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Fifth Wheel King Pin Dimensions and Questions


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#1 SKP

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Posted 25 September 2012 - 12:46 PM

May I ask for your help in measuring king pin dimensions? I don't have a king pin handy (yet), and there are no RV dealers nearby, otherwise I'd go out and measure.

If a King Pin, when looked at from the side, can be considered to have an "hour glass" shape that is more massive on the top than on the bottom, and where the "neck" of the hour glass is 2" in diameter by about 1.45" in height, I am curious about the height of the upper "heavier" part of this hour glass.

How tall is the upper part of your king pin... between the load bearing surface and the transition of the "hour glass" to the neck around which the jaws to your hitch clamp around?

I have one diagram that shows the height of the upper part king pin as 2.115", and yet I have a hitch that only has 1.56" (1 9/16") between the strike plate and the top of the jaws. Since the jaws clamp around the neck, and since the jaws close down to only a 2" opening, and since the upper part of the king pin is, according to the diagram, 2.875" in diameter (and 2" tall, as earlier stated), it appears that the jaws would bear all the vertical load of the king pin on the "shelf" created by the reduction in diameters at the neck, prior to the load bearing plate contacting the strike plate.

Probably every one of you will tell me that I am over thinking this, but I've never owned a fifth wheel trailer before, and my understanding of clearances has been so anemic, I've gone ahead purchased and rebuilt an old "practice" hitch just so that I could learn what I needed to know before ordering a new custom air ride hitch... and find that I still get puzzled about some details, mostly because I do not have immediate access to any trailer with a king pin.

But according to the diagram of standard king pin dimensions, compared to the dimensions of my fifth wheel hitch, it appears to me that the jaws will end up bearing all the vertical pin weight of any trailer with a king pin dimesioned as per the drawing... and I find that hard to believe possible. That's what the strike plate is for... to bear the weight. The jaws secure the pin, and handle the pull weight, not the vertical weight. Yet I only have 1.56 inches between the strike plate and the top of the jaws inside my practice hitch (PullRite Super 5th... old cable pull style).

Something isn't right. Even adding a 1/8" lube plate, which would increase the distance between strike plate and top of jaw to 1.69", that is still a good deal shy of 2.115" for the top half of that hour glass shaped king pin.

Maybe the king pin dimension diagram I have is in error? I assume that all king pins have a standard SAE specified dimension. Here are the dimensions of the king pin in this diagram:

Overall height: 4.13"

Overall width, determined by the upper portion of "hour glass": 2.875"

Upper portion height of "hour glass": 2.115" (Tthis is the crux of my question and poll... is the distance between the load bearing surface and the neck of your king pin really this tall?)

"Neck" height of king pin: 1.45" (This is the business portion of the king pin, around which the jaws clasp, and this height includes two RO.125 radii transisitions into the upper and lower halves of hour glass)

"Neck width of king pin" 2.000" (+/- 0.005 wear tolerance)

Lower portion height of "hour glass": 0.565" (This is the part of the pin that is captured below the jaws... the part that you look for to see that you are not high hitched)

Lower portion width of "hour glass": 2.812" (Slightly smaller diameter than upper portion of hour glass)



As you can see, it appears that the entire weight of any fifth wheel trailer with pin dimensions as specified above would ride on a bearing surface donut on top of the jaws with an annular ring width of only .4375"!!!!

(2.875 - 2.0) / 2 = .4375" annular ring land width riding on top of jaws, bearing all the pin weight, due to:

2.115 - 1.69 = .425" vertical gap clearance between load bearing plate of trailer pin box and strike plate of 5th wheel hitch.



Something isn't right. My hope is that many of you will report back that the upper portion of the "hour glass" of your king pins is really only 1.5" nominally.

Please let me know!

Thanks in advance for your help.

Edited by SKP, 25 September 2012 - 12:54 PM.


#2 Softail

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Posted 25 September 2012 - 01:28 PM

Top shaft length (pinbox to groove) is approx 1.25 on mine

I have one diagram that shows the height of the upper part king pin as 2.115"


I'm guessing this is the length of the top "pre-installation".

Edited by Softail, 25 September 2012 - 01:33 PM.

