What transmission is this?

[IslandGMCguy]

I have my 1966 GMC 910 (Canadian) 1/2 ton pickup at a transmission shop for having the transmission rebuild. They told me it was a rare and unusual transmission in it?? It is a 4 speed manual on the floor. They said they were having a lot of problems finding internal parts for it. They told me the trans was a MNNP420 transmission. I have never heard of this and can’t find any info online either. Anyone know what this is???

[FetchMeAPepsi]

Try looking at SM420 and see if the trans matches yours. I'm pretty sure they don't know butt from elbow at that transmission shop. You can rebuild them pretty easily on your own if they bugger it up. Sm420 was a very popular trans.

[Funky61]

Novak has the kits for it and I always thought they were fairly common

https://www.novak-adapt.com/catalog/transmission/transmission-parts/sm420-parts/

[Ed Snyder]

I have my 1966 GMC 910 (Canadian) 1/2 ton pickup at a transmission shop for having the transmission rebuild. They told me it was a rare and unusual transmission in it?? It is a 4 speed manual on the floor. They said they were having a lot of problems finding internal parts for it. They told me the trans was a MNNP420 transmission. I have never heard of this and can’t find any info online either. Anyone know what this is???

If it's actually an SM420, and not an "MNNP420", then it is neither "rare" nor "unusual" as your shop claims. They were installed in thousands, if not millions, of trucks from 1947 to 1967.

Good luck with your rebuild!