TH400 Identification: The Texas Pan and the Tag

Slide under almost any big-inch GM car from the late sixties and there's a decent chance you're looking up at a Turbo Hydra-Matic 400. It sat behind the 396 Chevelle, the 455 Buick, the Cadillac 472 and a long run of trucks and motorhomes, and it's still the automatic racers reach for when the power gets silly. That popularity cuts both ways. Because so many were built and so many got swapped, the transmission under a car you're about to buy is as likely to be a junkyard unit as the one the factory bolted in. TH400 identification comes down to two questions: is it a 400 at all, and is it the right 400.

Search the subject and page one is old forum threads, a parts seller's TH350 page, a Facebook answer, a teardown video and a 2018 Hemmings story. Each gets you partway. None of them puts the visual check, the tag and the VIN pad in order, and the charts that do try disagree on how to read the date. This guide does the whole job, in the order you'd actually do it on a lift.

TH400 vs TH350 vs Powerglide: The Pan Tells You First

Start with the oil pan, because you can see it without a single tool. The TH400 pan is the odd one, an irregular shape with a lot of angled edges that everyone describes as a lopsided outline of Texas. It's held on with 13 bolts. The TH350 pan also takes 13 bolts, so the count alone won't separate those two, but its shape is a near square with one corner clipped off. The aluminum Powerglide pan is a simpler shape again and takes 14 bolts, which is why Powerglide pan gaskets and bolt kits are sold as 14-hole and 14-piece parts.

A Texas pan means a 400 or a close relative, and the case backs it up. The main aluminum case on a TH400 runs 24-3/8 inches long and is essentially smooth, and it is visibly longer and bulkier than a TH350 sitting next to it. Overall length then changes with the tail housing, which came in different lengths for different chassis, so measure the case, not the whole unit.

Kickdown and Modulator: The Two Quick Checks That Settle It

The second tell is the kickdown. A TH350 uses a mechanical cable that runs from the carburetor linkage down to the case. A TH400 has no cable at all. It kicks down electrically, through a switch at the throttle and a wire that plugs into a connector on the driver side of the case, just above the pan rail. If you see that plug and no cable, it's a 400. On 1965 to 1967 Buick, Oldsmobile and Cadillac units, a two-prong plug in the same area also fed the variable pitch stator, the so-called switch-pitch converter. On 1970 to 1974 cars a plug there served the transmission-controlled spark system instead, so don't read a two-prong plug as proof of a switch-pitch box.

The vacuum modulator is the third check, and it is where some guides get turned around. Both transmissions mount it on the passenger side. On a TH400 it sits forward on the case, toward the bellhousing. On a TH350 it's back at the rear of the case, close to the tail housing. One widely copied how-to page has those two locations reversed, so trust the pan and the kickdown before you trust a single sentence about the modulator.

Chevrolet vs BOP Bellhousing: Which TH400 Do You Have?

Every TH400 is not interchangeable. The front of the case was cast in two main patterns. The Chevrolet pattern bolts to Chevrolet V8s and the other engines that share that bolt layout. The second pattern, often called BOP or BOPC, fits Buick, Oldsmobile, Pontiac and Cadillac engines. A Chevrolet case will not bolt to a Pontiac 400 without an adapter, and the reverse is true too. AMC and Jeep applications later used their own case. So a seller who says a car has the right TH400 should be able to tell you which pattern, and the pattern should match the engine.

The TH375, used from 1972 to 1976 behind smaller engines, looks almost identical but is a lighter-duty unit, and it's marked 375-THM on the underside of the tail housing. Check there before paying 400 money. The TH475 went the other way, an extra-heavy-duty truck and motorhome variant from 1971.

The tag tells you what the transmission was built for. Only the VIN pad tells you what car it went into.

How to Read a TH400 Identification Tag

Once you know it's a 400, go looking for the tag. On Chevrolet units it's a small metal plate riveted to the passenger side of the case. It carries four things: a model year, a two-letter application code, a three-digit production date and a serial number. A missing tag is common on any rebuilt unit.

