Category Archives: Torque Converters

TH350/converter service article

this is an article i ran across of a th350 tranny and converter service. now the refresh is nothing special but it is an excellent pictorial and brief  description of how a converter is serviced and how easily a lousy converter can screw up a transmission.

Torque Converter Damage/ I.D. your converter

393394926well, one of my friends brought a tranny into my shop a few weeks back and i literally had to take a sledge hammer to it to get it out of the tranny. the whole center of the torque converter had dislodged and exploded the bearing. i actually watched him pull his car off the trailer at fall brawl and pull forward to lose all gears after 10 feet. not one of the best things in the world to witness as a transmission builder.

After going through the transmission it was obvious that the converter had ballooned- tearing itself apart internally and messing up the front pump assy. Ballooning is primarily caused by prolonged operation under excessive loading, very abrupt application of load, or operating a torque converter at very high RPM. the shape of the converter’s housing is caused to be physically distorted due to internal pressure and/or the stress imposed by centrifugal force. In drag racing, ballooning is something that high hp cars with a trans brake experience due to shock load of launch off the line. in derby, this happens when you slam shift with your foot on the throttle. in this case the guy got suck in a hole with a couple of imperials comming after him in his 74 buick…..can’t blame him!

in extreme conditions like a nitrous oxide set up with a trans brake, you would get a converter specifically made with what is called an anti-balooning plate, along with furnace brazing the internal fins. Basically the ballooning plate is a re-enforcement of the housing to help take the shock load. a lot of guys also refer to them as nitrous converters. However for most of us, Furnace brazing the internals of the converter alone is more than enough to strengthen up a converter- and reasonably priced.
as you will see in the pictures below, the balloon victim was a stock light duty converter. I took the converter over to my friends at Midwest converter and discovered a few things i didn’t know that are worth sharing. there are two different th400 13″ non lockup converters. look at the pictures side by side and you will see by the fin grooves of the first one they tilt to the left, and the other tilts to the right. light duty 400 converters….they called them V-6 converters have the fins tilted to the right at the edge and extremely light duty bearings internally. V-8 or heavy duty converters tilt to the left from the bottom up.


our friend with the 74 buick put in a weak converter from the get go. the sad thing is that he probably either bought it from a new parts supplier for cheap or pulled it out of a driver with a v-8. since the v-6 converters are pretty much worth nothing on core and cheap, they got used more often than not where they should not have been. And although a bit off topic i am under the impression that some of these cheap reman converters you get from your local cheapo parts house may be slightly ballooned from the get go. a dead giveaway that you are having a ballooning issue is the thing wobbles all over the place at idle, or a bad vibration. when a converter balloons it will not center between the pump and the center of the crankshaft.

393394925Here is a picture of the stator and front section (flex plate side) of the housing. as you can see all the fins absorbed damage, and the front support bearing is completely missing from the stator as it exploded and causes most of the damage to the fins. according to Dennis @ Midwest, what happened is the converter ballooned, caused the stator to dislodge and explode the bearing- which in turn caused all the fin damage. in a heavy duty/furnace brazed converter, these bearings are nearly double the size of this unit and all the fins ypou see are brazed in place not just crimped into the pieces that hold em together. the picture at the top of the page is of the snout side that faces the front pump. notice that the fins to the outside on both of the pics have the same effect like running your finger across radiator fins bending over the sheet metal. some of the fins are completely ripped out of place.

So what is the moral to the story……well there are several points here. A well built furnace brazed heavy duty converter is absolutely critical for demo derby. light duty, stock replacement, cheap reman, and used converters will take you out….and it will piss you off completely when it does. so no matter who builds your tranny, take caution if they want to sell you a converter that is not heavy duty. i use converters from Midwest, B&M, TCI,and Coan. Most all of them make a stock reman converter for around 75-100$ that is not furnace brazed. so don’t be mistaken that just cause it has a big name manufacture name on it that it is good to go. AND if you go to your local parts house you may indeed get just about anything for 75 bucks including the weak P.O.S that was described earlier in this post. food for thought good luck!

How to measure converter clearance to engine / adapter plates.

First off, install the converter into the housing. on a Th350/Th400 you will install the torque to where you feel it click in three times and it spins freely. if it does not spin freely when it is fully seated in the tranny, you got a problem.

Next, lay a straightedge across the face of the bellhousing (engine side). I usually use a large carpenter’s square. With the converter installed in the transmission, take a ruler and measure the distance from the straight edge down to the mounting pad of the converter. The measurement should be as follows:
GM TH-350: 1 1/8″ from bellhousing to mounting pads
GM TH-400: 1 3/16″ from bellhousing to mounting pads
GM Powerglide: 1 1/8″ from bellhousing to mounting pads
Ford C-6: 1 1/8″ from bellhousing to mounting pads
Chrysler Torqueflite 727: 1 1/4″ from bellhousing to ring gear
Note: Distance may vary either way .050″.

when using a stock replacement converter, tranny, and engine there is usually no need to worry about this.

