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  1. Fifth Wheel King Pin
  2. CURT
  3. Lippert 1621
  4. 18000 lbs GTW
  5. Upgraded Pin Box
  6. Absorbs Road Shock and Reduces Chucking
Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 18,000 lbs

Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 18,000 lbs

Item # LC328492
Our Price: $1,447.65
You Save: $781.58
Fifth Wheel King Pin
Shipping Weight: 232 lbs
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Flex Air drastically reduces chucking, absorbs road shock, and minimizes bounce. The internal air bag and built-in strut work with the rubberized pivoting head to achieve a dramatically smoother ride. Replaces Lippert 1621 pin boxes. 1-800-940-8924 to order CURT fifth wheel king pin part number LC328492 or order online at Free expert support on all CURT products. Great prices and Fastest Shipping for Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 18,000 lbs. Fifth Wheel King Pin reviews from real customers.
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CURT Fifth Wheel King Pin - LC328492

  • Lippert 1621
  • 18000 lbs GTW
  • Upgraded Pin Box
  • CURT
  • Absorbs Road Shock and Reduces Chucking
  • Fixed Turret

Flex Air drastically reduces chucking, absorbs road shock, and minimizes bounce. The internal air bag and built-in strut work with the rubberized pivoting head to achieve a dramatically smoother ride. Replaces Lippert 1621 pin boxes.


  • Air ride pin box with a pivoting head replaces standard pin box on your 5th-wheel trailer to cushion your ride, improve handling, and limit strain on truck and trailer
    • Drastically reduces chucking
    • Virtually eliminates bounce and jarring
  • Internal air bag absorbs road shock
    • Scissor-like motion allows pin box to compress, compensating for jarring and bounce
    • Built-in shock ensures that pin box rebounds - keeps air bag from breaking down
  • Pivoting head limits chucking by allowing controlled movement of the king pin within your hitch
  • Rubber compound built into the head absorbs road shock
    • Reduces wear and tear on trailer
  • Black powder coat provides rust resistance
  • Sturdy steel construction
  • King pin connects to any 5th-wheel trailer hitch
  • Bolt-on installation with no assembly required
  • Made in the USA


  • Designed to replace Lippert 1621 5th-wheel pin box
  • Weight capacity: 18,000 lbs
  • 1-Year limited warranty


  • Do not use the Curt Flex Air pin box with any type of gooseneck adapter.

Getting there is half the fun, right? Not if you're being bumped, jerked, jammed, and pushed around from the chucking, road shock, and bounce that occurs as you tow your fifth-wheel trailer or RV. And imagine the costly wear and tear on your trailer as it endures that rocky ride.

Put the fun back into your trip and ease the stress on your trailer by replacing your factory pin box with the Curt Flex Air pin box. The internal air bag and built-in shock work together to cushion your ride, improve handling, and limit strain on both your truck and your trailer. But what sets the Flex Air apart from other air ride pin boxes is the addition of a pivoting head. This head, along with its built-in rubber compound, provides additional cushion, absorbs road shock, and reduces chucking for a significantly improved ride.

Rubber Compound and Internal Air Bag Absorb Road Shock

Curt Flex Air pin box has a rubberized pivoting head

A heavy-duty rubber compound is built into the head assembly of the Flex Air pin box. This rubber is the first defense against road shock. It helps to absorb the bumps and vibrations experienced when towing a fifth-wheel trailer over rough roads. Standard pin boxes without any type of shock absorption transfer road shock to your trailer, resulting in potential damage to the trailer and an overall bumpy ride. The shock-absorbing rubber inside the Flex Air takes the beating so there's less wear and tear on your trailer or RV and you get a smoother ride.

Curt Flex Air pin box has a shock and air bag

Because the rubber may not be able to absorb all of the road shock that you experience, the Flex Air also implements air ride technology to virtually eliminate shock. When you encounter uneven terrain on your trip or hit a pothole on the highway, the Flex Air pin box compresses vertically, similar to how a pair of scissor blades closes in on each other. This compression transfers the road shock and vibrations to the internal air bag, saving your fifth-wheel hitch, trailer, and tow vehicle frame from enduring a rocky ride.

A built-in strut, or shock, controls the Flex Air's compression so that the internal air bag doesn't burst. The strut also causes the pin box to appropriately rebound without any bounce, and positions it to be ready for the next bump in the road.

