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Discussion Starter #1
I recently swapped a sm420 in my jeep. I have heard of the common problem of the trannys popping out of 2nd. I was hoping mine wouldnt do it but I should have known better.

What exactly causes this problem. Does it have anything to do with worn clutch dogs. When installing novaks main shaft, I noticed that one of them was wore flat unlike the other two which had a bit of a lip on them.

Will a rebuild fix the problem

Even if the tranny pops out of second I still love the 7.05 first gear and the sound of straight cut gears!
 

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Does it do it going down hill after a while or something else? My dad's 63 Chevy carryall that he bough new would always do that on a ceartain spot going down the mile long hill we lived on. Also that tranny made a certain sound that I think was from second gear wobling slightly. I will never forget that sound. I grew up in that truck.
The sm420 I have in my Scout dosn't pop out of gear.
You could try putting new springs under the detent balls.
I love SM420's. What a great transmission!
 

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Chubby Chaser
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Shoot a PM to PinHead, he is a master of the SM420;)
 

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What ever it is, it can't be good. The possibilities are broken arms on the low gear slider and/or their detent springs, worn dog teeth or a worn out second gear bushing. Just get another one for $50. It isn't a common problem. It is more common to pop out of 3rd or 4th.
 

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Mine does the same thing, hell, I can hardly even use 2nd. All new brass, and I swapped out the original counter gears because the 1st set had some PTO abuse.

I've had it out three times now and it's still not right, I can shift to 2nd most times, but it will jump out seconds after I let the clutch out, sometimes I can't even get it into 2nd.
 

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Discussion Starter #7
I was hoping for an easy fix like hit it hard with a hammer :flipoff2:

I guess that I will look around for another one. Sucks that I took it apart to install the main shaft and put it in the jeep and now I get to do it all over again.
 

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Hey Pinhead while you are cureing all the SM420 blues...mine grinds when going from 3rd to 2nd unless I am almost stopped everything else works great. any Idea's?
 

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Unfortunately it is probably a worn out second gear synchro ring and the ring is built in to the low-second slider gear on the main shaft. They don't make them any more and a NOS part will cost more than another 420. The good news is that you don't often have to shift into second while you are moving, so it is easier to put up with than a tranny that pops out of gear. :D
 

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HANGTOWNCRUZR Dude, learn to double clutch and you won't need any sincro's. I double clutch every downshift and it goes in like butter. If you learn to do it well enough you can drive without the clutch if needed. Lots of big rig drivers drive without the clutch.
 
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