T-400 direct drum grinding on center support? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-06-2012, 08:23 PM
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T-400 direct drum grinding on center support?

I'm in need of some advice of TH400 gurus. I recently rebuilt a 400 in my 90 K2500 Chevy after I toasted it. What I actually did was get a good deal on another used 400 and rebuilt that, that 400 is an 85 model HD unit.

Long story short, I tore it down and put it together with new clutches and seals. Everything seemed to spec out fine and after I installed it in my truck it worked great for a couple dozen miles, until I lost 3rd gear, limped home, and then lost pretty much every gear. I just pulled the tranny and tore it down to find out what happened.

What I found was that the direct drum had been grinding down on the center support, creating a horrible mess of metal shavings. I attached pictures of the worn support and a known good one to show the wear.

Now, I'm trying to figure out why exactly this would happen. I did do the dual feed mod, leaving the second sealing ring and the piston seal out, as well as plugging the reverse feed in the case with a cup plug.

My best guess thus far is that the scarf cut teflon seals I used that came with the kit didn't seal correctly (or I also could have installed them incorrectly?) and couldn't keep the drum and support separated? I will admit however I'm not sure how the drum and support are separated while in motion, I'm assuming its fluid pressure from one of the feed holes between the seals.

I should also note that I noticed the plastic washer between the direct drum and forward drum was also chewed up and severely worn, which was definitely not the case when I installed it in the rebuild.

Any help or advice would be greatly appreciated, as I'm somewhat new to the tranny rebuild thing and would like to find the problem before I rebuild it again.
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Old 03-07-2012, 05:11 PM
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wow...this is one of those things that if it was on my bench,it would be soo much easier to diagnose,tough over the internet..so i'm just speculating from here on out...but for starts...it comes down to 2 things...lube prob or bushings ...i'll rule out the bushings if you didn't change them,and this problem wasn't there when you originally took the trans apart...and you didn't change the center support bushing when you were into the trans,right??.....and was the center support to case bolt tight when it went back together?? snap ring installed bevel side up,and fully in its groove??...intermediate clutches and steels and wave plate put back in in same order as removed,nothing omitted?? ok,since it worked good for dozens of miles,i'll rule out the teflon rings being the problem. That leaves us with a lube problem...so then,nothing wrong with leaving off the 2nd ring on the center support,as well as the center lip seal in the drum ....now.. the cup plug..was it a restrictor plug,or a solid one....i'll look for a response from what i said so far....and maybe someone else will jump in and join this party on this one....and remember,since this wasn't an issue when you first took it apart,it means that it turned into a problem that you created,[we've all done it]..so i'm thinking of what first comes to mind here.

Last edited by rick 427; 03-07-2012 at 05:37 PM.
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Old 03-07-2012, 05:53 PM
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sorry for the long post,but i'm just throwing out a bunch of mild to wild possibilities on that area of the trans.
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Old 03-07-2012, 06:00 PM
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I'm fully aware of the problem I created

I didn't change any bushings as they all looked fine, and even now don't look damaged. The center support bolt was still tight when I took it apart, and I did torque to spec when I installed originally. I did install the snap ring bevel up, I noted that when I was taking it apart, and was fully seated.

The intermediate clutches I was very anal about installing correctly, however I did not have a wave plate with it. Now, its possible it could have been missing from a previous rebuild by another hand, as I saw evidence of this (a shoddy one at that with over torqued bolts galore). The truck it came out of had been sitting for ten years so there weren't many if any miles on it. So this could be a possibility I suppose. I guess this one boils down to how to know exactly if a waved plate is supposed to be used?

I did use a solid cup plug in a case, 3/8" if memory serves me. The lube problem could make sense if maybe my pump had gone bad? I forgot to mention the pump had started whining after I lost all the gears limping home, I was thinking this was because the filter was so plugged from all the metal shavings that it was starving for fluid, but a dying pump losing pressure would make sense as well. I guess I'll have to check the pumps condition.

I will also mention that in my tireless reviewing of my rebuild I remember when I installed the scarf cut seals, for some stupid reason I made a seal compressor I had seen used for the SOLID teflon seals. I'm not sure if or even how that could have damaged or altered the seals function but I'll mention it regardless.
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Old 03-07-2012, 06:26 PM
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A flat steel was used to replace the wavy steel in a lot of rebuilds by guys...the whining noise was just the filter trying to digest all the metal i'm sure it ate...I've never put a solid cup plug in ANY port,unless a step in a valve body reprogramming kit told me to...restrictor plug, yes..solid,no....and every kit that I have installed,gave me a restrictor plug,never a solid one...unless we're talking full manual shift,which we're not this time...if it's the cause of the problem or not,i'd remove that plug if its not part of a kit....but thats just me...still thinkin...i've seen this same problem before,but its been a while.....wait...there were 2 different height center supports for the 400's....but i'm not sure what ill effect it would have by using the wrong 1 ...i've never mis-matched them....and if that trans was never run when you got it....but i'm thinking if the thin 1 was used where it called for a thick 1 that it would smack the back of the drum,even tho that would usually happen after it breaks the support to case bolt.....tell me, did that beveled snap ring go in easy?....or did you have to tap it into the groove fully with a hammer and screwdriver to fully seat it??

Last edited by rick 427; 03-07-2012 at 06:49 PM.
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Old 03-07-2012, 06:41 PM
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Hmm, good to know on the plug, I'll be sure to replace that then. I'm not 100% convinced that would create my problem though, I'll have to look into that.

