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  1. Fifth Wheel King Pin
  2. CURT
  3. Lippert 1621
  4. 21000 lbs GTW
  5. Upgraded Pin Box
  6. Absorbs Road Shock and Reduces Chucking
Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 21,000 lbs

Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 21,000 lbs

Item # LC369535
Our Price: $1,561.41
You Save: $369.54
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 LC369535 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 - 21,000 lbs. Fifth Wheel King Pin reviews from real customers.
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CURT Fifth Wheel King Pin - LC369535

  • Lippert 1621
  • 21000 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: 21,000 lbs
  • Overall width: 12"
  • Distance from bottom of king pin to top of box: 19-9/16"
  • Distance from center of king pin to back of box: 29-9/16"
  • Distance from center of king pin to front row of bolt holes: 20-3/4"
  • Distance from center of king pin to back row of bolt holes: 28-5/8"
  • 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

Curt 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.

369535 Curt Flexair Fifth Wheel Kingpin Box for Lippert 1621 - 21,000 lbs

Replaces 369535 Lippert Components (LCI) Flexair by Trail Air Fifth Wheel Kingpin Box for Lippert 1621 - 21,000 lbs

Customer Satisfaction Score:

Customer Reviews

Curt Flex Air 5th Wheel Pin Box - Lippert 1621 - 21,000 lbs - LC369535

Average Customer Rating:  4.6 out of 5 stars   (154 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.


Installed on my 2019 Grand Design Momentum 351 and it fit perfect. Shipper came on time and helped me slide it on to my tailgate, it came on a small pallet. I backed it up to the camper and used a car jack to lift up the aft end. Once I got the lower holes on either side aligned I installed bolts loosely. Then I jacked up the forward side and installed bolts once they lined up. Installed very easy by myself. I did purchase grade 8 replacement bolts from [farm store]. Original bolts were 1 1/2" I put in 2" because the metal is thicker on the new pin box and I wanted threads to come through the nuts. I installed the new bolts with the nut on the outside to make it easier to torque the bolts to the required xxx lbs. I haven't got to try out how it helps out for chucking yet. I will add to this review later to tell how it went.


Believe the hype! This kingpin is the “kingpin of kingpins”. I went from my straight 1621 to this. The difference was astounding. It was like going from being on the SS Minnow on rough seas to the USS Nimitz on a calm bay. Very little chucking. True night and day difference. Yes, it is a little pricey, but if you consider the potential outlay of cash to the incurred maintenance of wear and tear from travel to BOTH the tow vehicle and trailer-I felt it was worth the investment. If you’re looking into upgrading-do it. It’s worth it!


Easy install. Took 2 people due to the weight of the Flex Air pin box. The Lippert 1621 is pretty light comparatively. We placed the pallets the flex air was shipped on (Estes Freight) in the back of the Dually, then labored the Flex Air on top of the pallets. Backed the truck under the 5th wheel and were able to start getting bolts through with small adjustments. Too heavy to lift and put bolts through. Tools used: 1/2 in High Torque Dewalt Impact, 15/16 impact socket, 15/16 wrench, 50-250 torque wrench, impact driver to get the breakaway removed and remount. The grade 5 bolts I have required xxx ft lbs torque. Completed install in 45 minutes.


I'm sorry I can't give a 5 star rating for this order. As always etrailer service was 5 star. But, unfortunately the pin box was received with some damage. Originally I thought the damage was limited to cosmetic only. There were some gashes in the metal and deep scratches in the paint. However, after about 5 months of use I noticed a bolt on the jaw that appeared to be misaligned. Upon inspection it was obvious that the bold was striped. It must have taken a direct hit when dropped during shipping.

The good news is LCI did replace the pin box for me under warranty. However, I had to pay for the installation which was understandable.

In retrospect I should not have accepted the shipment. I'm not sure where the packaging was done but this would have been avoided if this 230 lb product was packaged appropriately.


The reason I purchased the pin box is I needed some extra distance between my truck bed and 5th wheel. It did the trick, I got an extra 5" (had 4"). But the ride is amazing! Very pleased with towing, I almost can't tell I'm pulling a 44' toy hauler cause it's so smooth. My ordering experience was top notch, I will definitely purchase again from


Formerly I had a hitch with air bags. Changed trucks with different mount. Looked to replace the easy ride and fell in love with this king pin. Now have several thousand miles and found this king pin better than the shock hitch! I would buy it again if something happened or we changed trailers!!!!!!!! I ordered the longer one and it allows me to let my tail gate down when hitched or hitching. Wonderful.

Dennis P.


Absolutely Wonderful! After a year and over 10,000 miles I could not be happier with this hitch. Easy to hook up and unhook with safety features that allow you to easily see it is hooked properly. Over the past 25 years I have purchased 3 5th wheel hitches and by far this is the top of the line without the top of the line price. Several people have ordered because of seeing our hitch.


Right to the garage door. Thanks


Great product with quick delivery. Does not come with mounting hardware, so if your existing pin box only bolts on with a couple of bolts you will more. Good idea to just order them at same time. Also did not come with glide (lube) plate as listed in info videos.



Very happy with my pin box. Does not seem to be much difference on the truck, but the inside of the trailer tends to get shook up a lot less.


Great product for any fifth wheel! Really smoothes out the ride for both the truck and trailer. Easy bolt on install! All RVs should come with one of these!



Best thing we ever did!


Definitely a two person job, looks better attached to our momentum than the stock. Only wish the arrow was more visible.


product smooth's out ride over our great highways (cattle paths) here locally. only issue I have is that the pin box once tightened down into the original bolt holes is leaning to one side and is noticeable in the rear view mirror as I haul.

Faith E.


has trouble keeping air in the 2 rubber doughnuts. seems that it needs top up a couple times a week when travelling. have heard that others have the same issues and have upgraded the rubber doughnuts to the 20K with better success.


Really nice piece, install was a breeze other than it being really heavy!
It definitely improved the ride of my 45 foot toy hauler but it also raised the front of my unit about 3 inches (with the airbag inflated to 90 psi) and that’s after I lowered my hitch to its lowest setting.


The hitch pin box did exactly what I wanted it to do. It gave me more bed clearance and a better ride for the fifth wheel. The only drawback, is that it touches the side of the bed on sharp turns.


Packaging and delivery was spot on!! Truck backed up and dropped the new king pin right in front of my fifth wheel trailer for install. I'll follow up with more details once its installed.


Smooth out the right of the fifth wheel so well reduce chucking I will never buy another fifth wheel without this hitch


This is an awesome upgrade it takes the chucking away 100% smoother than the standard king pin, the only issue is that after setting at one location for awhile I have to top off the air?
It is definitely a great addition to the HL Landmark


Christina was wonderful - walking me through issues involved with the purchase, and following up throughout the process all the way up to arrival at my door. 5-star customer service by her and etrailer!


Works great, did install myself. Used measuring method in instructions but had to take back off and raise as high as I could get it. Makes pulling much nicer


replaced original FlexAir pinbox design with this new model. easy swap out and much improved pinbox performance. great service and delivery.


Great product. No problems after one year.
Helps a lot with the bucking on rough roads.
Highly recommend.


I haven’t got to try it yet but I’m very excited to try it everything went great ordering and shipping thanks again


Great website quick shipping and god se rvice


Made a huge difference in towing comfort in the truck.


Amazon quick delivery! Outstanding!!


Customer service was great!

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