GM SM420 4-Speed Manual Transmission for Sale | OEM Used GM Heavy-Duty 4-Speed Manual Produced 1947 to 1967 with Ultra-Deep Granny Low
$1,499.00
At a Glance
- Condition: OEM used, all 4 forward gears and reverse tested
- Availability: In stock, GM SM420 4-speed manual transmissions available
- Shipping: Free shipping to all 50 states via freight pallet, 5 to 10 business days
- Donor truck era confirmed (1947-1967 production range)
- Granny low (7.05:1) engagement verified specifically (non-synchronized)
- Bell housing pattern confirmed (235 / 261 Inline-6 or early SBC)
- Input shaft spline count verified
- 4WD vs 2WD configuration confirmed
- No core charge required
- Backed by a 15 day replacement warranty against internal defects
Description
About the GM SM420 4-Speed Manual Transmission
The SM420 is a heavy-duty manual transmission introduced by General Motors in 1947 that remained in production for two decades before being replaced by the SM465 in 1968. This SM420 Transmission for sale at Part Nests was a favorite among classic truck enthusiasts, off-road builders, and heavy-duty truck owners during its production era, and remains highly sought after today for classic Chevrolet and GMC truck restorations. The SM420 was also used in various military applications, providing reliable heavy-duty service in demanding conditions.
Features
- Cast iron case construction: Suitable for off-road and heavy applications, nearly indestructible when properly maintained
- 4-Speed Configuration: 4 forward + 1 reverse. 1st gear is a deep non-synchronized granny low (7.05:1) for stopped-vehicle starts; 2nd / 3rd / 4th for on-the-move shifting. NOT a 5-speed transmission despite some product descriptions incorrectly listing it as such.
- Non-Synchronized First Gear: The 7.05:1 first gear is non-synchronized; vehicle must be at complete stop to engage smoothly. On-the-move shifting requires double-clutching to match RPMs.
- Simplicity: Mechanically straightforward design without electronic complexity. Direct mechanical feel that classic truck enthusiasts strongly prefer.
- Off-road capability: 7.05:1 first gear combined with 4.10 or 4.56 rear axle gives crawl ratios in the 28-32:1 range, ideal for rock crawling and heavy towing on steep grades.
Compatibility
Vehicles:
- Chevrolet Advance Design Trucks (1947-1955)
- Chevrolet Task Force Trucks (1955-1959)
- GMC New Design Trucks (1947-1955)
- GMC Blue Chip Trucks (1955-1959)
- Various military and commercial GM-based vehicles
Compatible Engines:
Note: The original product listing incorrectly labels the 235 and 261 engines as V8s. These are actually Inline-6 (straight-six) engines in Chevrolet famous Stovebolt family. Correct compatibility:
- Chevrolet 216 cubic inch Inline-6 (Stovebolt 6, pre-1953)
- Chevrolet 235 cubic inch Inline-6 (3.9L) (Blue Flame Six, 1953+)
- Chevrolet 261 cubic inch Inline-6 (4.3L) (Jobmaster Six, 1954+)
- GMC 248, 270, 302 Inline-6 engines (in GMC truck applications)
- Chevrolet 265 cubic inch V8 (1955+ Task Force, the original small block)
- Chevrolet 283 cubic inch V8 (later Task Force applications)
Advantages
- Off-road capability: Exceptionally low first gear (7.05:1, even deeper than SM465 6.55:1) ideal for rock crawling. With low-range transfer case, delivers crawl ratios approaching 60:1.
- Durability: Built for heavy-duty use in trucks, buses, military vehicles, heavy equipment of the 1947-1967 era. Cast iron case is virtually indestructible.
- Period-correct restoration: Original equipment for 1947-1967 Chevrolet or GMC truck restoration.
SM420 vs SM465: Which Should You Choose
The SM420 was replaced by the SM465 (our vaz133 listing) starting in 1968. The two transmissions share similar architecture but differ in important ways:
| Specification | SM420 | SM465 |
|---|---|---|
| Production | 1947 to 1967 | 1968 to 1991 |
| Speeds | 4-speed | 4-speed |
| 1st Gear Ratio | 7.05:1 (deeper) | 6.55:1 |
| Synchronization | Non-synchronized in some gears | 2nd, 3rd, 4th synchronized |
| Application Era | Advance Design, Task Force | C/K Series (1968-1991) |
| Bell Housing | 235/261/early SBC pattern | Later SBC, BBC, diesel patterns |
For authentic restoration of a 1947-1967 Chevy / GMC truck, choose the SM420 (this listing). For 1968-1991 C/K Series truck, choose the SM465 per vaz133. For LS swap classic truck build, either can be used with appropriate adapter, but SM465 offers smoother on-the-move shifting via synchronized 2nd-4th gears.