With the transmission installed to the engine ,you should have between 1/8″ to 3/16″ of clearance between the torque converter and flexplate before pulling the converter forward and bolting it to the flexplate. So, when using an adapter plate/distributor protector, you can use this as a guide to select the right thickness of shims to properly set your converter to not bind the engine, housing, or most of all the pump gears into the back face of the pump. for derby i like to lean more toward 3/16″ as the converter needs as much room room to expand as possible as it heats up quickly. It may seem like if isn’t in the pump that far but trust me, as it gets hot it will be in there just right.

commonly an adapter plate comes with shims of the correct thickness and these measurements become somewhat pointless when using a factory converter on a factory tranny. However, when you mix and match aftermarket performance converters it can be an issue. the only time i had a real problem with one though is when using a powerglide core custom built with a turbo spline installed in a th400.

also when using washers from the hardware store as converter shims you gotta be careful. most of the cheap washers like that don’t have identical thickness and can really screw things up. you are usually better off cutting shims from the same material as the adapter plate. you should also be aware of how the snout of the converter centers in the end of the crankshaft. some of these ungodly thick adapter plates i have seen can pull the converter out of the end of the crank and the converter never centers- causing a lot of problems and vibration.

Torque Converter Selection

One of the most overlooked and common failures of a derby transmission is the torque converter. quite often if you run a used converter, it is a crap shoot whether or not you get one that has some life left in it.

I always recommend running a new or rebuilt torque converter with any new transmission build. Stock converters are not furnace brazed and/or fully welded in most cases. when you beat on em hard, they will eventually come apart. A used converter in a new tranny can not only cross-contaminate the tranny with debris from the tranny that it came from, but if it does have internal damage it can create a frustrating overheating situation even though the fluid looked good when you pulled it from a working tranny.

i cannot tell you how many people called me completely pissed off that i fucked up their transmission, and come to find out they ran a bad converter….usually cause they got cheap shit. If you lean on a stock converter by slam-shifting, the stock tach welds(similar to how body seems are spot welded) usually bust apart. the stator sprag can also dissintegrate. when this happens you get the all too familiar,” i pulled onto the track and when the flag dropped nothing happened”…..” i have this noise comming from your tranny that sounds like bolts in a grinder even in park.” ……or,” My tranny lost all gears 10 minutes into the derby at the red flag , but when i gunned the engine and played with the shifter for a few minutes all my gears came back for the rest of the derby.” You pull the tranny and converter after the race and see nothing wrong with it…..hell fluid looks good you hear nothing rattling….stick a screwdriver down the neck and check the sprag it’s fine….but when you drain the fluid out of it finally you can hear a sound like wing nuts in an empty beer can.

The stall speed of the converter you chose mainly has to reflect the engine you put it behind….I.E the camshaft profile. if you run a common low hp/high torque derby built engine, your best choice is a 13″ truck converter with a stock or low stall. low stall creates less heat as compared to a high stall converter,plus you take full advantage of all the low end torque and 6-8 qts of fluid capacity!! Truck converters are usually heavier built to take abuse from towing/hauling and are easily idetified by 6 torque converter bolts to the flywheel.

Now if you are running a more radical camshaft that idles up over 1000rpm and has a higher rpm band, then of course you have no choice but to run a stall converter. For drag racing, the 11″ and smaller powerglide style converters accell for this application as it is smaller and creates less drag on the engine, creates a lock-up at a higher rpm to give you a good launch, and heat is a non-issue as you are only running for a short period of time.

Whether or not it’s for derby, drag racing, or heavy street……buy a quality converter if you can afford it. The guys at Coan engineering really have their shit together as far as converters go and can build pretty much whatever you want. A cheaper alternative to a Coan for derby applications is a Midwest Converter. affordable and well built for the price for low hp applications. B&M and TCI converters work well for the street/strip if you don’t buy the cheap line- you want a better furnace brazed converter like a holeshot B&M rather than a Tork Master as an example . I recommend having somekind of heavy duty converter for derby use. i have not had very much luck with the cheap light duty stock replacement car converters. As i stated earlier in this post, a cheap and easy solution it to run a 6 bolt truck torque converter, even though not all truck converters may be brazed/welded internally they seem to hold up better over a cheap 60-80$ replacement.

for you guys that run the ultrabell, you are pretty much stuck with a custom 11″ converter. For derby use Punisher and I have been trying out some built by Midwest performance made from a powerglide core with a turbo spline adapted internally to fit the th350-th400, and have them stall it as low as possible. with a 4.56 gear the poweglide custom converters seem to lock up around 1500rpm but they are intended to stall around 2000.

and finally….all th400 13″ converters are non-lockup converters. Th350 converters are 12″ factory and come both lock-up and non lock-up converters. 11″ converters came in powerglides originally, and are non-lockup. 11″, 12″, and 13″ non lockup torque converters all interchange and fit both the th400 and th350 non-lockup trannys. a lock-up clutch th350 DOES NOT interchange with anything else. A LOCK UP CLUTCH CONVERTER OUT OF A LOCK UP TH350, 200R4, 700R4, 4L60, or 4L80 WILL NOT FIT A TH400 OR TH350 NON LOCK-UP.