SM 420 new synchros and bearings

This is not a rebuild, just a refresh. I have only fixed one other transmission, a t-90. I used the universal manual for that and a chilton book that just copied a lot of that service manual.

I am using this manual for my reference. http://kinzers.com/don/landcruiser/transmissions/

I hope to not destroy this transmission and I really hope to make it work again after I take it apart. The reason why I am dissembling it is because it grinds going into high gear. Double clutch and its ok, but something is wrong. I think its the brass synchronizer cone.

I like to learn. I would appreciate if you have experience rebuilding these transmissions that you would share your knowledge. Certainly if you see me doing something wrong or just dumb, speak up! I have minimal experience rebuilding transmissions.

This is my 1953ish SM420, bearings, gaskets and synchros with the attached Novak adapter for the d18. This 135 buck kit did not come with the synchros and I am not sure they are correct, as they call for a couple different ones. I did some research and ebay'ed one and the other was some heavy truck supplier. I was cheap and didn't get the Novak kit that comes with the synchronizer rings. Just buy the novak kit.

shift tower is off and these are the gears without me messing with them. Take off the reverse lever and don't lose the bushing the lever rides on.

this transmission has been apart before as the holes for the spanner wrench are egged out

some of you know what happens when you do this.

there is no turning back now. roller bearings in the bottom of the case, semi panic attack, this aint no game no more.

this clutch gear synchronizer cone might have something to do with poor shifts

the third and 4th gear clutch keys are riding out of the clutch gear synchronizer cone. I might need new clutch keys.

pull off the main shaft bearing to get it all out

This is the mainshaft assembly out of the transmission case. Bearing is just placed on the shaft for reference.


The kit didn't come with a cup to seal the countershaft front bearing and to remove it from the outside I would destroy it, so I used a wooden hammer handle to tap out the countershaft assembly toward the rear. Might still wreck that cup seal when I replace that front bearing.

now I can remove that bearing, pop out the front seal/cover and replace that bearing too.

done for tonight.

 

Here is a link to some other good pics.
http://www.pbase.com/dennygraham/sm420_rebuild&page=all

 

After you pull off that rear countershaft bearing the countershaft assembly goes back out of the front bearing and the assembly can be lifted out of the case.

Front countershaft bearing with spacer(plug) intact. This bearing has quite a lot of lateral movement.

drive out the spacer(plug) with appropriate sized socket and extension.

pops out with no damage. service manual says to drill hole and use a slide hammer to remove.

drive bearing into the case to remove with appropriate socket.

Take apart mainshaft assembly. removing 3rd and 4th speed clutch sleeve with clutch keys intact

remove snap ring and pull off 3rd and 4th speed clutch hub. light pressure to do this.

3rd speed gear synchronizer cone. worn.

3rd and 4th speed clutch sleeve and hub assembly. wear on the mating surface. Wasted 4th speed synchronizer cone in background.

worn clutch keys. Not chipped, just rounded corners.

take off 3rd speed gear assembly. has special washer and brass/bronze bearing/sleeve.

take off 2nd speed constant mesh gear

take off 1st and reverse speed assembly

here is 1st and reverse speed disassembled.

here is everything in its reference positions

 

Can I file the clutch keys to make a more defined edge? Anyone got a line on new keys?

 

These are the clutch keys. There is a bit of a ding on one corner. They are worn, but look serviceable to me.

The wear is consummate with all the parts throughout the transmission. The chance I took with the research I did for pre '54 sm420s seems to be correct for the synchro cones. They are the same as the ones that were in the transmission as found. Now, maybe those weren't the correct ones and have caused this failure?

All the parts are cleaned and ready for reassembly. Unless someone says otherwise I will reuse those clutch keys.

 

Can you find pictures of new ones, to make a comparison?

Here are mine, out of my T18. Don't know that it will help. All my edges were pretty square and looked good. The hump in the center of yours seems to be a different design than these have.

 

Thanks Dave,
Those look a lot nicer than mine. I get the feeling that mine might not have been manufactured as nicely as yours and mine are more worn for sure. I cannot find any mention anywhere online about much other than the rebuild kit minus the synchro cones for the sm420. Parts for these seem to be hard to come by in new form. If I could find pics of new ones I would have ordered them already for sure. There is no online "take it apart" guide that I could find other than the link to the pics I posted. I thought it might be nice for someone else to use this as a reference. So far its a pretty simple transmission. Anything will come apart though, we shall see how well I do putting it back together.

