jpwalt1987
Well-Known Member
- Joined
- Nov 9, 2003
what exactly is the cure for a th-400 taking out the thrust bearing on the turbo6? thanks, jeremy
th-400 and thrust bearing?
- Thread starter jpwalt1987
- Start date
Every so often you come across a 400 that does not control converter pressure well. I believe it's caused by casting variations of the pump assembly. I haven't been able to prove it though. A good way of spotting one is to monitor cooler line pressure. Assuming that there is not any abnormal restrictions in the transmission cooling system, cooler line pressure should read 40 psi. No higher! Lower is OK. If yours has a pressure higher than 40 psi, congratulations, you have a potential crank killer.
Pressure in the torque converter causes it to try to spit out the front of the transmission. Some forward pressure on the crank caused by the T/C trying to spit out is normal with automatic transmissions. It's when that pressure is excessive that the crank thrust bearing has a hard time dealing with the high pressure and it wears out. This is a very common occurence with the THM400 in motorhome and HD truck applications where heavy load is constantly on the powertrain during normal use. Heavy throttle positions translate to higher line pressures in the transmission which means higher converter pressures in those units that don't control T/C pressure well. If your using a manual VB, you have high line pressure at all throttle positions. Even at idle. I realize that there are other theories floating around that try to explain this problem. I've heard them all. This is the 'true' reason for this problem and for about 20 years, since I started modifying torque converter feed, I have not had one single problem and have in fact helped some well known people on this board solve repeated crank thrust problems. I feel for those people that ruin 3 or more cranks before they learn the truth behind the problem. This is a mod that should be done on every single 400 built.
Pressure in the torque converter causes it to try to spit out the front of the transmission. Some forward pressure on the crank caused by the T/C trying to spit out is normal with automatic transmissions. It's when that pressure is excessive that the crank thrust bearing has a hard time dealing with the high pressure and it wears out. This is a very common occurence with the THM400 in motorhome and HD truck applications where heavy load is constantly on the powertrain during normal use. Heavy throttle positions translate to higher line pressures in the transmission which means higher converter pressures in those units that don't control T/C pressure well. If your using a manual VB, you have high line pressure at all throttle positions. Even at idle. I realize that there are other theories floating around that try to explain this problem. I've heard them all. This is the 'true' reason for this problem and for about 20 years, since I started modifying torque converter feed, I have not had one single problem and have in fact helped some well known people on this board solve repeated crank thrust problems. I feel for those people that ruin 3 or more cranks before they learn the truth behind the problem. This is a mod that should be done on every single 400 built.
jpwalt1987
Well-Known Member
- Joined
- Nov 9, 2003
awesome
Don, that is some very good info there. i am gonna print this all out and bring it to my tranny builder. he is the best in town when it comes to stock applications and diagnostics. but hasnt done a th400 with a brake for a turbo6. any other special info that would be useful in putting the trans together? you know... tricks of the trade.
thanks for all the help. jeremy
Don, that is some very good info there. i am gonna print this all out and bring it to my tranny builder. he is the best in town when it comes to stock applications and diagnostics. but hasnt done a th400 with a brake for a turbo6. any other special info that would be useful in putting the trans together? you know... tricks of the trade.
thanks for all the help. jeremyIt's not hard to check if the mod was done. Pull just the pump and unbolt the halves to check. Have someone familiar with trans work help you so you get the pump aligned properly when you put it back together and you don't drop any clutch plates. Tranny guys know what I'm talking about.CHANCE said:
I cannot urge this modification enough. Do it regardless of whether you think you have a crank killer or not. It is not worth the chance of damaging a very valuable engine.
jpwalt1987
Well-Known Member
- Joined
- Nov 9, 2003
don, what do you mean "drop any clutch paltes?" i found the passage that you are talking abotu in the ATSG manual i have. what do you use to restrict it? thanks, jeremy
DonWG said:
Don,
I was one of the people you helped with problem along with Jeff Harrington. I bought a 400 from a well know tranny guy on the board who said the trans was modified to eliminate the thrust
failure. Well it failed shortly after. I checked the converter feed pressure and sure enough it was too high about 100 psi. I did the mod and had 40 psi. Hence to say so far so good. Thanks again.
Quick Turbo V6
Define stock!
- Joined
- May 24, 2001
What is need to do this mod? Where can I find a picture of the passage and where it is located?
