Help to I.D. 4 speed please. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Myself · 03-17-2006, 11:22 AM

I got a '67 lwb parts truck last week. The trans is a 4 speed. The option on the glove box door says 4M20AN . The reverse gear is all the way left and down, never seen one like this. Anybody know?

**CPNE** · 03-17-2006, 11:28 AM

SM420

Myself · 03-17-2006, 04:34 PM

So it looks like with those ratios, as far as street driving, that trans is junk. Right??

JAKES 68GMC · 03-17-2006, 04:52 PM

Depends what you are going to do with the truck. If you want to shift fast and hot-rod around, than probably not the tranny of choice. If you just want to cruise around once in awhile, then I think it would be fine.

I guess it would be more like a personal preference thing.

**dennislbrooks** · 03-17-2006, 05:07 PM

If it is decent--I have interest--I have a PTO that fits it.

TP from Cntl PA · 03-17-2006, 06:39 PM

What is wrong with a 420 for driving? With the granny 4's, you only actually ever use 2nd, 3rd, and 4th. Loaded is the only time you ever need 1st, and it better be a big load.

Ratio's aren't much different than a 3 spd sag.................I can't tell much difference between them and the SM-465's I run.

Scrubby · 03-17-2006, 07:56 PM

4th gear is still 1:1 which is the same as you will get with any non overdrive transmission manual or auto. This trans should be fine for the street.

72armyswbtruck · 03-17-2006, 08:30 PM

I agree and if you are looking for someone to trade or sell it to, besides dennis, Jeep owners love them because they use the tranny and mount it up to a 350 during the 350 swap.
BTW Scrubby I agree totally

Old Yeller 1970 · 04-15-2006, 08:49 PM

My truck has the SM465. It's similar to the SM420. 2nd, 3rd and 4th are all geared about the same. However, the SM420 has a very steep 1st gear. Like 72Army said, Jeep drivers and even first gen Bronco drivers really like it for it's rock crawling capabilities.
However, take off in 2nd gear with a 3.07 rear end is close to what an auto trany in 1st gear with a 3.73 rear end gets. Thus, I have surprised a few of my auto trany car friends over the years with a good beatin' at the hands of an old Chevy truck in the 1/4 mi.
With that in mind if you run a 3.07, someone with a 3.73 would have to have a .82 OD to match the engine RPM's of the 3.07 SM420/465 in 4th gear.
SM420-465's run well with a 350 that has a long flat torque curve.

*wolfthing2000* · 04-15-2006, 11:30 PM

Best tranny for any Jeep!!!! I had a Muncie 4 in my CJ-5 which was fine for drags or hilol climbs, then I put a SM420 behind it's Buick V-6 and it is a Jeepers dream!!! Very low geared and TOUGH!!! Meant to be!!!

I drove with one in a 66 Chevy 3/4 ton that I purchased 2nd owner and 50,000 miles in 1970 and I hot rodded and drove that truck until 1993 with campers , towing, snowing and blowing !!! That was the best truck ever. I would still have it except my EX killed it in a divorce let some boyfriend hack it and got sold after setting dead for 3 years . Someone got it for prolly a few hundred or less.

Excellent tranny. Probably still an option in those years because Chevy truck people weren't ready to change just yet, even though there was the 465. The granny gear is lower and straight cut and you actualy engage that whole mesh, not like the 465 with dogs. They sound way cool too!!!!

*wolfthing2000* · 04-15-2006, 11:34 PM

67 must have been a transition year when both were available. I have a SM420 in the garage right now mated to a Rockwell 221 Timken transfercase that came out of a 67 4x4 3/4 ton.

*wolfthing2000* · 04-15-2006, 11:42 PM

1947 -1967 chevy and GMC trucks. used also in a military vehicle called the Gamma Goat I believe . It was like a cabover and the tranny had a remote style cover shift set up rather than a cane shift. THE one i have in mynJeep was one of thjese and came in a box that was poured with a foam packing that surrounded the tranny. It was olive drab and slathered in cosmoline bran spanking new!!! That's right a brand spanking new 1947 technology tranny packed new in a box and manufactured in the mid seventys for the military ( not GM) and unpacked in 2002 and installed with a top from a wrecdking yard for the cane shift.

What a gear box!!!!!

68C15 · 04-16-2006, 12:40 AM

what did GM use in the 1/2t in 68? did they have some of the 420s left over?
the reason I ask is I think mine had one. I'll have to check in the garage in the morning.

