TH400 Bolt on yoke problems

[kfactor239]

I have an 86 dually and I have replaced the 454 with a chevy HT502 crate engine and the stock TH400 with a B&M Traveller TH400 transmission. Everything seems fine but my bolt on yoke seal keeps destroying itself. The first time I changed the seal I reinstalled the yoke but left the bolt a little loose to move the yoke away from the seal a bit (it seems if I tighten the bolt up it does not leave enough clearance for the seal). Well that didn't last long and the seal failed and it leaked. The next time I decided to change out the yoke I found a replacement yoke that was for a bolt on yoke but it was designed to be used as either a bolt on or a slip with a blank out plug where the bolt goes that you can either leave or remove. I decided to leave the plug in and install it without bolting it on as a slip yoke. Well that turned out even worse. What happened is my center bearing for my driveshaft came apart and Ive got one on order. My problem is that for some reason my output shaft is not sticking out long enough to allow the yoke to be bolted on and still leave clearance for the seal. The extension housing that came with the new transmission is identical to the one on the old one I had checked that before I sold the old transmission. I have also noticed that the slip for the driveshaft with the truck unloaded and sitting static isn't showing much spline. I think that as I am driving the truck the driveshaft slip is maxing out and pushing my front driveshaft forward. Does anyone know if a chevy 502 is longer than a 454? Thats all I can figure for the lack of slip clearance on the driveshaft. Oh and if the yoke has shoved the output shaft into the transmission I guess this could account for the lack of clearance for my yoke seal but I am not a TH400 expert at all if that is the case what would be damaged and what would the fix be.
Thanks
Kenny

 

[bucket]

The block dimensions are the same.

Does the output shaft have any play in it?

 

[kfactor239]

With the yoke off I have a little side to side play but no forward aft play.

 

[Blue Ox]

Can you put shims inside the yoke so it doesn't go in too deep?

You're definitely going to need to be sure the system is not running out of travel. Bad things will happen. Usually you can use O-rings, paint, or zip ties to verify travel. Ideally it should be done by fully flexing the suspension, but that can be challenging on a heavy truck.

 

[Matt69olds]

The bolt on yoke should bottom out on the output shaft, and just barely clear the seal. There also shouldn’t be any side/side, or up/down yoke movement. If so, the bushing is shot and probably tearing up the seal. As far as I know, all bolt on yoke style output shafts are the same, and all short extension housings are the same 4 inch length.

The bolt on yoke doesn’t move, the driveshaft is a 2 piece with splines to change length with suspension movement. If the driveshaft doesn’t fit because it seems too long, I’d start by making sure the splines are clean and lubed, make sure the driveshaft splines don’t hang up or bind.

 

[kfactor239]

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Ok this may have been the problem. Picture #2 here is showing the old bearing and where it was pressed on to. It was pressed in the middle area of the three different diameters of this shaft. Picture #1 is showing my new center bearing which is much larger in diameter than the one that came off making me think that it is to be pressed onto the largest diameter the most aft part of this driveshaft. Picture #3 is showing a little ring that came off when I got the center bearing off and I have no idea where it goes, if it was suppose to be there, and what its purpose in life is. I have the service manual for this truck but it does not go into detail about any of this. Please give me some help I could surly use some. Thank you

 

[kfactor239]

Ok I am sorry I haven't been keeping up with posts. And thank you all for your help. Ok as far as putting shims on the yoke I thought of that however the way it is designed the splines go all the way through the yoke and there is a big washer that caps off the yoke and the bolt goes through the washer and the bolt and the washer keep the yoke from moving aft. It is weird but as I can see there is nothing that keeps the yoke from moving forward other than it hitting the extension housing which in turn destroys the seal.

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[kfactor239]

With the yoke on the outputshaft and inside the extension housing there is no movement at all. I had checked the movement of just the outputshaft without the yoke installed and there was only a slight side to side movement not fore and aft movement at all. Of course with the yoke installed and riding on the bushing of the extension housing there is no movement side to side.

 

[bucket]

Is it known for sure that the output shaft of the new trans is the same length as the old one? Being a bolt-on style yoke, the driveshaft and carrier bearing should have nothing to do with the output shaft seal issue.