Words by Bryan Irons @bryanirons
Every time we go to find a component for a 4x4 in our stables, we strive to select parts that don’t carry monikers like “rare” or “obscure”. General Motors had a 15-year production run starting in 1969, the with the Turbo Hydromatic 350, or TH350 automatic 3-speed transmission. They are plentiful, strong, compact, easily adapted and a GREAT choice in an off-road vehicle for all these reasons. Found in anything from 2 Wheel Drive grocery getter midsize cars to 4WD trucks, our buddy Derek scored a sweet deal on a TH350 to land firmly under the hood of his TJ based snow wheeler.
That was a few years ago and Derek’s luck with the 3-speed automatic finally ran out. As is normal for Derek’s luck, the TH350 died AFTER a deep snow wheeling trip only a mere metre from his house. If only we had his luck. In order to keep costs in check and get him back into the fluffy stuff, Summit Racing was tapped for a rebuilt replacement unit (PN SUM-700310 $1199.99 USD) and a torque converter that matches the weight, gearing and planned usage (PN SUM-G2699 $129.99). The only fly in the ointment was that Summit only sells their house brand TH350 transmissions in 2WD form. Luckily, we have one of the best transmission rebuilders not far from us to help make the changeover to 4WD.
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Externally, the only difference between the old and new is the extension housing on the new grey unit in the background.
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Ivar decided that we should go in and get the short rear output shaft we needed to make the new transmission work. He started by removing the pump assembly. All components either come out this end of the transmission or the pan area in a TH350.
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Prybars, screwdrivers, picks and specialty tools from here on out.
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Our first “Eureka!” moment came after removing the pan. Ivar proclaimed “Torque convert’s dead” after looking at the fluid. Good thing we ordered a new one!
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Although it looks like this filter gasket may be positioned correctly, it isn’t sealing the hole above it and is allowing air and unfiltered fluid into the system.
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The valve body needs to be removed to release a few bands and allow the rest of the internals to come out.
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Problem #2 made itself be known as the Direct Drum came out in multiple pieces of shrapnel. Good thing we don’t need it to continue.
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Apparently, this is a relatively rare occurrence. Well done Derek!
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Ivar decided that he would pop apart the other drums and clutch housing to see what else may be lurking. We were not disappointed…
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“You should not be able to shave clutch material off this easily” we were told. “This is what happens when water gets to the clutches”. This old transmission was running on borrowed time since it was installed. YIKES!!!!
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A few more components still had to come out before the rear output shaft would be released to us.
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After some struggling, the reverse cluster and rear output shaft came out.
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The struggle was due to the rear bushing being severely side loaded at some point. This can happen when a transfercase is not perfectly aligned to the transmission.
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Like a botched game of Operation, there was no hope for this patient. Just load it all into a box and ship it back to Summit Racing as a core.
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All that work just to retrieve the short output shaft seen in the background. We need to delve just as deep into the new TH350 to get the output shafts swapped.
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If only it were as simple as popping off that 2WD extension housing and swapping the shaft out. Nope. Other than this housing, the rest of the work getting into the new TH350 is exactly the same as the old one.
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Once we were at “ground zero”, Ivar check all the clearances and components in the new transmission. All the refurbished components looked to be in excellent shape.
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An aftermarket “case saver” was installed by Summit to keep wear to the transmission housing to a minimum - a very nice touch!
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After adding all the “clutches” and “steels” to the drum assemblies, Ivar would check for clearances and then align all the components so that reassembly would be that much easier.
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This particular TH350 from Summit came with a few upgrades including the hardened sprag race shown here.
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When inserting the loaded drums, Ivar can tell by feel and sound if everything dropped into place properly. This is where I know I would have screwed up.
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The steels and clutches here are name brand Raybestos. We are told they may not be “race spec” but are a major step above stock replacements.
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After all the snap rings, doohickeys, thingamabobs, unicorn farts, hens’ teeth and whatever else it takes to make our wheeling hopes and dreams come true are in place, Ivar takes yet another measurement.
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This ensured that clutch pack engagement is within spec. Once the jig is set up, it gets moved to the pump unit itself while it is still on the bench.
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Technically, our unit was within spec, but fortunately for us, Ivar is a perfectionist and shimmed the bearing until it was at the height he preferred.
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The valve body went back on and all the linkages were re-connected. We learned that there are three holes on a TH350 valve body that are a smaller diameter than the others, these are to align it perfectly. And for once, the engineers made things easy; all the valve body bolts are the same size!
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Summit includes this very nice aluminum oil pan with a drain hole with their rebuilt TH350’s complete with stainless Allen head cap screws. An aluminum pan with a quality paper gasket should not require any sealant. This is good as many failures Ivar sees are from excessive sealer clogging passageways.
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The final part of our reassembly was reinstalling the governor and cap. The washer was installed here to put a little extra tension on the retaining clip. Then it was returned to the Jeep before the winter wheeling season came to an end!
After expressing our gratitude and muttered something about “owing favours”, Ivar Hoiland of Kelowna Transmission & Auto Repair (KT&AR) finally broke down. He caved and let us hang out at his “healing bench” while he dissected and determined the cause of death of the old transmission and swapped the required parts to convert the new Summit TH350 to a 4WD unit. Typically, we’re not afraid to tackle just about any automotive job we have at home, but automatic transmissions require a “feel” and very strict attention to detail that you only get after decades of hands on work. We just don’t have what it takes and are happy to shell out the clams required to do this kind of work. The best advice if you are searching for a shop is do your research and ask questions. KT&AR is one of a select few where we can confidently drop parts and vehicles off and KNOW it’s done right, and the only one for transmission work.
You would expect that with a 15-year production run, there would be a multitude of variants of the TH350 but there are only a few; Obviously a 2WD version and a 4WD version are out there, but this is only a difference of a few parts. Bellhousing bolt patterns consisted of a Chevrolet version, a BOP (or “Buick, Oldsmobile, Pontiac”) and in some rare cases, a dual pattern. And for a few years during the end of its productions run, there was a version with a “lock up” style torque converter known as the TH350C. Know what version you have as there are a few critical parts in the TH350 and TH350C that are not interchangeable.
External connections, or lack thereof, is another reason the TH350 is a great swap candidate as it is almost entirely controlled internally. The only “required” connections are a vacuum source to determine engine load and a gear selector. The TH350C version had an electrical connector to actuate the lockup torque converter and gain some extra mileage. A kickdown cable attached to the engines carburetor or throttle body and lets the transmission automatically “gear down” when the motor is floored. The kickdown is a simple hookup but it is not required and often omitted on many off road vehicles for simplicity sake and better driver control.
So with our overview out of the way, let’s hang over Ivar’s shoulder and see what it takes to make a good deal on a TH350, a GREAT deal when we make it 4WD compatible.
Summit Racing – www.summitracing.com
Kelowna Transmission & Auto Repair - https://kelownatransmission.com/