**'68OrangeSunshine** · 04-16-2006, 06:36 AM

I heard Canadian production lines used the 420 into 1969 -- maybe later. U.S Chevy was supposed to change over to the 465 in the '68 model year. GMC sometimes used up older components after Chevy had moved on.
My '68 C/10 Stepside has the 465. [Fremont truck]

03-17-2006, 11:22 AM	#1
Myself Registered User Join Date: Oct 2005 Location: Northern Arkansas Posts: 1,144	Help to I.D. 4 speed please. I got a '67 lwb parts truck last week. The trans is a 4 speed. The option on the glove box door says 4M20AN . The reverse gear is all the way left and down, never seen one like this. Anybody know?

03-17-2006, 11:28 AM	#2
CPNE Resident Curmudgeon Join Date: Dec 1999 Location: NH Posts: 6,662	Re: Help to I.D. 4 speed please. SM420 __________________ Currently on or near the homestead: 67 Chevy SWB 2WD stepside 350/3 on tree (Pat's) 67 GMC SWB 2WD Fleet 402/auto (Brian's under construction) 67 Chevy 3/4 ton 2WD 402/auto (Business Hauler) 67 Chevy 1 ton dually 2WD 396/4 speed (Former business hauler, Needs TLC) 68 Chevy 1/2 ton Suburban 2WD 250 six/3 on tree (Brian's Needs TLC) 70 Chevy 3/4 ton 4WD 350/4 speed (Pat's - Disguised as a 68 GMC) 71 Chevy SWB stepside (Crushed by tree - parts donor) 72 Chevy 3/4 ton 4WD (Parts donor) 72 Chevy 3/4 ton 4WD Suburban (Parts Donor) 72 GMC 3/4 ton 4WD 292 six/4 speed (Mine - Disguised as a 67 GMC) 81 GMC 4WD Dually Dump Body 350/4 speed (Business Hauler) 82 Camaro Z/28 355/Super T-10 (Pat's toy) 93 Caprice 9C1 (Brian's Cop Car) 02 Toyota Camry (Reliable but a souless steel and plastic hulk) 2011 2SS RS Camaro M6 Factory Hurst Shifter Maybe I need to sell some of this crap Yet another Bozo with a sawz-all

03-17-2006, 04:34 PM	#3
Myself Registered User Join Date: Oct 2005 Location: Northern Arkansas Posts: 1,144	Re: Help to I.D. 4 speed please. So it looks like with those ratios, as far as street driving, that trans is junk. Right??

03-17-2006, 04:52 PM	#4
JAKES 68GMC Aluminum slot wheels rule... Join Date: Jan 2006 Location: Maine Posts: 6,211	Re: Help to I.D. 4 speed please. Depends what you are going to do with the truck. If you want to shift fast and hot-rod around, than probably not the tranny of choice. If you just want to cruise around once in awhile, then I think it would be fine. I guess it would be more like a personal preference thing.

03-17-2006, 05:07 PM	#5
dennislbrooks Senior Member Join Date: Feb 2004 Location: Williamsburg, Ohio Posts: 1,798	Re: Help to I.D. 4 speed please. If it is decent--I have interest--I have a PTO that fits it.

03-17-2006, 06:39 PM	#6
TP from Cntl PA Registered User Join Date: Apr 2003 Location: Central PA....In the Berg of Nisbet Posts: 790	Re: Help to I.D. 4 speed please. What is wrong with a 420 for driving? With the granny 4's, you only actually ever use 2nd, 3rd, and 4th. Loaded is the only time you ever need 1st, and it better be a big load. Ratio's aren't much different than a 3 spd sag.................I can't tell much difference between them and the SM-465's I run. __________________ '72 K-20 EFI'ed 250 Inline/4spd stake body, Farm Truck '71 C-20 Cummins Diesel Powered, In storage thanks to $5.00 diesel! '69 3500 GMC 305V-6/4spd, Still under reconstruction.......... Inlines Rule! 6 soldiers standing is better than 8 laying down!!!!!!!!

03-17-2006, 07:56 PM	#7
Scrubby Check out GMTrucks.org Join Date: Sep 2003 Location: Boise, Id Posts: 284	Re: Help to I.D. 4 speed please. 4th gear is still 1:1 which is the same as you will get with any non overdrive transmission manual or auto. This trans should be fine for the street.

