Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Transmission - Rearend (
-   -   M-21 Muncie - What Ya Using For Lube? (

lakeroadster 11-11-2012 06:06 PM

M-21 Muncie - What Ya Using For Lube?
Just as the title states: What Ya Using For Lube on a M-21 Muncie? My '65 C-10 has red trannny fluid in it from the previous owner. I need to change the lip seal in the tail housing, so nows the time to choose the right lube.

cobalt327 11-11-2012 06:39 PM

Here is a post covering it:

Good luck.

lakeroadster 11-11-2012 07:03 PM

Thanks for the link.

I am wondering how big a deal it is in regards to flushing the trans to get the ATF fluid out before filling with Sta-Lube GL4 from Napa?

cobalt327 11-11-2012 09:11 PM


Originally Posted by lakeroadster (Post 1610116)
Thanks for the link.

I am wondering how big a deal it is in regards to flushing the trans to get the ATF fluid out before filling with Sta-Lube GL4 from Napa?

If the trans in in the car, do two fills.

First fill run for a short distance just to thoroughly mix the new lube w/the remaining ATF (if that's what it was and not some sort of synthetic gear lube), then dump. Second fill keep for the duration.

The smell will tell you if it was ATF if there's any question, obviously if it was piss thin you know it's not right because the correct gear lube is very viscous compared to any ATF.

66GMC 11-11-2012 10:16 PM

I can't say that I've ever heard a recommendation to use ATF in an M21, but I do recall Ford's recommendation of using it in a NP 203 transfer case on a Ford 4x4 in order to improve cold-weather shifting.

Those T-Cases also had a little oil pump in them as well, and I suspect that they mave have been another reason behind using a lower-viscosity lubricant than the 80W90 gear oil, which is thicker than molasses at -30.

cobalt327 11-12-2012 04:54 AM

FWIW there are several common manual transmissions that use ATF type fluid (like the T-5 for one), but like you said- sure not a Muncie 4-speed.

The OP could have a synthetic manual trans lube like Castrol BOT 402- it's red and has a GL 4 rating. But it's also thicker than ATF: SAE 75W-85. But at least it would be the right lube.

I'd do the smell test before assuming it's ATF, though. The two (ATF and gear lube) smell markedly different. Hopefully it's been filled w/gear lube and it just looks like ATF.

lakeroadster 11-12-2012 05:10 AM

Thanks guys, I 'll switch it over to Sta-Lube GL4. I found this article while searching the internet.

ATF in a muncie? - THE H.A.M.B.

I had a 1985 S-10 with a 5-speed manual, it used ATF in the tranny. But on my ole Muncie that thicker GL4 will help to keep my truck from "marking it's territory. Well the thicker lube will help, but the new lipseals and shift arm seals will no doubt help more. :D

Thanks again,


gino21 11-12-2012 06:36 AM

m 21 muncie
What ever you run steer clear of synthetic gear oil it doesn't allow the synchros to do their job especially at higher rpm cause it's. too slippery.

AutoGear 11-12-2012 06:44 AM

Gino is correct.
Also, The fill/drain hole is located incorrectly. I wish I still had the service bulletin but, the actual measurement is to fill it till you can insert the tip of your finger and bend it at the first knuckle and touch the fluid. They designed it around a 1qt capacity. Over filling it is actually as damaging as under filling it.

You can run a Muncie on 1qt with no averse effects. It will run cooler and shift faster. The P/N for the Sta-Lube is SL24229 @ Napa. I have a lot of customers who also use Brad Penn and the only synthetic I can recommend (mostly for people who can't find the Sta-Lube) is Royal Purple's MAX GEAR in a 75w-90 or there abouts. Only the MAX GEAR though. You do not need an additive package with any of these oils.

All times are GMT -6. The time now is 10:03 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.