SM420 Noisy Third Gear

[BigSix]

Putting my SM420 into 3rd makes much more noise than 4th and I think more than 2nd. Its a sort of "rattle" style noise I would call it. Trans shifts into gears fine. Is there a particular cure to this issue or is it the nature of the beast?

[markeb01]

It should really be opened up and inspected. The loud howling noise typical to early Chevy transmissions is usually caused by chips or pits in the face of various gears. Over time this will cause additional damage to the mating gears. In a passenger car transmission this was amplified by the use of bronze bushings instead of ball bearings.

If it has a rattling sound, you could have a bad ball bearing that needs replacing or too much clearance from worn bushings. These are very sturdy transmissions, and can continue working for decades making noise with defective parts. But if neglected indefinitely, repairs are far more extensive when the time comes to fix it.

[BigSix]

Ok I will do an inspection. Can 3rd gear be inspected with just the top cover off? Basically can I leave it in the truck and go through the floor hump?

[morethanstock66]

I had an SM420 for 7 years. It is a louder transmission due to its gearing, but one gear shouldn't be drastically more than the others. You could have rounded or missing teeth

[BigSix]

I have no mat, or carpet, and the transfercase boot is missing and the shifter booter is loose - and yet the tranny is pretty quiet in every gear...except in 3rd. I'll add "rebuild trans" to the list of items I have to do the truck.

[morethanstock66]

IMO I say rebuild it, even if you end up swapping it later. They used to be buttoned up to big blocks and are incredibly strong, they will last forever if taken care of. If you do sell it later, Jeepers and other off roaders love them- obviously. I used mine because it could handle the power and would launch.

[61K10]

if your gears look good this is whats in the rebuild kit. Novak says the gears are almost non existent.

[BigSix]

It is my assumption, and only an assumption, that a bearing or something of the sort would cause the rattling sound. Of course a worn gear could too but for some reason I am thinking that is less likely. Ive been wrong many times before!