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Topic: spline engagement - how much?  (Read 241 times)
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Mad Max
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... this vehicle under construction ...
« on: November 28, 2009, 07:38:00 PM »
When I converted to the 6BT/518 I also did a 203/205 doubler, and one of the issues I encountered was what I felt was too little spline engagement between the 518's output shaft and the 203's input shaft.  When I mocked it all up I found there was 'only' 3/4 inch of spline engagement between the two, and I thought that was too little...that there needed to be more (like 1 or 1-1/4 inches).  To achieve more I had to space the tranny and t-case apart about 2 inches and using two 1" spacers and a stub shaft I achieved about 1-1/4" of mesh on both ends, and I feel that is 'plenty'....but I don't really like using the spacers.  The burning question I've had ever since is....how much spline engagement is really needed?  I really have no idea, so, I offer the question up to the congregation.

The basics for my chassis are a relatively stock 6BT and 518 o/d, 203/205 doubler x-fer case, 4.56 gears, and 38" tires.  Truck weighs in at ~7000 lbs, and it will be a big, strong, trail rig.  I have no plans to bomb the engine over 200hp/500tq - just over stock.  No sled pulling...but I will be working it pretty hard off-road, and I'm 100% positive I will be yanking folks out of deep holes, etc.

So, thoughts?  Is 3/4" of spline engagement 'enough', and if not, what are some recommendations?  Has anyone out there actually broken the end off of an output shaft as a result of two little engagement?  I would much rather bolt the 203 to the back of the 518 'clean'.  I'd like to hear some experiences out there in "real-world land".

Thx,
- Sam
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« Reply #1 on: November 28, 2009, 09:15:17 PM »
The big question is :  Where will you be when you find out 3/4" is not enough?  it would not be an easy trail fix. going from 1-1/4" to 3/4" the torque rating loss will be well over 1/2,  most likely you will be at the point of having to figure out the new rating based on the individual splines, not the shaft diameter.

A similar thing has been discussed on here in the past, mounting a 208, on a transmission shaft designed for a 203.  several members have stated that they did it, and not have any problem, while others have said they broke the TC input shaft.   Which do you want to believe, and how much do you want to gamble?  behind a 318, on the highway, you would probably be ok, but behind a diesel?  Go look at a diesel clutch disk, and measure how long the splines are, and thats just engine torque, not multiplied by transmission gears.
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« Reply #2 on: November 28, 2009, 10:49:42 PM »
Sam, I've had a similar debate recently mating a '96 1500 transfercase to a 727.  I talked to Novak's techs and they claim they get away with 1x shaft diameter spline engagement on Jeeps, but they recommended at least 1.5x shaft diameter spline engagement in a fullsize rig.  For a 23 spline shaft (~1 3/16") that means about 1.75" of engagement.  I've measured a couple factory Dodge setups and they have about 1.75"-2" of spline engagement.  3/4" is not enough.
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... this vehicle under construction ...
« Reply #3 on: November 29, 2009, 12:15:42 AM »
copy all, and that is what I figured as well which is why I did the stub shaft - just didn't 'look' like enough, but I did not have any emperical data to back up my brain.  So unless I can figure a way to either extend the 518's output shaft or the 203's input shaft I guess the stub shaft and spacers will have to do.

Thanks fellas
« Last Edit: November 29, 2009, 10:42:41 AM by Mad Max » Logged
...there's always a way, and if it's worth doing...is worth over-doing.

78 Cummins M350 - "B.u.d." (trail rig)
93 RC W150, 5.2 Magnum - "Blu Oxx" (driver)
93 Cummins Ram D250 - "Big Mack" (tugboat)
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« Reply #4 on: December 1, 2009, 11:10:04 AM »
have you considered dumping the 203 range box for something else like Klune V?
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... this vehicle under construction ...
« Reply #5 on: December 1, 2009, 01:05:56 PM »
I have considered it (or an Atlas or a Black Box), but my cradle, linkage etc are already set up and unless I can find someone that would want the whole doubler gig I'd just as soon keep it.  Besides, the 203/205 gig is bulletproof...and I like bulletproof Grin.
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...there's always a way, and if it's worth doing...is worth over-doing.

78 Cummins M350 - "B.u.d." (trail rig)
93 RC W150, 5.2 Magnum - "Blu Oxx" (driver)
93 Cummins Ram D250 - "Big Mack" (tugboat)
68 Charger R/T - "Trigger", 71 Demon GTS-R - "Chubby"

Vendor - ASA Modifieds
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