Pivoting Head Reduces Chucking

Lippert Flex Air pin box has a pivoting head that reduces chucking

The pivoting head on Flex Air allows for controlled back and forth movement of the king pin to reduce the uncomfortable jarring known as chucking. Unlike a rigid pin box that has little give at the point where the trailer and the hitch are connected, the Flex Air can pivot slightly to compensate for those times that your trailer suddenly pushes or pulls on your tow vehicle. Because the pin box head - not the king pin itself - is moving, the chucking that often occurs when the king pin jerks around inside the hitch is significantly reduced.

Simple Setup and Installation

First, simply remove your existing Lippert 1621 pin box from your trailer. The Flex Air uses the same bolts that held on your original pin box so no additional hardware is required for installation. Plus, the Flex Air pin box comes out of the box ready to be put on your trailer, so no assembly is needed. Because the Flex Air is quite heavy, you should put it on an elevated surface or have a friend help you support its weight during installation.

The internal air bag should be inflated after the Flex Air pin box has been mounted on your trailer. Using the air intake valve, inflate the air bag until the outer cylinder of the shock rises high enough to fully expose the yellow inflation indicator sticker.

328492 Curt Flexair Fifth Wheel Kingpin Box for Lippert 1621

Replaces 328492 Lippert Components (LCI) Flexair by Trail Air Fifth Wheel Kingpin Box for Lippert 1621

Installation Details LC328492 Installation instructions

Customer Satisfaction Score:

Customer Reviews

Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 18,000 lbs - LC328492

Average Customer Rating:  4.6 out of 5 stars   (158 Customer Reviews)

Flex Air drastically reduces chucking, absorbs road shock, and minimizes bounce. The internal air bag and built-in strut work with the rubberized pivoting head to achieve a dramatically smoother ride. Replaces Lippert 1621 pin boxes.


I bought the Flex Air to replace the standard Lippert pin box on our Cedar Creek fifth wheel that weighs 12K lbs. Just looking for something to smooth out the ride and the Flex Air seemed to have good reviews and recommendations.

I went to the Lippert website to download the installation instructions because I couldn't get the link on the etrailer website to work. The instructions will give you the torque values required for installation and the air pressure for the inflatable airbag on the hitch.

Installation was straight forward, but you better have a plan because this thing is heavy (300 lbs) and it will take some manpower to move around and install.



Really glad we switched out pin boxes, the Flex Air was a great choice and definitely smoothed out the ride.



Really glad we switched out pin boxes, the Flex Air was a great choice and definitely smoothed out the ride.


This is the best money I have spent on my camper hands down. My hitch weight is a bit over 3200 lbs. I use a PullRite Super Lite hitch, photo attached. Once locked on the king pin, four bolts are set from the adapter plate against the king pin plate. The plate on the Flex Air is twice as thick as the original pin box and I have way more confidence in a solid set of these four bolts. I was blown away by how this hitch absorbed road shock. I will never have another 5th wheel without this set up.



Since this original post I have gone back to a regular style 5th wheel hitch new truck with factory puck system but the Flex Air is still doing its thing and I will never pull a trailer without it unless something better is invented.


This Curt flex air 5 th wheel pin box was probably the best investment that I’ve made in pulling my fifth wheel. It takes approx. 75% to 85% of the rebound bounce out of hitting irregularities in the roadways, of which there are many. I’ve pulled a 36’ couger and now a 41’ reflection. It makes pulling a trailer, almost, pleasant.


4 stars as I have just recently installed this although I received it over a year ago.
I just put 750 miles on it and experienced almost every PAVED road condition imaginable and it worked great. I can't imagine what that would have been like with the stock pin box.

4 stars because I still need to see how it performs off road and I am just a few days into using it on a trip covering OR, ID, MT and WY so I still have my personal evaluation to complete and need more drive time to see if anything fails on it.

Installation was a breeze but it's a heavy hitch so caution is advised.

Arman K.


I just finished a 3,000 mile trip from Oregon to South Dakota. The hitch works like a dream. Very few road issues are transferred to my truck from the trailer. This is a great hitch. Totally satisfied with this purchase.If anything I wish they made an offset pin box so that I could adjust it to make my trailer ride correctly, as it is with my 4x4 it runs a tad nose high.


Install went pretty smooth. Did it from the bed of the truck, using a motorcycle jack in front and a small hydraulic in back. Make sure your torque wrench can dial to 160 ftlbs. I will update after I tow it!!!

James S.


