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The infamous SM-420
#1392912 Sat Jan 16 2021 07:22 PM
Joined: Jan 2021
Posts: 16
D
'Bolter
I hope I'm posting in the right area, I wanna participate. My sweetheart(1964 C-30 Suburban) So it kept popping out of 2nd gear even if I held the shifter down, it seemed like it was gear related as opposed to Syncro related. I have 2 of them and Pop had one he didn't need. Internals Look brand new in there, and it feels beautiful. It's the older model and it didn't have the hand brake. So I swapped out the yoke and rear cover assembly and between the two I had I was able to recreate/build the propeller shaft hand parking brake. Which I had to have the band relined(I hate paying to have stuff done. I don't call a plumber, electrician, or carpenter....ever! But brake lining is difficult because materials and procedures are top secret). Anyhow a tranny swap turned into a clutch, rear main seal, the pilot bearing was stuck like chuck and I had to cut it & chisel it out in pieces. Found out I have a 1980-85' 350ci power plant with all of the 1964 attachments and housings. The person in there before me didn't know there's a pin hole in the crank and flywheel for a reason which causes me some concern. This covid thing has hit us financially and since I hate to have stuff done by someone else I decided to resurface the flywheel myself......go ahead and breath everyone I have a max runout of .003(pretty darn good if you ask me). I mounted the bell housing and followed my 1963 Chev truck shop manual(supposed to be a chev factory shop manual??🤬) Which I find a little confusing. I think they pull engine/tranny when they do a clutch because there doesn't seem to be a procedure for engine in truck. Although there's reference to "use transmission section to remove tranny" in the clutch section?(I accidentally ordered the 1964 shop supplemental for cars and not trucks so maybe it's in there?) By far the most in depth procedure for a clutch I've ever done(on an Automobile. I'm getting to finish driving the pilot bearing home and see if I can squeeze the pressure plate and disc between the crossmember and bellhousing.'m gonna have to rebuild another sm-420 probably end up with 1out of the 2 I have. Does anyone know if syncros are available yet as an aftermarket item? or have any other advice or knowledge regarding the procedure?

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Re: The infamous SM-420
Dexstar 64 #1392921 Sat Jan 16 2021 08:11 PM
Joined: Sep 2001
Posts: 31,205
Bubba
I would first check with Novak Conversions for SM420 parts. [novak-adapt.com]

They might sell some parts without buying a full kit?


Tim
1954Advance-Design.com [1954advance-design.com]
1954 3106 Carryall Suburban [stovebolt.com] - part of the family for 49 years
1954 3104 5-window pickup w/Hydra-Matic [1954advance-design.com] - part of the family for 15 years
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engines are not "splash oilers" - this is a splash oiler. [chevy.oldcarmanualproject.com]
Re: The infamous SM-420
Dexstar 64 #1392923 Sat Jan 16 2021 08:19 PM
Joined: Feb 2004
Posts: 23,004
H
'Bolter
.003" runout on a flywheel is huge- - - - -get ready for lots of clutch chatter. I hope you used a tool post grinder to get the right surface finish, also. There are complete overhaul kits for the SM 420 on Ebay with bearings, gaskets, and other small parts, along with plenty of gears, synchronizers, and other hard parts. Those transmissions are so bulletproof that rock crawlers install them in virtually everything. If you're having clearance problems with the crossmember while installing the clutch, unbolt the rear motor mounts and jack up the bellhousing a little, and/or install mounts that aren't worn out and collapsed. I've installed dozens of those clutches with no problems whatsoever.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: The infamous SM-420
Dexstar 64 #1392925 Sat Jan 16 2021 08:29 PM
Joined: Jul 2000
Posts: 1,311
G
Insomniac
To be clear, you have 2 transmissions and they both keep popping out of second gear. So, you would like to rebuild one of them and you need parts. Novak has rebuild kits. See this link:
https://www.novak-adapt.com/catalog/transmission/transmission-parts/sm420-parts/

My SM420 pops out of second when under deceleration. Plus, I can only shift into second from third while moving very slowly. Maybe it's a common problem with these units.