Vehicle Compatibility
| Vehicle | Compatibility |
|---|---|
| Chevrolet Advance Design Trucks (1947-1955) | Classic post-war pickup trucks |
| Chevrolet Task Force Trucks (1955-1959) | Second-generation classic Chevy pickups |
| GMC New Design Trucks (1947-1955) | GMC equivalent of Advance Design |
| GMC Blue Chip Trucks (1955-1959) | GMC equivalent of Task Force |
| Military / Commercial GM-Based | Various military and commercial applications |
| Classic Truck Restoration | Period-correct restoration of 1947-1967 GM trucks |
Why Buy From Part Nests
- Donor truck era confirmed (1947-1967)
- All 4 forward gears and reverse tested
- Granny low engagement verified
- Bell housing pattern confirmed (Inline-6 or V8)
- 4WD vs 2WD output configuration confirmed
- Free freight pallet delivery to all 50 states
- 15 day replacement warranty on internal defects
- Classic Chevy and GMC truck experienced support team
- Call (855) 634-4447 to confirm GM SM420 (heavy-duty 4-speed manual produced 1947-1967, NOT a 5-speed despite some incorrect descriptions), donor truck era (Chevrolet Advance Design 1947-1955, Task Force 1955-1959, GMC New Design 1947-1955, or GMC Blue Chip 1955-1959), bell housing pattern (235 / 261 Inline-6 or early SBC 265 / 283), 4WD vs 2WD output configuration, engine pairing (Chevrolet 216 / 235 / 261 Stovebolt Inline-6, GMC 248 / 270 / 302 Inline-6, or 265 / 283 small block V8), and SM465 per vaz133 successor (1968-1991, shallower 6.55:1 granny low, for later C/K Series and LS swap) before ordering
15 Day Replacement Warranty
Every used GM SM420 4-speed manual transmission purchased through Part Nests carries a 15 day replacement warranty starting on the delivery date.
What Is Covered
- Internal defects already present when the transmission arrives
- Gear failures within 15 days from installation on any of the 4 forward gears or reverse
- Granny low (1st gear) engagement failures within 15 days from installation
- Bearing failures within 15 days from installation
- Case crack failures not disclosed at shipping
- Incorrect bell housing pattern shipped (Inline-6 vs early SBC mismatch) or incorrect 4WD vs 2WD configuration due to an error on our end
What Is Not Covered
- Damage caused during installation
- Damage from attempting to engage 1st gear (granny low) while the vehicle is moving (non-synchronized; designed for vehicle-stopped engagement)
- Damage from attempting on-the-move shifts without double-clutching
- Damage from clutch operation issues (slipping, dragging, incorrect adjustment)
- Damage from neglected gear oil (GL-1 or GL-4 spec per pre-1968 GM service specification)
- Damage from overloading beyond SM420 design torque capacity
- Damage from incorrect bellhousing alignment at installation
- External components (clutch assembly, bellhousing, driveshaft, crossmember, shift linkage, fluids drained for transit) unless itemized
- Labor expenses of any kind
To start a warranty claim, reach us within 15 days of delivery at (855) 634-4447.
- Donor Truck Era Confirmed: 1947-1967 production range documented
- All Forward Gears and Reverse Tested: Function verified before payment
- Granny Low Engagement Verified: Non-synchronized 7.05:1 first gear confirmed operational
- Bell Housing Pattern Confirmed: 235 / 261 Inline-6 or early SBC pattern documented
- 4WD vs 2WD Configuration Confirmed: Output configuration documented
- 15 Day Replacement Cover: Internal defects protected from delivery onward
Additional information
| type | Manual Transmission |
|---|---|
| model | SM420 |
| maker | General Motors (Saginaw) |
| speeds | 4-speed (NOT 5-speed) |
| 1st-gear-ratio | 7.05:1 (granny low, non-synchronized) |
| case-material | Cast iron |
| production-years | 1947 to 1967 |
Heavy-duty 4-speed manual transmission, GM truck transmission 1947-1967. The SM420 is a 4-speed manual produced by General Motors from 1947 to 1967, the truck transmission of the classic Chevrolet Advance Design (1947-1955), Task Force (1955-1959), GMC New Design (1947-1955), and GMC Blue Chip (1955-1959) era. Cast iron case construction, 4 forward + 1 reverse, with an ultra-deep 7.05:1 non-synchronized granny low first gear (the deepest first gear of any GM truck manual transmission). Replaced by the SM465 per vaz133 starting in 1968.
