UPDATE: Help with SM420 (1 Viewer)

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I discovered an ugly sounding noise coming from my SM420 after driving at 40-65 mph for about an hour. I didn't hear anything while driving and everything seemed fine, but I had to crawl underneath the vehicle to check on something else, and I heard a lot of noise coming from the Tranny..... it sounded like a bunch of gears rattling around. I freaked out and took it straight back home (about 50 miles) without a problem, keeping it under 60 mph. When I got it home, I took the clutch housing off and it seemed like the clutch was spinning a little wobbly. The noise was rather pronounced and seemed to definitely be coming from the Tranny, and not the TOB. I shut it down and let it sit overnight. Today I started it up and the noise was gone. I ran the engine for 5-15 minutes, and shifted the gears, but I didn't hear the noise and the clutch seemed to be spinning normally. I was listening with a screw driver today while a friend depressed the clutch, and I could hear a slight change in the sound but nothing significant. (NOTE: I couldn't do this some test yesterday when it was making all the noise because I didn't have anyone around to disengage the clutch).

I know this isn't much to go on, but does anyone have any thoughts on what might be happening in that Tranny? I did pull this Tranny 9 months ago and replaced the front seal, but I'm certain the Bearing Retainer was properly torqued.

The only thing I can think of is that at the time I took the Bearing Retainer, the (Clutch) Output Shaft slid out of the Tranny slightly. Since the Tranny was sitting at a downward angle, the shaft and bearing had a natural tendence to wany to slide forward. I didn't know, and hadn't been told that this was BAD until it already happended. I was then told "DONT DO THAT!
You run the risk of collapsing the needle bearings inside that shaft if it moves too far forward, as well as having the forward synchro get unsynchronized."


So that is one possibility of what has happened. I don't know how to determine if that is the case. I assume those needle bearings would fall to the bottom of the case, so draining the fluid might provide the answer??

Does this sound like I am going to need to pull the Tranny to find the answer? Would driving just to more damage to the Tranny at this point?

Thank you for your advice!!!!
 
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I drained the gear fluid from the Tranny: No metal parts were detected. The oil came out completely clean. I inserted an inspection light (fiber optic) into the drain plug and couldn’t identify any roller bearings or metal at the bottom of the housing. I was able to insert it all the way to the front where it meets the BH.

So, no explanation for the noise I was hearing, at least not caused by any loose needle bearings unless they are just loose and banging in the retainer??

Next step prior to pulling the Tranny is to take the lid off and see if there is any signs of issues. I’m no expert so I’ll be posting photos. Of course if there is anything obvious….

7BF86002-1BF2-40F9-83CE-FC3DC6E32664.jpeg
 
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Well, after taking the lid off, there isn’t much to see. No obvious issues. However, after watch this video

on YouTube it seems to me that if all or some of the Roller Bearings fell out, they would fall to the front toward the clutch shaft bearing and not into the the Tranny case, therefore there wouldn’t be visible but they would make the noise I was hearing.
I’m just surprised no one mentioned this.
It seems to me that the only way to know is by pulling the Tranny and removing the Clutch gear shaft from the case.
 

pb4ugo

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The 420 is one of the easier trans to work on. If your curious youll have to remove it. Fyi, never grind going into second.
 
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I love guys who say: “one of the easier trans to work on” and assume every person on this list has the tools and the mechanical knowledge to work on a transmission. That kind of thinking is what got me in this situation in the first place. THANKS for your deep insight!
 

pb4ugo

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The dogs or splines for 2nd gear that the shift collar slides over are part of the shaft and cant be replaced. If you grind 2nd the dog's where down and the collar has less engagement. When they wear too far the tranny will start to pop out of 2nd. It will progressively get worse.
 
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What does this have to do about noise coming from the the tranny e.g. like loose gears or bolts in a washer? It’s simply not helpful information.
 
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If the needle bearings in the input gear pilot got loose, they would end up inside the transmission and probably be stuck to the magnet. There is no way for them to get outside without a big hole in the tranny case. The 420 is reasonable easy to work on, but I wouldn’t assume it is responsible for the noise If there is no obvious loose metal inside
 

pb4ugo

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Rereading your 1st post, you drove it for an hour and the tranny made noise. The 90wt warmed up during that hour and it's viscosity is thinner. The next day you said you ran it for 15 min. The 90 wt was not warmed up so it wasnt as noisy. How many miles or how old is the tranny,? Is there a stub shaft in the adapter? Does the output shsft have a locking collar?
 
