Decode a Opposed Twin Stamp
In production: c. 1979-2003 (opposed-twin L-head line; the garden-tractor fleet it powered stayed in service well beyond production).
The opposed twins are Briggs & Stratton's flathead twin-cylinder line - the 400000, 420000, and 460000 series, two L-head cylinders opposed across a shared crankshaft, in vertical and horizontal shaft builds. They were the standard garden-tractor engine of the 1990s: a large fleet of lawn and garden tractors from most major brands shipped with one, and a large share of that fleet is still running.
The stamp is the standard Briggs format: a six-character model, a type, and a code. The model decodes by the published chart - displacement class, design series, crankshaft orientation, bearing, starter. The type is where this family's data concentrates: crankshaft dimensions, and in places the power rating and oil specification, are set by the Type and its parts list, not by the Model alone.
Two things follow from that. First, identification and parts work on this family is Model + Type work - the full stamp, never the model alone. Second, every tired opposed twin is a repower candidate, and the replacement question (usually a modern V-twin) is answered from the original Type: its shaft, the mounting pattern, and the muffler and hood clearance.
Where the stamp is
On opposed-twin engines the Model-Type-Code is stamped into the blower housing - the wide shroud over both cylinders - or carried on a metal tag attached to the housing; check the top face of the housing and the edge over each cylinder bank. (Per Briggs & Stratton identification literature and the twin-cylinder repair manual 271172.). It is stamped into the metal, not printed on a sticker - wipe the area clean and read it at an angle if it is faint.
Worked example
A real stamp from this family: 422707 1705
| Part | Value | Means |
|---|---|---|
| Model digits 1-2 | 42 | Displacement class: 42 cubic inches nominal (the 420000 opposed-twin block). |
| Model digit 3 | 2 | Basic design series. |
| Model digit 4 | 7 | Vertical crankshaft (chart range 5-9 = vertical). |
| Model digit 5 | 0 | Plain (sleeve) PTO bearing. |
| Model digit 6 | 7 | Electric starter with alternator. |
| Type | 1705 | This Type's documented build carries a 1 in x 3-5/32 in straight crankshaft. The same model's Type 1510 carries a 7/8 to 1 in stepped shaft - on this family, check the Type before any shaft or repower decision. |
| Code | YYMMDD + suffix | The code is stamped per engine: the first six digits are the build date (two-digit year, month, day) and the trailing characters are the assembly plant and line. A code gives a build date, not a sale date or the tractor's model year. |
Reading the type
The Type is a four-digit trim code (often followed by a revision suffix) that pins down the exact build - on this family, more than on any other here. The crankshaft is a Type attribute: model 422707 is documented with a 1 in x 3-5/32 in straight shaft on Type 1705 and a 7/8 to 1 in stepped shaft on Type 1510. Power ratings and oil specifications also vary by Type and oil-filter fitment, which is why published figures for the same model can disagree - they are usually quoting different Types. The Type does not decode position-by-position; its mapping lives in Briggs & Stratton Illustrated Parts Lists. If a Type is not in a verified table, we decode Model + Code and report the Type trim as unlisted.
The model-number rules
Reading the model number
- Digits 1-2Displacement (cu in)
- Next digit/letterBasic design series
- Next digit/letterCrankshaft orientation: 0-4 or A-G horizontal, 5-9 or H-Z vertical
From the Briggs & Stratton model number key.
PTO bearing / drive digit
| 0 | Plain bearing |
| 3 | Ball bearing |
| 4 | Ball bearing, pressure lubrication |
| 5 | Plain bearing, gear reduction (6 to 1), CCW rotation |
| 6 | Ball bearing, gear reduction (2 to 1), CCW rotation |
| 7 | Plain bearing, pressure lubrication |
| 9 | Plain bearing, auxiliary drive |
Starter digit (last)
| 0 | Without starter |
| 2 | Rewind (recoil) starter |
| 5 | Electric starter |
| 6 | Alternator only |
| 7 | Electric starter with alternator |
The code: build date
The code's first six digits are the build date, read YYMMDD - two-digit year, month, day. The trailing characters are the assembly plant and line. A code gives a build date, not a sale date or the year of the equipment it powers.
Opposed Twin engines in the database
- Spec records for this family are being verified.
Quick answers
Where is the model number on an opposed twin?
Stamped into the blower housing - the wide shroud over both cylinders - or on a metal tag attached to it. Check the top face of the housing and the edge over each cylinder bank. You need all three groups (Model, Type, Code).
Why is there no spec page for my opposed twin's exact stamp?
Because on this family the data itself lives at that level. The crankshaft changes with the Type - one documented model, 422707, carries a 1 in x 3-5/32 in straight shaft on Type 1705 and a 7/8 to 1 in stepped shaft on Type 1510 - and power and oil figures also vary by Type and filter fitment, while Briggs never published a dry weight for the family at all. A stamp-level page would have to invent or average figures; this page covers the family, and Model-Type data is added where it is verified.
Can a V-twin replace my opposed twin?
That is the standard opposed-twin repower question, and it is decided by the Type on the old engine: match the crankshaft from the original Type, expect the V-twin to run wider (about 1-3/8 in on documented conversions), and check muffler and hood clearance before anything else - on some tractors the replacement's bolt pattern needs drilling. Repower kits for this conversion are documented for Woods, Grasshopper, and White applications.
What are the valve clearances on an opposed twin?
Intake 0.004-0.006 in, exhaust 0.007-0.009 in for the opposed-twin L-head series, per Briggs & Stratton form MS-3992 and OEM tractor documentation. Some spec databases print wider clearances (0.006-0.008 / 0.009-0.011); those do not match the manufacturer's published checking specifications for this family.