Adding Tern Overland Windows

We decided on Tern Overland windows for our ProMaster’s two sliding doors.

Pros and Cons

The main positive factors that drove us to choose the Tern windows are:

  • Double pane: we plan to use this van for camping and as a ski rig, so we wanted the thermal performance of double panes
  • Integrated screen and black-out shade
  • Fully opening awing style hinge with secure latches — offers huge ventilation while keeping out the rain

The downsides of the Tern windows:

  • Acrylic, special care is needed and we expect them to scratch or haze eventually
  • They look like conversion van windows
  • They are flat, and the ProMaster side panel is curved (more about this later)

The other popular choice for ProMaster is the CR Lawrence. They are glass and look like OEM, but the opening is tiny and it seems a bit hokey in my opinion.

Forum talk

Looking at dttocs post on their install of Tern windows on the Promaster Forum gave us the confidence to give the install a shot.

We also leaned on this post for confirmation of the largest size that would fit in the door.

Based on that research we ordered 450×1100 with the 33mm-44mm bezel thickness. Later, we had some second thoughts on whether the 450×900 would have been a better choice – see below.

Installation and measuring

So on to the actual install. Using measurements from the Tern Overland measurements table we made a cardboard template so we could check whether the window would fit in the door:

Tern Overland install in a ProMaster: Cardboard template

As I was carefully measuring the door for the window, I thought that I would document my approach to measuring, in the hopes that it would be useful to other DIY installers.

In-depth measuring tips

First up, I used some wide blue masking tape to cover the area where the marks (and eventually the cuts) would be made:

Tern Overland install in a ProMaster: Taping before measurements

The ProMaster door has an inner frame that is pressed into the trapezoidal shapes that make the structural reinforcements, covered with a skin that gives the door its outside curve. I decided to use the edges of the openings in the inner frame as my reference. The idea is to carefully mark out horizontal and vertical center lines, then measure half of the total distance in each direction to give a perfectly centered hole. So the first thing is to measure the cutout of the inner frame accurately:

Tern Overland install in a ProMaster: Measuring centers

Once you have those measurements, mark the horizontal and vertical center lines, then measure left & right and up & down using 1/2 of the measurements from the Tern dimensions document to find the extents of the opening you will eventually cut:

Tern Overland install in a ProMaster: Measuring cutout extents

Next, I used thin masking tape of a different color to make straight edges for the upper & lower and left & right edges. I then found a paint can lid that had ~ 2.75″ radius and used that to mark the corner radii:

Tern Overland install in a ProMaster: Marking cut lines

I learned (the hard way, of course) that my jigsaw will curl small bits of metal on the backside (outside the van in this case) that can curl back into the paint and scratch it. So (italics) this time (/italics) we also put masking tape on the outside of the van in the cutting area to protect the paint:

Tern Overland install in a ProMaster: Outside protective masking

In other builds, I have seen them cut the center brace out at this step. I elected to just cut right through the center brace with the jigsaw, and then to remove the small bits that remain later by using an air-powered abrasive cutoff tool to carefully cut through most of the top layer, then just bend it back and forth until it breaks away:

Tern Overland install in a ProMaster: Scoring and cutting the center brace

On to the actual cutting. I used my trusty vintage Craftsman jigsaw (new ones here) with a good quality, superfine 36 TPI blade. After cutting for some distance, I would stop and put some heavy tape across the cut to help stabilize the panel to keep it from flopping around. For my marking, the cut is just inside the green tape:

Tern Overland install in a ProMaster: Cutting the opening using a jigsaw

Now that the hole is cut, we made a stiff frame by cutting and gluing three rings of 1/2″ (reused, vintage) plywood into a stiff, strong backing ring:

Tern Overland install in a ProMaster: Plywood backing frame

We then glued that ring to the inside of the door using SEM Seam Sealer (my favorite glue, by the way) and clamped the body metal against the ring, using 2×2 boards to spread the load and masking tape to protect the paint:

Tern Overland install in a ProMaster: Gluing plywood frame to body

The SEM has a strong enough cure after 24 hours (even in our cold barn) to keep the bodywork snug against the frame. Then there is the relatively straightforward job of the actual window install:

Tern Overland install in a ProMaster:  Finished install, inside view

Uh oh

This is the time where I kinda freaked out.

Backstory: On the driver side window I shimmed between the curved outer panel and flatter inner support to maintain the body panel curve, counting on the clamping force of the window frame to slightly bend the window, following along the body curve of the ProMaster.

That was a major fail — there is no way that the Tern window frame was going to conform to the curve of the van. The center of the frame completely compressed the gasket, and the upper and lower corners were so far out that the gasket did not make a good seal against the body panel. It was only with great effort that we were able to remove that first attempt to re-do it as outlined here.

Back to our story: The idea of all the clamping and gluing is to give a flat surface for the flat Tern windows to seal against. In that sense it worked perfectly — there is a flat region for the window to seal against. However, it is impossible to make a flat region in a curved piece of body metal without some flexing and bending. In this view, you can see the area around the window that is a bit depressed compared to the surrounding body metal:

Tern Overland install in a ProMaster: Deflection of body panel around the window

In the flattened area, you can see that the window frame gasket is evenly compressed, which is great:

Tern Overland install in a ProMaster: Gasket compression in flattened area

This composite image shows that there is still a slight curve in the metal where the window is mounted and compares it to the much larger curve of the original body panel:

Tern Overland install in a ProMaster: Comparing the curve of the flattened area to native body curve


After talking with Tern support, it might have been better to use the 450×900 window, this would have left more room at the right and left edges to make the transition from curved to flat over a larger area, thus perhaps making it less noticeable.

What I do not get is how dttocs’ install has no visible distortion:

In Pnieder’s post they do mention “I had to buy and use a thicker rubber gasket, as the curvature was a bit too much for the stock gasket,” so maybe that was the easier solution.


I hope that the deflection is not so noticeable once I am not looking for it. At the end of the day, what we have is a great window that is about as large as could be possibly be stuffed into a ProMaster door, the install is solid and I have no fear that it will leak.

Get a Tern Overland Discount

If you decide to use Tern Overland windows, be sure to mention “VanMargrit” for a discount on your order.

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