There were 2 different height center supports, in my ATSG manual it said the difference was whether or not you used the fretting ring beneath it (I think, I know it was the difference of one snap ring) and I do remember measuring to make sure I needed the extra snap ring. I tried doing some research and couldn't find anything about mismatching a direct drum and center support, so I'll shelve that idea for the time being.

EDIT: I can't remember exactly how the ring went in. I want to say I had to wiggle it in a little but I don't remember it being stubborn enough I had to tap it in. But for what its worth it didn't just slide in like butter either.

Last edited by Mr. BBQ; 03-07-2012 at 06:52 PM.
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Old 03-07-2012, 06:51 PM
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did you check end play when you got the back half together?
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Old 03-07-2012, 06:54 PM
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From what I remember of rear endplay it was in spec, but on a looser side. The exact number I can't remember. Front endplay was was directly in spec.
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Old 03-07-2012, 07:03 PM
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What makes this situation bad is that it was rebuilt by who knows who before you got your hands on it and you being,addmittedly, inexperienced.....if this trans was never run,you could have inherited a problem from the previous build......man,I don't know,wish I was looking at it. Instinct alone tells me if something isn't right when putting them together. Little harder trying to visualize it.

Last edited by rick 427; 03-07-2012 at 07:27 PM.
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Old 03-07-2012, 08:18 PM
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How do the splines fit from the tube into the direct drum? Are they loose ? The drum can wobble around if the splines are loose

Typical drum wear of this type is from loose bushing in the center support on this tube. It allows the drum to drop down. The sealing rings are not there to support this drum.

You say the end play was slightly loose? If you are talking about the front end at the pump, this can allow the drum to move around some and forward slightly. This can allow the drum to wobble.

There is a bushing available from CK Performance in New York area that fits into the direct drum on the front side and aligns with the inner drive hub that sits inside the forward drum (input shaft drum). This is not a drop in repair, some machine work is required.
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Old 03-07-2012, 08:54 PM
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good point Crosley...but it still throws me off on why the problem wasn't there when he took it apart the first time...only after 25 miles or so of driving after the rebuild.......ok Mr BBQ...listen up....I have a gear train in my hand...the teeth on the top of the sun gear shaft spline into the drum...the drum bottoms out at the end of the teeth,preventing it from making contact with the support,now heres the good part...on the other end of of the sun gear shaft is a 3 piece torrington bearing that it, and the sun gear,rides on..if that bearing or a part of it, is missing,the sun gear shaft will be lower than its supposed to be...and when the sun gear shaft is lower,it is lowering the back of the drum allowing the drum to make contact with the support....you might be missing a bearing back there....please double check that

Last edited by rick 427; 03-07-2012 at 09:05 PM.
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Old 03-07-2012, 09:14 PM
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I'll double check the splines tightness in the drum, but I don't remember it being particularly loose. I do remember the splined shaft being a good fit in the center support however, I'm still certain that bushing was good.

I did check that bearing under the sun gear as I originally thought that was the problem, but it was in place in good working order, all 3 pieces were there. I even made sure I put the sun gear in correctly, all was according to direction in the ATSG with the chamfer end facing up.

I did note how the splined shaft looked to support the drum where it seated. I mean at the base of the splines where it goes into the direct drum. it looks like the direct drum is worn on that surface as well but the shaft isn't. I have 2 shafts to compare it to. Whether it was worn there before or not I cannot say for sure, but it is worn enough to let the drum drop down onto the center support. This seems like another possibility?

EDIT: Omit that last idea, I had a total brain fart. I was thinking of where the drum was hitting the center support, not where the splined shaft seated. That's what I get for thinking at night I'm going to double check the splines however for wear anyway.



Front endplay was perfectly in spec, rear was in spec as well but towards loose. I've been reading its better to spec it towards the tight side now, but I wouldn't know if this could cause this much problem?

Tomorrow I'll have to dig into it more and see if I can find more plausibilities. It is a pain in the butt being inexperienced with this, but dammit I will learn one way or another!

Thanks a million for the help thus far guys! I do appreciate it!

Last edited by Mr. BBQ; 03-07-2012 at 09:23 PM.
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Old 03-07-2012, 09:28 PM
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and you got the 3 piece bearing on the 1 side of the output shaft,and the 4 tang washer and the 3 tab case washer[that goes on the case] on the other side?? Is the case worn where the 3 tab rides?? Is it the right thickness 3 tab washer?

Last edited by rick 427; 03-07-2012 at 09:44 PM.
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Old 03-07-2012, 09:40 PM
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And if you say yes to that,then i'm gonna have to put my bat away...I've hit enough balls to the warning track on this one....other than mis-matched parts,and i don't believe the trans would even go together that way,sans the center support,,if you find what I haven't covered,you win the home run derby......oh,and my 1st posts was a mis-read from me,as i thought that the drum was eating the teflon rings,not the drum kissing the center support,,,sorry about that.

Last edited by rick 427; 03-07-2012 at 10:03 PM.
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Old 03-08-2012, 11:54 AM
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Something was mis-assembled. Just gotta find out what.

As for the cup plug, it is used when dual feeding and using an OEM separator plate to prevent loss of 3rd gear oil out the reverse passage/circuit, and exhausted at the manual valve. It needs to be there when dual feeding with a stock plate.

The direct drum can be supported on the forward hub using a 4L80E forward drum bushing and some minor machining.
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