03-17-2006, 08:30 PM	#8
72armyswbtruck Registered User Join Date: Feb 2004 Location: Idaho Posts: 4,033	Re: Help to I.D. 4 speed please. I agree and if you are looking for someone to trade or sell it to, besides dennis, Jeep owners love them because they use the tranny and mount it up to a 350 during the 350 swap. BTW Scrubby I agree totally __________________ "Remember, They can Kill You, but they can't eat You, That's against the law!!!!!!!!!"

04-15-2006, 08:49 PM	#9
Old Yeller 1970 DON'T TREAD ON ME Join Date: May 2005 Location: Crosby County, TX Posts: 989	Re: Help to I.D. 4 speed please. My truck has the SM465. It's similar to the SM420. 2nd, 3rd and 4th are all geared about the same. However, the SM420 has a very steep 1st gear. Like 72Army said, Jeep drivers and even first gen Bronco drivers really like it for it's rock crawling capabilities. However, take off in 2nd gear with a 3.07 rear end is close to what an auto trany in 1st gear with a 3.73 rear end gets. Thus, I have surprised a few of my auto trany car friends over the years with a good beatin' at the hands of an old Chevy truck in the 1/4 mi. With that in mind if you run a 3.07, someone with a 3.73 would have to have a .82 OD to match the engine RPM's of the 3.07 SM420/465 in 4th gear. SM420-465's run well with a 350 that has a long flat torque curve. __________________ Old Yeller

04-15-2006, 11:30 PM	#10
wolfthing2000 Account Suspended Join Date: Oct 2004 Posts: 1,276	Re: Help to I.D. 4 speed please. Best tranny for any Jeep!!!! I had a Muncie 4 in my CJ-5 which was fine for drags or hilol climbs, then I put a SM420 behind it's Buick V-6 and it is a Jeepers dream!!! Very low geared and TOUGH!!! Meant to be!!! I drove with one in a 66 Chevy 3/4 ton that I purchased 2nd owner and 50,000 miles in 1970 and I hot rodded and drove that truck until 1993 with campers , towing, snowing and blowing !!! That was the best truck ever. I would still have it except my EX killed it in a divorce let some boyfriend hack it and got sold after setting dead for 3 years . Someone got it for prolly a few hundred or less. Excellent tranny. Probably still an option in those years because Chevy truck people weren't ready to change just yet, even though there was the 465. The granny gear is lower and straight cut and you actualy engage that whole mesh, not like the 465 with dogs. They sound way cool too!!!!

04-15-2006, 11:34 PM	#11
wolfthing2000 Account Suspended Join Date: Oct 2004 Posts: 1,276	Re: Help to I.D. 4 speed please. 67 must have been a transition year when both were available. I have a SM420 in the garage right now mated to a Rockwell 221 Timken transfercase that came out of a 67 4x4 3/4 ton.

04-15-2006, 11:42 PM	#12
wolfthing2000 Account Suspended Join Date: Oct 2004 Posts: 1,276	Re: Help to I.D. 4 speed please. 1947 -1967 chevy and GMC trucks. used also in a military vehicle called the Gamma Goat I believe . It was like a cabover and the tranny had a remote style cover shift set up rather than a cane shift. THE one i have in mynJeep was one of thjese and came in a box that was poured with a foam packing that surrounded the tranny. It was olive drab and slathered in cosmoline bran spanking new!!! That's right a brand spanking new 1947 technology tranny packed new in a box and manufactured in the mid seventys for the military ( not GM) and unpacked in 2002 and installed with a top from a wrecdking yard for the cane shift. What a gear box!!!!!

04-16-2006, 12:40 AM	#13
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	Re: Help to I.D. 4 speed please. what did GM use in the 1/2t in 68? did they have some of the 420s left over? the reason I ask is I think mine had one. I'll have to check in the garage in the morning.

04-16-2006, 06:36 AM	#14
'68OrangeSunshine Senior Member Join Date: Feb 2002 Location: Tucson, AZ USA Posts: 7,517	Re: Help to I.D. 4 speed please. I heard Canadian production lines used the 420 into 1969 -- maybe later. U.S Chevy was supposed to change over to the 465 in the '68 model year. GMC sometimes used up older components after Chevy had moved on. My '68 C/10 Stepside has the 465. [Fremont truck] __________________ Every 25 years I like to rebuild that 292, whether it needs it or not. Last edited by '68OrangeSunshine; 04-16-2006 at 06:39 AM.



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