at the flexplate. I worked for years in transmission shops, and this is VERY common. they like to break around the bolts, either the tc bolts or the crank bolts.

I have, for the record, never heard a TH350 make a knocking noise.

HTH
JB

02-20-2005, 11:32 PM	#1
TugOwar Registered User Join Date: Sep 2004 Location: Pleasanton, TX Posts: 1,723	TH350 spun a bearing! You guys ever heard a th350 "knock"? Developed a knock on the way home from work saturday night. Today I went out to try to find it. Cranked it up and slid under it and it sounded like it was coming from right between the motor and tranny. I removed the torque converter bolts and slid the torque converter back away from the flexplate and cranked it up..... I can barely hear a low end "bump" in the motor. Its a tired old 305 V-8 so I know the mains have wear on them. But it was nothing like what I was hearing with the tranny hooked up. Put TC bolts back in and the knock is back and worse than before. What is inside the front end of a th350 that could cause a knock? Maybe something inside the torque converter?

02-20-2005, 11:38 PM	#2
Hotrod70C10 Low & Slow Join Date: Dec 2002 Location: Oklahoma Posts: 1,047	My buddy has a 84' 1/2 ton and it was doing the same thing, He drove it like that for a good six months. He just put a new tranny in it this week. I'm not sure what's up with it though. __________________ 70' C-10 LWB Fleetside 4.5/6 drop

02-21-2005, 01:23 AM	#5
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	a blown torque convertor will make almost the exact same sound as a spun rod bearing. theres also two sprags in that thing that will make a loud racket when ya blow em apart. the only way to find out for sure. is to yank the tranny and tear it down and look __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

02-21-2005, 04:09 AM	#6
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Torque converters have moving parts in them. A sprag clutch can lock up preventing the center from spinning, but any other time both halves are moving against each other. If it begins to fail by either the sprag clutch or some damage to the impellers the halves can start to strike each other causing particles and other bits of material to end up in the fluid. A good indicator that a torque converter is failing is the transmission fluid comming out of the torque converter looks like silver paint. I wouldn't keep driving it until the transmission fails. That fluid works its way into pan via the clutches and the valve body. The more damage the torque converter receives the more particles end up in the fluid. Replacing the torque converter is a cheap fix compared to the need to rebuild the transmission. __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

02-21-2005, 07:00 AM	#7
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	I would look at the engine again. You still heard the noise after the TC was disconnected. Did you pull the dip stick and see if it had any bearings on it. With no load the knock would get lighter. Rod knocks are when you let of the rpms you can hear or feel them. If I remember right wrist pin noise is all of the time. Don't write off the engine as a noise maker. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

02-20-2005, 11:48 PM	#3
TugOwar Registered User Join Date: Sep 2004 Location: Pleasanton, TX Posts: 1,723	hmmm did he change the TC as well?

02-21-2005, 01:15 AM	#4
dons454s Registered User Join Date: Nov 2004 Location: Claysburg, Pa Posts: 408	sounds like the front pump bushing and possibly the pump is out of it.

02-21-2005, 07:19 AM	#8
modifiedGMC Registered User Join Date: Oct 2004 Location: Walker Co. Alabama Posts: 474	You could tke a length of rubber hose and with the engine running pull the engine dipstick out and stick the hose over the dipstick tube. Then put the other end to your ear. You should hear a noise something like a sewing machine running but if a rod knocks you'll hear it. Just take your RPM's up a little and back off while using the hose and it will make the noise more noticeable.

02-21-2005, 07:45 AM	#9
GMC AMI Registered User Join Date: Apr 2004 Location: The Other Maine! Posts: 2,236	A mechanics stethoscope works well for this. You can isolate the noise better. __________________ 1968 Chevy 4x4 307 4 speed

02-21-2005, 11:05 AM	#10
TugOwar Registered User Join Date: Sep 2004 Location: Pleasanton, TX Posts: 1,723	Great information guys thanks very much. I wondered about the decreased load idea. When I had the TC unbolted I ran the motor up to about 3500 rpm and there was no knocking. I will try the hose and see what I can hear. Here is another possible clue. Sitting in the truck, holding the brake, engine at idle, when I pull it into reverse the knocking stops until I speed up the motor a bit.

02-22-2005, 07:09 PM	#11
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	I believe (someone correct me if I'm wrong) the torque converter is locked up when spinning in the opposite direction. If that's the case then I bet it might be that front pump bushing has opened up, and you're hearing the squealing of the converter kneck on that brass bushing. Is the torque converter loose in the transmission when it's completely seated in the front pump? Does it wiggle around at all? __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

02-22-2005, 07:43 PM	#12
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,709	Another way of checking for a rod knock is to pull the spark plug wire on the suspected cylinder and if the noise goes away then that rod is bad. VV __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.

02-22-2005, 09:01 PM	#13
JBstor Registered User Join Date: Feb 2005 Location: Winnipeg, Manitoba, Canada Posts: 66	If the engine stops making noise with the TC unbolted, get a trouble light and take a good hard look at the flexplate. I worked for years in transmission shops, and this is VERY common. they like to break around the bolts, either the tc bolts or the crank bolts. I have, for the record, never heard a TH350 make a knocking noise. HTH JB

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