The application code is the field that matters. On Chevrolet units the two letters start with C, and the second letter ties the transmission to an engine and horsepower rating. MACSWEBS documents the A-body codes this way: CC for the 325 hp 396 from 1967 to 1969, CE for the 350 hp 396 in 1969, CX for the 375 hp 396 in 1969, CS for the 360 hp 454 in 1970 and CY for the high shift point unit behind the 450 hp LS6 in 1970. CD and CF changed meaning from year to year as the engine lineup changed, which is exactly why you read the code against the model year printed beside it, never on its own. Pontiac, Oldsmobile and Buick ran their own code lists, so a Chevrolet chart won't decode one of their tags.

The TH400 Date Code Is Not a Plain Julian Date

Here's the field most guides get wrong. Plenty of decoders, including the summary Google now shows at the top of the results, call the three-digit number a Julian build date. Read it that way and you'll hit numbers you can't place, like 395 or 521, because a Julian date stops at 365 or 366. On Chevrolet TH400 tags the count doesn't restart in January. It starts on January 1 of the calendar year before the model year and keeps running.

So a 1970 tag reading 304 was built on the 304th day of 1969, October 31, 1969, which is exactly when 1970 cars were being built. A 1967 tag reading 521 is 156 days into 1967, June 5. Leap years need one extra step. A 1969 model counts from January 1, 1968, and 1968 had 366 days, so you subtract 366, not 365. MACSWEBS shows a 1969 CX tag dated 395, which is 395 minus 366, January 29, 1969. ChevelleStuff's year pages walk through the same arithmetic, and both sources agree.

The date has to make sense against the car. A transmission built after the body cannot be original. A few weeks before is normal. A year before needs an explanation.

Finding the VIN Pad on a TH400

The tag proves what the transmission was built for. It doesn't prove which car it went into, and tags can be swapped. The VIN derivative does that job. From the 1968 model year on, Chevrolet stamped a shortened VIN onto the transmission, and on a TH400 you'll find it on a flat pad on the driver side of the case, just above the oil pan rail. It carries the division number, the model year, the assembly plant letter and the last six digits of the car's sequence number.

Compare it digit by digit with the VIN plate. Factory characters sit in a consistent font and depth. A pad ground smooth, or digits that wander, is the classic restamp. A service replacement unit from the dealer typically came with no VIN at all, which is honest history rather than fraud, but it's still not the original transmission. For more on how far a matching stamp actually gets you, read our numbers matching guide.

Where the TH400 Charts Disagree

When three or more charts cover the same transmission, compare them, because they'll disagree. These are the conflicts worth knowing. First, the Julian date claim, covered above. Second, the Powerglide pan bolt count: one popular GM transmission ID guide, which Google cites in its own summary, gives the Powerglide pan 13 bolt holes, but the aluminum Powerglide pan takes 14. Third, the modulator location, which at least one how-to page reverses.

Fourth, availability. The MACSWEBS CorvetteWorld TH400 page, which ranks on page one, states that the TH400 was only available with a 396, 402 or 454. That's the rule for an A-body Chevelle, where the TH400 arrived for 1967 and sat behind the big block only. It isn't the rule for a Corvette. For 1968 the Corvette dropped the Powerglide and its automatic became the Turbo Hydra-Matic, offered behind the base 300 hp 327. Read the rule for the car in front of you, not the car the chart was written for. If you're checking a Chevelle, our Chevelle hub covers the model years, and a manual car gets the same treatment in our Muncie 4 speed identification guide.

Frequently Asked Questions

How do I tell if my transmission is a TH400?

Look at the pan first. A TH400 pan is an irregular Texas shape held by 13 bolts. Then confirm two things: there's no kickdown cable, only an electrical plug on the driver side of the case, and the vacuum modulator sits forward on the passenger side. All three together mean you have a TH400.

Where is the ID tag on a TH400?

On Chevrolet units the tag is a small metal plate riveted to the passenger side of the case. It shows the model year, a two-letter application code, a three-digit production date and a serial number. The shortened VIN is stamped separately, on a pad on the driver side of the case just above the oil pan rail.

How do you read the date on a TH400 tag?

The three digits count days from January 1 of the calendar year before the model year. If the number is 365 or less, it falls in that earlier year. Above that, subtract 365, or 366 when the base year was a leap year, and you get the day in the model year.

What is the difference between a TH400 and a TH375?

The TH375 was a lighter-duty version used from 1972 to 1976 behind smaller engines. From outside it looks almost identical to a TH400, with the same Texas pan. The giveaway is a 375-THM marking cast into the underside of the tail housing, so check there before paying TH400 money.