No. The SM420 is a 4-speed transmission. Some product descriptions and aftermarket listings incorrectly label the SM420 as a 5-speed; this is incorrect. The SM420 has exactly 4 forward gears (1st through 4th) plus 1 reverse gear. The first gear is the deep 7.05:1 non-synchronized granny low primarily used for stopped-vehicle starts under heavy load; the remaining three forward gears are used for on-the-move shifting.
The deepest first gear of any GM truck manual transmission. The SM420 first gear is a 7.05:1 ratio, even deeper than the SM465 successor 6.55:1 ratio. The non-synchronized design means the vehicle must be at a complete stop to engage 1st gear smoothly. By design for: stopped-vehicle starts under heavy load, off-road crawling from a stop, severe-grade launches with heavy loads, and very low-speed maneuvering of heavy commercial trucks. Combined with a 4.10 or 4.56 rear axle and a low-range transfer case, the SM420 delivers crawl ratios approaching 60:1 with appropriate gearing.
SM465 replaced SM420 in 1968. The SM465 (our vaz133 listing) replaced the SM420 starting in 1968 as GM truck production transitioned from the classic Stovebolt Inline-6 era to the modern Small Block V8 and Big Block V8 era. The SM420 has a deeper 7.05:1 granny low vs the SM465 6.55:1, the SM420 has bell housing patterns matching pre-1968 Inline-6 and early SBC engines, and the SM420 lacks synchronizers in some applications. For period-correct 1947-1967 restoration, choose SM420; for 1968-1991 C/K Series or LS-swap, choose SM465 per vaz133.
Chevrolet Stovebolt Inline-6, GMC Inline-6, and early small block V8. Chevrolet 216 cu in Inline-6 (Stovebolt 6, pre-1953). Chevrolet 235 cu in Inline-6 / 3.9L (Blue Flame Six, 1953+). Chevrolet 261 cu in Inline-6 / 4.3L (Jobmaster Six, 1954+). GMC 248, 270, 302 cu in Inline-6 (in GMC truck applications). Chevrolet 265 cu in V8 (1955+ Task Force, the original small block). Chevrolet 283 cu in V8 (later Task Force applications). Important: the 235 and 261 are Inline-6 engines, NOT V8s, despite some product listings incorrectly labeling them as V8s.
Chevrolet Advance Design and Task Force trucks, GMC New Design and Blue Chip trucks. Chevrolet Advance Design Trucks (1947-1955): classic post-war pickup trucks. Chevrolet Task Force Trucks (1955-1959): second-generation classic Chevy pickups. GMC New Design Trucks (1947-1955): GMC equivalent of Advance Design. GMC Blue Chip Trucks (1955-1959): GMC equivalent of Task Force. Various military and commercial GM-based vehicles using these chassis. Classic truck restoration is the dominant SM420 application today.
SM420 for pre-1968 trucks, SM465 for 1968-1991 trucks. Production span: SM420 1947-1967 vs SM465 1968-1991. First gear ratio: SM420 7.05:1 (deeper) vs SM465 6.55:1. Synchronization: SM420 non-synchronized in some gears; SM465 has 2nd / 3rd / 4th synchronized. Application era: SM420 for Advance Design and Task Force trucks; SM465 for C/K Series 1968-1991. Bell housing: SM420 patterns for 235 / 261 Inline-6 and early SBC; SM465 patterns for later SBC, BBC, and diesel. For LS swap into pre-1968 chassis, either can work but SM465 offers smoother on-the-move shifting.
No. There is no core return required.
Penelope J. –
Got an SM420 for a 1953 Chevrolet Advance Design 3100 pickup restoration. Donor era correctly identified as 1947-1955 Advance Design range, bell housing pattern confirmed as Stovebolt Inline-6 (235 cu in), all 4 forward gears and reverse tested, granny low (7.05:1) engagement specifically verified, 2WD configuration confirmed. Honest pre-purchase information about the SM420 being a 4-speed (NOT a 5-speed despite some incorrect descriptions) and the SM465 per vaz133 successor (1968-1991 replacement with shallower 6.55:1 granny low for later C/K Series chassis).
Augustus K. –
Sourced an SM420 for a 1957 Chevrolet Task Force 3200 with 283 V8 swap-back restoration. Donor era correctly identified as 1955-1959 Task Force range, bell housing pattern documented as early SBC (matching 283 V8 destination engine), all 4 forward gears tested. Honest pre-purchase information about Chevrolet engine identification correction (235 and 261 are Inline-6, NOT V8 despite some product listings incorrectly labeling them) and the SM420 deeper 7.05:1 granny low vs SM465 per vaz133 6.55:1 for extreme off-road crawl ratios.