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#3 Mark & Dale Bruss

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Posted 25 September 2012 - 01:48 PM

I have 1-1/4" from the trailer plate to the lower edge of the collar.

On the hitch the distance from the top of the hitch plate to thr top edge of the claws is 1-3/8"

Edited by Mark & Dale Bruss, 25 September 2012 - 07:02 PM.

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#4 Raquel

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Posted 25 September 2012 - 05:42 PM

On the hitch the distance from the top of the hitch plate to thr top edge of the claws is 1-33/8"


Mark, is that 5-1/8? :lol:


Sorry. :(

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#5 SKP

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Posted 26 September 2012 - 01:58 PM

Softail and Mark&Dale... THANK YOU!

Your measurements of 1.25" and 1.375" respectively have offered quite a bit of relief.

Softail, you may be right about the diagram. Perhaps the 2.115" is allowing for a .5" (nominal) weight bearing plate that the pin gets weld anchored into.

Anyway, it matters not. I now know that the 1.5" between the strike plate of this hitch and the top of the jaws is normal. Whew! Thanks again.

#6 armyret

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Posted 27 September 2012 - 05:50 AM

Your hitch would not lock around the narrow portion of the pin, if as you say the wide portion of the

pin is sitting on top of your locking jaws.



I think your looking at it wrong, the verticle weight, (pin weight) is carried by the plate the pin is welded to.

the narrow portion of the pin is were the jaws close around the pin, the wider portion of the

pin ride in the cutout of the hitch.




If you have questions go to the nearest RV dealer they shoudl be more than happy to

show you how it works. Although there might be minor differences in size, any hitch will

hook up to any pin and vice versa.

Edited by armyret, 27 September 2012 - 05:55 AM.


#7 jblscooper

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Posted 27 September 2012 - 08:40 AM

IMHO yes, you are overthinking it.

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#8 SKP

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Posted 27 September 2012 - 01:25 PM

Your hitch would not lock around the narrow portion of the pin, if as you say the wide portion of the pin is sitting on top of your locking jaws.


Thanks for your feedback.

Note that nothing is sitting on top of my jaws right now, as I do not have a trailer yet. I repaired an old hitch I picked up used to learn from, where the bar that triggers the jaws to close had separated at the weld bead. It was interesting to observe how the factory weld itself had failed, not the parent metal around it. Anyway, after grinding it out, I thought it prudent to make the fillet a little bit deeper and broader than was there initially, so as to create a "gusset" that would endure a bit more abuse. However, it occurred to me that, since this weld was on top of one of the jaws, I would need to account for clearance for the upper (fatter) ridge of the pin... what I called the top half of the "hour glass."

And that was the thought that lead me to order an SAE drawing of the exact dimensions of a fiver pin, so that I could fillet as fat of weld bead as possible without creating interference. However, once I got the drawing, I did a double take on the height of that upper half of the pin, as it exceeded the vertical distance between the top of the jaws and the load bearing strike plate. With the upper part of the pin exposed as high as the drawing illustrated, there is no way that the plate on the trailer itself would make contact with the plate on the hitch if the narrow part of the pin was encapslated by the jaws, leaving all the weight to be borne by the ridge of the pin. Something wasn't right...

... the verticle weight, (pin weight) is carried by the plate the pin is welded to the narrow portion of the pin is were the jaws close around the pin, the wider portion of the pin ride in the cutout of the hitch..



Exactly. And that is why I put the question to the folks here, because even Ray Charles could see that the weight of the trailer is intended to by borne by the plates, not the jaws nor the upper ridge of the pin, and that the jaws close around the narrow portion of the pin. But the diagram I was referring to in order to size my weld correctly made proper weight bearing seem impossible, due to the 2.115" height of the upper half of that pin that rides in the cut out of the strike plate. I was concerned that if indeed all pins were dimensioned as per the drawing, that I had other problems with this hitch far beyond what I was fixing. Turns out however, that a couple of members here understood my question, and reported back that the upper half of their pins were less than 1.5"... which is what I needed to know.

If you have questions go to the nearest RV dealer they shoudl be more than happy to show you how it works.


The "nearest" RV dealer is about an hour away, which is why I am so indebted to and appreciate Escapee'ers like yourself who take a moment of time to help on a technical question like this.

Thanks again! :)