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If the needle bearings in the input gear pilot got loose, they would end up inside the transmission and probably be stuck to the magnet. There is no way for them to get outside without a big hole in the tranny case. The 420 is reasonable easy to work on, but I wouldn’t assume it is responsible for the noise If there is no obvious loose metal inside
No, there is no obvious loose metal inside. From the videos I’ve seem, it seems to me that they could fall out forward toward the clutch and not fall into the tranny housing….. Is that not correct?

Something was rattling in the tranny and it wasn’t normal. There isn’t any metal on the magnet of any consequence. The clutch was wobbling. Everything suggests the roller bearings. I can’t think of what would cause the gears to start rattling. The tranny does not have high mileage or wear. I have the lid off but I don’t know what to look for in terms of slop. Things look/seem normal.
 
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Rereading your 1st post, you drove it for an hour and the tranny made noise. The 90wt warmed up during that hour and it's viscosity is thinner. The next day you said you ran it for 15 min. The 90 wt was not warmed up so it wasnt as noisy. How many miles or how old is the tranny,? Is there a stub shaft in the adapter? Does the output shsft have a locking collar?


Good questions. The Tranny is 20 years old but has very few miles. The adapter is an Advanced Adapters, and I believe it does have a stub shaft. Not sure what you mean about a locking collar. The AA Adapter is 50-2602 SM420 to LC 4SPD TC, it combines the 10 Spline GM shaft to the 16 Spline TLC 4Spd TC Input Shaft. I believe there is a Set Collar on the Adapter.

Are you thinking the problem is the Adapter or TC?
 

pb4ugo

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It's hard to say what's wrong. Didn't you recently have the tcase off? What was done to it or why? The collar goes on to the 420's output shaft. It needs to be against the rear bearing. It keeps the rear bearing in place on the shaft. If it's not against the bearing, I believe you will have some slop in the mainshaft. It's just a guess. It could have worn bearings. Idk.

By the way. I always tell an owner of a 420 to avoid grinding going into 2nd.
 
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It's hard to say what's wrong. Didn't you recently have the tcase off? What was done to it or why? The collar goes on to the 420's output shaft. It needs to be against the rear bearing. It keeps the rear bearing in place on the shaft. If it's not against the bearing, I believe you will have some slop in the mainshaft. It's just a guess. It could have worn bearings. Idk.

By the way. I always tell an owner of a 420 to avoid grinding going into 2nd.
“ I did pull this Tranny 9 months ago and replaced the front seal,”

(NOTE: I was advised to replace the seal when I was having the TC rebuilt…. I should have followed my own instinct and philosophy of: “if it ain’t broke, don’t fix it!”)
 
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Does anyone know for a fact if the Needle Bearings can drop out and NOT fall into the case, I.e, can they get retained in the recess behind the front bearing? Is it possible that a few needle bearings dislodged, fell into the recess and are the source of the noise I heard? Would missing needle bearings explain the clutch wobble? Is it possible this doesn’t show up when first starting because the gear oil hasn’t been spun around from the gears cranking at higher speeds, and the oil isn’t moving the loose roller bearings at that point, and the shaft is still turning somewhat concentric. After driving, the shaft gets out of balance and the bearings start moving.

Is this possible?

Is there anyway to determine if any of the needle bearings are loose? What could you test for?

Maybe I’ll never know. Once you pull the shaft, all the bearings fall out.
 
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If they fell out it will eventually cause shifting to second issues and input bearing issues, fwiw, IMO the 420 would eat those up and youll never know, I too had same issue, took front bearing retainer off and input moved forward, tink, tink, tink, what was that, learned how to rebuild a 420.
 
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It seems to me that if the roller bearings dropped into the case and the gears chewed them up you would get a substantial amount of metal on the magnet. I didn’t.

Hearing a noticeable noise like bolts in the dryer or loose gears rattling around coming from the Tranny at idle while in neutral from several feet away with the engine running doesn’t seem like a normal condition.

I pulled the lid and inspected the gears the best I could and I couldn’t detect any noticeable issue, but I’m no expert either. There was no metal, and no noticeable damage Ilike bolts in the dryer or loose gears rattling around uld see. I may try driving it and seeing if I can replicate the noise for a better diagnosis. The hump is off so I can pull it without a problem once I better determine what the source of the noise is.
 

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