350 gm identification number transmission

This is April 23, , M21 ratio.

Identifying a GM TH250 automatic transmission

Date code format for — model years was just a month and day. Because and shared the same casting, sometimes it is hard to tell the production year of the case. The and vehicles received a year, month, and day designator. Because the case was used in and , the case without the year designator is a The —s received a year, month, day, and ratio designator. Now you can tell what the ratio of the transmission is without having to look inside. The first Muncie case was number All ball bearings have standardized numbers used by the bearing industry. The T10 front bearing size was reduced in diameter in only.

The smaller type bearing replaced the larger bearing in The first Muncie copied the smaller bearing and retainer combination of the T I believe the move to a smaller bearing was to reduce the width of the front bearing to make room for wider gears. By , wider gears improved the T10; this meant that the front bearing had to get narrow to compensate for the size difference of the front portion of the countergear. The N bearing has a mm bore, an mm outside diameter, and is 21 mm thick.

The bearing is the next smaller standard size that uses the same bore. However, it is thinner by 4 mm and smaller in diameter by 8 mm. By late the rare case started to show up in cars with a larger bearing bore. Therefore, they kept the width of the smaller style bearing of 17 mm but went with the standard outside diameter of a The casting clock codes are at right above the fill plug.

The number represents the month and the dots represent the week. Most of the time the other cloc is blank, but occasionally it represents numbers and deviations to the original blueprint.

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Later cases seem to only have one clock. Their number was B. It is unique to the Muncie and was used until the very end of production in all main cases with the exception of the case. By the case appeared. By late the case was cast with GM Patent Number The case with the same casting number has a squared-off upper extension housing boss. The left case is a T The bearing bore is smaller than the Muncie case on the right. The case used the same bore as the T The main case was used during and Most of these castings look very rough. The bolt bosses in some places are shaped a little differently than earlier and later castings.

It may have been from a different foundry. These surface from time to time but are rare. Corvette production determined how Muncie design evolved. By late , the Turbo Jet engine, rated at hp, was introduced. A very small number of engines also found their way into the Impala. I learned from speaking to many builders that the rare case seems to be in a few Corvettes and full-size GM cars with big-block engines.

The case with the last numbers milled off is in some full-size cars, such as the Impala. This usually indicates that the gears used in the milled cases might be of a different steel alloy. The case appears to be identical to a case so perhaps it is made of a different aluminum alloy. It appears that more milled cases are in circulation compared to unmilled cases.

They all seem to come from big-block cars. The case, with a December 17, , build date, was probably for an early car. These cases were run in conjunction with the case. This might have been a different alloy because its appearance is identical to the case and it was to be used strictly with the new big-block engine. Photo Courtesy Ed Hartnett. The main cases saw some additional design changes. With the introduction of the case, the countershaft bore was increased to 1 inch.

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The case countershaft bore usually stretched out so the increase in shaft diameter spread the counter-shaft load over a wider area. These two milled cases have March assembly dates. They are obviously cases with the portion of the number machined off.

Coincidentally, March seems to be a time when the engine started to migrate into more vehicles, such as Z16 Chevelles, Corvettes, and full-size Impalas. These transmissions probably were assembled with special alloy gears, because the case casting is identical to the as well. Usually when an external modification is made to the case, such as milling, it means a special internal mod. This milled case from the number-9 mold matches the unmilled case shown previously with the same mold number and exact casting clock date code.

What you can see is that before March these cases were not milled. The versions of this case are identical except that the upper middle extension housing bolt boss on the rear of the is square rather than round shown on the painted orange case. This case was used to the end of The way to differentiate a case from a is by the serial number. This case casting was used during the and model years; was the first time a year designator was added to the serial number. This one has a January 19, , date code.


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This is a case dated June 2, The rear pad where the numbers are stamped is wider than earlier versions of the case. The mounting ears also have thicker gussets than earlier cases. This casting design evolved into the and-up castings. The cases were used primarily in to Some spots also have more material and the case has a thicker look to it. Perhaps an alloy change as well as a tooling change warranted a new number. By the start of these cases and all future cases had the drain plug boss drilled and threaded. The casting seemed to surface by the last quarter of Stamped serial numbers for all model year cases also included the year designator.

The last Muncie case was the It is identical to the case but definitely has a different color. This main case has a P8R04 assembly date and was cast in a number-2 mold.

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TH Transmission Identification | Drivetrain Resource

The VIN in the left picture is 28G; it decodes as Pontiac 2 year 8 and G plant followed by the sequential production number. Therefore, this is a LeMans case. Here is a rare casting from a Camaro. The assembly date of P9P16A translates to September 16, Notice the odd casting clock above the fill plug and that it was drilled for a drain plug. This was mold pattern number 1. It was used primarily with the spline input shaft and larger spline output-shaft models, so it appears that they were looking to optimize strength everywhere possible since the engine was introduced in However, there seems to be a big overlap from to , with both of these cases.

This is a case from a Corvette M20 transmission. It has an August 7, , build date.

Helpful Resources

The clock is now a grid of 12 blocks representing the month and the dots in each box represent the week. This one was cast in the fourth week of January in mold number This is a M20 case. Location G - Power Steering Option. Location H - Power Brake Option. Location I - Radio Code. Location J - Disc Brake Code.


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    Tech Vault

    Not available on Camaro. P9D28 F. QXG D. Chevrolet Owner.

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