I have a sm420 mated to an early (3spd) case, behind a new generation (vortech heads, 1pc rear seal, ect.) 350, in a 72 fj40.
The problem i am having is with the 420, it was VERY noisy in 1st gear from the start, (had new bearings put in by a toyota/chevy tech, said all else looked good), but i didnt worry bout it cause its no pavement pounder only gets driven to the trails(untill i get a tow rig) Then, it began to pop out of 2nd and 4th, and more recently 1st. I've been told it could be caused by engine/tranny misalignment? pops out mostly on decell.
thanks

If it is caused from mis alignment, it's too late and has already caused damage. Usually it will wear the mainshaft.
Sounds more like bad synchro's, and bearings.
What type bell housing are you using?
It should be the cast iron truck version.
Butch

I would guess you got the wrong year shift cover on it, I hear that it will cause it to pop out of gear lilke that, I guess the shift forks don't align with the gears/sincros and such. Changing your cover is real simple, just get another complete tranny (you need the extra hard parts anyway) and swap covers and see what it does.

-Scott

You might have parts from two different SM420's connected. Not sure of when the trans model changed, but I know one did have a reverse light on the top. The two different types will NOT work together. I have an older one that doesn't use a front seal, so just make sure you don't over fill it, or it will leak and make your clutch slip...and don't ask how I know this stuff:rolleyes: (The first SM420 I had in mine would pop out of 2nd also, finally found out it was a mixed & matched trans)

Joseph A.

Thanks, where do i look for mainshaft wear problems? i already have the top plate off and didn't see anything obvious. I'll definately try the top plate/ shifter from my spare 420, or build it, and try that one. I actually have no clue what kind of bellhousing it is, the truck was a sloppy 305/3spd conversion when i bought it, was told it was a chev. belhousing, so i reused it with the 420. Also pirate 4x4 kicks a%%!, you guys got some sweet rigs, this is like porn for me! Again thanks for your help! kyle

I'll take a pix of the bell housing tomorrow, have one in the back of my truck, but the digital camera is at work. You have to tear the tranny all the way down and remove all the gears and bushings from the shaft to check it for wear.
Does you bell housing fit the bearing retainer on the tranny snug?
Butch

Originally posted by 72rockcruiser
VERY noisy in 1st gear from the start, pops out of 2nd and 4th, and more recently 1st.
thanks

That is a lot of problems for 1 tranny. Just get another one at the pick and pull and put a rebuild kit in it for $125. If is a 2 bannana job that your average person can do.

BTW, you can find the correct bellhousing in the pick and pull behind 6 cyl engines in big GM trucks from 1952 to 1967. The average person doesn't know that the 6 cyl used the same bellhousing.

Here is a couple of shots of the cast iron truck bell housing, look for the angled sections where the truck had mounts, the car cast version did not have these and it is quite a bit lighter in weight than the truck version, My daughter broke one of the car versions, too much strain with the transfer case mount at the rear in my thinkin'
Butch

'Nother angle

Last picture. BTW it is bolted to a 3 spd LC tranny and X case combo.
Came out of one of my parts rigs that had a 250 inline Chevy 6 in it.

Originally posted by TxCruzr
You might have parts from two different SM420's connected. Not sure of when the trans model changed, but I know one did have a reverse light on the top. The two different types will NOT work together. I have an older one that doesn't use a front seal, so just make sure you don't over fill it, or it will leak and make your clutch slip...and don't ask how I know this stuff:rolleyes: (The first SM420 I had in mine would pop out of 2nd also, finally found out it was a mixed & matched trans)

Joseph A.
Reverse lights were only available in 67 and newer trannys (effectivly one year) The change in SM's came about around 56 I think.
Mine pops out of third when I let off the gas hard. It will be replaced by a new junkyard tranny in the near future..

So i was wondering... the 420 i have was supposrted by the belhousing mounts as in the picture, and the 3speed xcase is ment to be hung off the back of the tranny, so if one were to use a tranny xmember to support the tranny/xcase, where would you put the mounts?

Matt:confused:

texican, you rock! i really think you hit the nail on the head! i am actually pretty sure the bearing retainer is not snug in the bellhousing, that along with the aluminum bellhousing(i looked at it today, its aluminum.)is causing too much tweak on the tranny, huh? I guess its time to freshen up my spare and plug it in! With a cast truck bellhousing and proper bearing retainer, of course!!
Thank you very much! kyle
p.s. how much is too much to pay for that stuff, i.e bellhousing and bearing retainer? I dont have much loot, I have been trying to do saginaw for 2 years, Should be starting on it next month, Whoooo Hooo!!!

GM did make some aluminum bell housings back in the 60's and 70's that will fit the 420. The Muncie & Borg Warner 4 spds, and the 3 spd car trannys share the same bolt pattern and center hole size. However this one I have pictured is the strongest. I can still locate the used casy iron ones in my rural area for $30-50, but I would imagine in metro areas they might be rare. The bearing retainer is on the front of your 420......
Fixxer, all 420's were hanging out in the wind via the bell housing just as the Toyota stuff was, I've often thought we ought to ditch the wing mount at the rear of the Xcase and fab up bell housing mounts like the engineers designed.....
Butch