 

So, doing a bit more searching I have become across some terminology differences. Clutch keys, strut keys, shift dogs, others... these are all terms for the same thing. SM420 may also be a "muncie" transmission. Searching for SM 420 gets a lot of nothing for those keys, but muncie sm420 gets these http://www.thegearbox.org/catalog/item/3864782/9454027.htm I bought them. 15 bucks shipping on 3 bucks for the keys and springs and a tub of transmission assembly lube. oh well.

They might work.

 

The SM stands for synchronized muncie transmission.

 

Stated here on Novaks site they say it stands for "synchro mesh". http://www.novak-adapt.com/knowledge/transmissions/manual/sm420
There seems to be confusion about the terminology. I don't really care, just looking for parts. The manual says "clutch keys" but, struts, dogs, etc work. The keys I have ordered are different than the ones I have presently . If they are the same physical size they should be fine.

 

I took apart the countershaft assembly because after i put it in the case with new bearings there was a bit of mechanical noise while spinning the assembly that was not there when I took it apart.
Pulling it apart

apart

more apart

together

more together

pulling the counter shaft into the rear bearing

put the spacer over the front bearing

tap the bearing into the bore


The front bore for the bearing in the case allows me to pretty much just push the bearing in by hand. The bearing is new and seems to be where the mechanical noise originates. It is the same size as the old one and is clean. Checked everything about 6 times. There was nothing wrong with the counter shaft assembly other than the normal wear I am finding throughout this transmission.

 

Replaced input shaft bearing. in this blurry pic you can see the threaded nut that needs a special spanner that i don't have. I used a punch. They are left handed threads and were staked real well kind of a PITA.

press off the old bearing and clean it up. That bearing made a bunch of noise.

press on the new bearing, grind off the burrs you put on the spanner holes, put oil slingers on, tighten up the nut, wreck new flat slinger, take apart, put good old flat slinger on an tighten. tower of power

 

Thanks Chuck.
Yeah, those clutch keys might not work.. But I contacted http://touristtransmission.com/index.php and someone responded with specific questions about my clutch keys. I shall see what they have for me.

 

I have pulled that countershaft assembly about 20 times trying to find the source of the bearing noise. Not the rear bearing. Its the front. The bore might be oversize since I can push the bearing in most of the way with hand pressure. The old bearing fit the same way. New bearing is good, spins free with no issue, put it all together and there is a slight moan. I pulled off the front"spacer" cup and can move the bearing in and out on the countershaft while inside the case. Bearing locktite?

 

Noise wouldn't come from a bearing not fitting snug, it comes from non-smooth ball or race surfaces, right? Is it a new bearing?

 

I can't believe anyone is reading this. Thanks.

I went a bit nuts today trying to get to the bottom of this. I decided that i needed to compare this SM420 to another. I happen to have a spare. Thought it might be the countershaft, or the transmission case.. I took the other one apart, cleaned it up, swapped cases, swapped bearings, swapped countershafts from one case to the other. everything is the same size, bearings, bores, shafts, etc. and guess what? The noise occurs no matter what. Its raining. i did it in the rain outside.

The bearing is new, i cleaned it in new diesel fuel twice, blew it out twice, its just noisy. Both the cases allow the bearing to move around with old bearings and new. When I spin the bearing on the countershaft out of the case its silent. Put it it in the case and its noisy.

The other transmission that came out of a dump truck is in very similar shape to this one. I was going to break it down, but I can't get the synchronizer hub off without heat and effort. If i break this one, unlikely, I will just put that one in.

 

Got those Muncie synchronizer clutch keys. The WILL NOT work. They are dimensionally different, thinner with less of a protruding bump. Looks like I will be reusing those old ones. The ones in the spare transmission look exactly like the ones I wanted to replace, same wear but those synchronizer cones are in pretty good shape. Probably If i just swapped the transmissions, my shift issues would have gone away.

But then you guys would have missed out on the transmission excitement!!!!

 

Pressed on the clutch hub. I used the old clutch keys and slid the clutch sleeve on. The old synchronizer ring for 3rd gear looked ok, but upon further inspection, and when compared to a new one, the inside where the grooves catch on and cut the oil to slow the gear down or up for synchronization was worn. The new one is inside the clutch hub that is pressed on. Snap ring gets put on the mainshaft to keep the hub in place.

The ring on top of the main shaft was not on this assembly, but was on the other one. The loose roller bearings on both transmissions were the same length and diameter as is the bearing surface of the main shaft itself. It either doesn't belong or was left out of a previous repair of this transmission.

These ARE the correct synchronizer cone part numbers for a pre 1954 SM420

can't remember which is which