I cannot believe it has been a year....and a trip to Perdido Key, FL and Breckenrigde CO. In addition to taking care of us, I have to tell you, it will take care of your trailer too! Improved our towing experience old pin box was an upgrade Rotoflex greatly. Worth every penny! If you did not see my earlier post, my friend and I installed it ourselves. Motorcycle jack in the back of a pick up bed. :


So far it's performed great. Really reduces the bucking and normal bumps. My only issue is the kingpin angles DOWN too much for me and not sure how to correct it. the plate where to pin is, is not level when hitching up which has caused gouges in my 5th wheel plate.

Etrailer Expert

Katrina B.


I have gotten in contact with the manufacturer and they have confirmed that the 1621 longjaw pinbox is going to have more downward angle to it because of its length. You can try to adjust the pinbox andor hitch height. The pinbox itself is not defective, you need to try and level out to get the pinbox as straight and level as possible.


First, about etrailer. As always, their service was outstanding. They honored their low price guarantee without question and shipping was fast.
I upgraded from a Trailair Air Ride pin box with a fixed turret. That pin box helped with bouncing and jarring, but I still got a lot of "chucking". This FlexAir pin box solved that problem as well. Installation was a breeze, although you must make sure you have enough adjustment on your trailer to mount this pin box higher up than your existing pin box. I had to mount it at the very highest set of holes on my trailer. Lipperts web site has specs on this pin box and installation instructions, including measurements and torque specs. With the help of a friend and using the bed of my pickup truck, we unbolted the old one and bolted the new one in. Took about an hour and that was not hurrying.
I took the trailer out this past weekend with the new pin box and hit some pretty bad roads. Wow! What a difference this pin box makes. I can only describe the ride one way. Smooooooth. I am very pleased with both etrailer and this product. It's pricey, but to me, pulling a large fifth wheel trailer, it's worth every penny. Highly recommended.


Have not had a chance to take it for a road test yet. But it fit well, was easy to mount, the issue I ran into was getting the pin height. As my truck is 4x4, and the B&W hitch is as low as it can go, I still needed another 2 inches to get the pin box high enough, could not raise pin box any higher, and with the 5th wheel already at 13'4" did not want to lift the trailer, so ended up lowering the truck 2 inches to get everything back level again. But will find out next week how it works as we start out on a close to 12,000 mile trip to the west coast and back across through canada and back to florida.


This item arrived much faster than etrailer said it would. It came by truck, and the freight company driver was very nice and made sure I opened the box before he left.

I installed it myself with some help from my 15 year old son. We used our forklift tractor attachment to put it in place. The Curt Flex Air totally changed the experience of pulling our 5er. It eliminates most of the chucking, and acts like shocks on a car, so ride returns to normal immediately after a bump in the road...which the Flex Air absorbs most of.


I have used my Flex Air for over one year and put around 3000 miles on it. I purchased this for one reason to reduce bucking when I it bumps in the road. I can safely say that it has virtually eliminated the bucking. It has also made the ride a lot smoother. I would highly recommend the Flex air. I did my own install of this product by my self.

My rig
2018 Toyota Tundra with a Husky 16 KW Hitch
2017 KZ Durango 30 ft.


I will be on the third flexair pin box when it gets here on the 21st of November 2016. The problem iIm having is with the lower roto flex portion. The rubber block shifts and I have talked to others with the same problem. Lippert has been great about the replacement, But the the hassle of replacing this three times in one year is not worth it.


Best thing since slice bread,the way the roads are not repaired and full of pot holes and patches this has smooth out the ride, A must to have on your 5th wheel,,



Works Great. put appx 20,000 miles on it in last 2 years, 2 trips . with all the Potholes on the roads it helps, also put the Lippert Center Point air ride on the axles .. Like Night and Day for the Ride......


Reflection 320MKS
Dealing with etrailer
was very easy. Starting with getting the correct information on which pin box would be suitable for our trailer.
The unit arrived in 10 days from ordering it. Which was less than the estimated timeframe when placing the order.

The only down side, on inspection when arrived, the air bag valve was damaged in transit. A quick email with photos, a call to etrailer & they have a replacement part ready to dispatch.
Great customer service & i would definitely recommend them.

Kevin T.