Gord
----
1954 1/2 ton 235 4 speed
Re: The infamous SM-420
Dexstar 64 #1392938 Sat Jan 16 2021 10:27 PM
Joined: Sep 2001
Posts: 31,205
Bubba
I just now noticed that this topic is about the "infamous SM420".

"infamous" = adjective. having an extremely bad reputation: an infamous city. deserving of or causing an evil reputation; shamefully malign; detestable: an infamous deed. Law. deprived of certain rights as a citizen, as a consequence of conviction of certain offenses.

I do not think of that transmission as having a bad reputation?

I think of it as the famous and favored SM420 transmission.

"The SM420 has proven to be one of the best transmissions ever built." (see the Novak Conversions links, in two prior posts, above)


Tim
1954Advance-Design.com [1954advance-design.com]
1954 3106 Carryall Suburban [stovebolt.com] - part of the family for 49 years
1954 3104 5-window pickup w/Hydra-Matic [1954advance-design.com] - part of the family for 15 years
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engines are not "splash oilers" - this is a splash oiler. [chevy.oldcarmanualproject.com]
Re: The infamous SM-420
Dexstar 64 #1393138 Sun Jan 17 2021 11:39 PM
Joined: Feb 2002
Posts: 3,009
Socket Breaker
sm 420 info:

http://kinzers.com/don/landcruiser/transmissions/index.html

also has info about the sm 465 as well. (for future reference, if anyone wanders in here)

---

novak was mentioned above,

https://www.advanceadapters.com

has various things listed as well, dunno if they have what you need tho'. worth a look.

---

duckduckgo.com

transmission parts sm-420

looks like lots of pieces are still available.

If you can't find it online, stop by a local transmission shop and ask them.

Re: The infamous SM-420
Dexstar 64 #1393156 Mon Jan 18 2021 01:10 AM
Joined: Jun 2011
Posts: 4,198
E
'Bolter
Last I heard, 420 synchros are hard to find, unless someone has started making them again.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
Re: The infamous SM-420
Dexstar 64 #1393165 Mon Jan 18 2021 02:20 AM
Joined: Sep 2001
Posts: 31,205
Bubba


Tim
1954Advance-Design.com [1954advance-design.com]
1954 3106 Carryall Suburban [stovebolt.com] - part of the family for 49 years
1954 3104 5-window pickup w/Hydra-Matic [1954advance-design.com] - part of the family for 15 years
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engines are not "splash oilers" - this is a splash oiler. [chevy.oldcarmanualproject.com]
Re: The infamous SM-420
Dexstar 64 #1393302 Mon Jan 18 2021 10:48 PM
Joined: Jul 2014
Posts: 284
1
'Bolter
Novac kits have both 3 & 4 syncros for the later transmissions that use the same size. As of last summer they did not have the one that is different on the earlier trans. I don't remember which one that is. I lucked out and had enough good syncros to finish the job. I don't remember if the kit contained the 2nd gear syncro. In any case I didn't replace it.

One thing I noticed was that the clutch-gear bearing supplied with the kit has fewer, larger balls than I have seen before on that part number bearing. I have a new bearing of the same size with more balls but no snap-ring groove which is probably why I still have it. Be careful installing the 2nd gear detent springs and shoes. If you don't get them positioned correctly the trans will jump out of 2nd gear. Jumping out of 3rd gear is usually because of worn clutch-gear or rear mainshaft bearing or both. This was the first SM420 I had worked on in about 10 years so my memory for the details is a little fuzzy.


51 3800 PU, 55 235 (w/cam, headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
Re: The infamous SM-420
Dexstar 64 #1393303 Mon Jan 18 2021 10:51 PM
Joined: Jul 2014
Posts: 284
1
'Bolter
PS on above post. Novac did allude to having the NLA syncro manufactured for them. Don't know if they did.


51 3800 PU, 55 235 (w/cam, headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
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