Thanks,
Scott
Thanks,
Scott
the converter charge hole exits the pr valve and pees oil out of the pump and into the converter in the vicinity of the front seal area thru an angled passage .recent testing has shown that installing an allen head set screw and drilling it out to .175" is enough to reduce converter charge psi to 55 psi.we used to crossdrill the hole .100'" to exhaust oil at the back of the pump but this generates increased stall speed from the torque converter due to restricted charge and will increase operating temperature of the transmission and some pumps didnt reduce pressure if the casting port of decrease oil was machined too wide,very common on most 6 bolt 895 /896 combinations with induction heat treated stator tubes.if the car is to use a fixed line pressure the line should be kept to below 170 psi by using an unshimmed stock pr spring (worn out is even better) .we have went even further with this concept by making the only externally adjustable th400 transbrake valve body that uses the modulator valve and a drilled and tapped modulator plug that exerts a force on the modulator valve to raise or lower operating pressure.the clutch apply area in a th400 is capable of running reliably at 160 psi hot with up to 1000 horsepower.keeping crankshaft endplay to the low side of factory specs and doing the mods will ensure trouble free operation with the th400 / 3.8 combination.the thrust area of the crankshaft in inches squared wasnt designed to work with the normal thrust of the th400 converter.then theres this stuff thats needed:http://www.ckperformance.com/resources/GMTH400COMPETITIONCOMPONENTS.html
ijames said:
It is very important to realize that your standard cooler line pressure found with most automatics is 40 psi. Even the vast majority of 400s run 40 psi. A few will test at over 40 psi. Those units are the ones to watch out for. Just a little of my background. I worked at the only shop in San Diego that had a transmission dyno for 13 years. I studied a lot of cooler line pressure readings on 400s on the dyno because of the stories and problems people were experiencing with crank thrust wear. Mainly in motorhomes with 454 BBCs. Another important spec to remember. Cooler line flow should not fall below 1 quart in 20 seconds measured after the cooler. Pressure is not the important factor. Flow volume is. That doesn't mean restrict to 0 psi. Just enough to give you 40 psi at all loads. You'd be surprised how small you can go with the orifice and still maintain 40 psi and 1 quart in 20 seconds. To get an idea of converter feed restriction sizes, just study some of the late model transmission hydraulic circuits. They will give you an excellent idea of what size is needed.
ijames said:
I have heard the cavitation scare before. Even from a very well known transmssion man in the Buick community. All I will say is this. I have been doing the same mod, same size orifice for almost 20 years on stock, motorhomes, heavy commercial trucks, diesels, light racing and max effort racing and have yet to see an overheating problem or cavitation problem or crank thrust problem yet. Some of those customers run temp gauges too and monitor it. You can talk theory all day. In my mind it is proven.
I would imagine that if you go too small on the orifice size, then yes, cavitation is a possibility. Just stick to the specs I outlined and you'll be OK. Don't worry, be happy. No more crank repairs.
Some are asking what size orifice to use. I prefer not to freely give that info out. I have outlined the results to shoot for, some transmissions to study that will give you important clues. Now it's just a matter of doing some good old fashion experimenting and figuring it out for yourself. Not trying to be a hard bottom ender. I just like to see people figure things out for themselves. It's like dealing with my children and my employees. I prefer to see them learn on their own with a little guidance. It teaches good problem solving skills.
heres the deal.the hole is usually around .300 ",tap it to 3/8 coarse and install a 3/8 by 1/4 " allen head set screw and drill a hole through it to .155" to .175".with a stock unshimmed pr spring this will give 50 to 55 psi cooler pressure hot .period.
Lonnie, Don,
Could you have lets say 40 psi with a certain size orifice say 3/32" and 40 psi at 1/8" ? But now the difference is the volume the converter sees with the 2 different sized orifices? I'm thinking larger the size with same psi would supply more volume as to not raise the stall speed? Does this sound correct?
If you have one quart in 20 seconds, you're fine. That's all you need.
When I was playing with different orifice sizes there was a case where a small block Ford that someone was building kept eating the crank thrust. It is very uncommon to hear of a Ford having this problem. I helped him out with it by restricting the converter feed. I think it was a C4. On the dyno we got it to 20 psi. Still ate a crank. I restricted it further to 0 psi. Still ate a crank. The interesting thing was, even at 0 psi cooler line pressure, it still had at least one quart in 20 seconds flow. In the end it turned out he started having crank thrust problems with other SB Fords he was building and found the true cause to be bad cranks. Can't remember what the exact problem was with them. I believe they were aftermarket cranks.