Hi, I installed the Curt 5th wheel king pin unit in March 2022 on our Reflection 320MKS. What a difference it made on hauling the 5th wheel. We have traveled about 12-14,000 miles this summer towing the 5th wheel on highways &amp many country roads. The kin pin unit has reduced most of the stock feedback through the 5th wheelking pin & chucking has seen a big reduction as well on uneven roads. I would highly recommend this Curt king pin unit. Thanks E-trailer for recommending this unit.


perfect fot my application, and the ride is much smoother, eliminates chucking


I received my new pin box in less than a week, I’m very pleased



This pin box made the ride so much smoother for the truck and 5th wheel. With the shape the roads are in these days this pin box is a must


Smoothest ride that I've ever had with a fifth wheel trailer


I have put about 4,000 miles on my camper since I installed this flex air pin box and it doesn’t disappoint. No more chucking or teeth rattling on those bumpy roads, you sometimes forget you’re cruising with a 40 foot trailer. Don’t think twice about it you won’t be disappointed.


This 250 lb item was shipped in a cardboard box that was just sitting on a pallet. When I opened the box I found the Flex Air was sitting upside down with the foam pad on top. I have no idea how the air filling nozzle wasn't damaged. Luck I guess. There were no "UP" markings on the box. No instructions were included.
Installation went smooth, it's just heavy so be careful. Instructions and videos can be found on-line.
The Flex Air is at least 4" taller than the OEM box that came with our Grand Design Reflection. I mounted the Flex Air as high as I could and lowered the receiver on the truck. The trailer is still nose high but seems to tow fine. My test ride was noticeably smoother. You can see a lot of motion on the Flex Air that would have otherwise been transferred to the truck.

Steve S.


Fifth Wheel tows smooth. After 5000 miles from Minnesota to the south east US. Freeway and bumpy back roads. Wind gusts up to 25 mph. The trailer tows smooth. No chucking. The trailer is still higher in the front but it has no noticeable effect on towing and the leaf spring suspension can handle it. The tires seem to be equally loaded, no uneven wear.


I can’t rate this product as we have yet to install it on our trailer. The pin box dimensions aren’t what is specified in description. The opening to mount the pin box is 12” on our trailer and the Curt Flex air that we received is 11 3/4”. We have talked to Lippert and etrailer and have gotten mixed answers about the safety of mounting a product that is off by 1/4” on to a 16,500 lb trailer.

Etrailer Expert

Heather A.


Hi Denise, I reached out to the manufacturer for you. Once the bolts are torqued down it will make up the difference in width. These are built a little narrower to fit into the opening easier in the event the wings have a little distortion when removing the old pinbox. Just follow the torque specs in the instructions and everything will be safe!


Product arrived in a timely fashion in an un damage condition. Quality was better than expected. Installation for me took about two hrs, with my wife helping. I placed the pin box in the bed of my truck with the help of a frontend loader and let it down on a floor jack. From there install was fairly easy.

Pros: Good quality Heavy Duty

Cons: You will need 3 or 4 strong friends to handle it
No instructions included I did find a pdf on the bottom of the page however that listed the torque for 1/2" bolts



Works great after a year of use preforms well. I haven’t had to put air after the initial start up.


Awesome price! Being installed on a Grand Design 375RES. Shipment went great this is a heavy unit, arrived banded on pallets in excellent condition. Installation was simple and as instructions stated. Waiting for the next road trip to be able to test effectiveness. Always been a pleasure dealing with E-Trailer.


Item was as described and has made a huge difference in the overall ride of m truck and 5th wheel.

Thank you very much for the timely delivery although the lift gate on the delivery truck did not work so we needed to slide it down some 2x8's I had on to the back of my truck. All good the driver was a great help.


Night and day difference in ride quality for both the truck and trailer. Smooths out rough bridge bumps and eliminates chucking.


My pin box arrived in excellent condition!! Installation was easy with good help. Thanks E-trailer

Pete M.




Dealing with you guys and your suppliers ment all kinds of information going in different directions and nothing ever jiving. Worst experience I've ever had with your delivery companies. Making promises never fulfilling them never calling ahead of time me having to chase them down when you chase him down you talk to somebody that is in the orient. They say will have someone call you in 1 hour. Never did they ever call back. I'll just stop is here because I don't want to get angry it's too upsetting. Finally got delivery 5 days after it was supposed to be here 3 days waiting all day long as promised and nobody showed up when you call back they say will have someone call you in an hour. They still have not called driver showed up here finally 5 days late said he didn't have my number you're the reason I know that it would be was here cuz he had a 40 ft tractor trailer

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