M725 Cowling weather-strip replacement

Yes, this has turned into way more of a project than I expected, and it ceased being fun after I was about 10 hours into it. I think I may be one of the few folks on here that completed this job, so I took some pics and did a little write-up for anyone else that wants to replace their worn out M725 cowling weather-strip. I found that the windshield and cowling parts of the M725 are very different than the M715. They share very few parts except the windshield glass. The M725 specific weather-strip is not being reproduced and its physically different than the M715 weather-strip. That said, the reproduction M715 weather-strip is likely the closest you are going to find to the factory M725 weather-strip and it can be modified to fit.

To start with, some before and after pictures of the cowling area. . . . .

Cowling gasket removal:
The original 50-year old cowling gasket was cracked and brittle and broke like dry twigs when I moved it. Also, the PO had covered it in tub and tile silicone caulk to hold it in there. I removed it from below the windshield frame by pulling it out by hand and gently breaking it up the more tightly wedged-in ends with a small flat blade screw-driver and carefully prying the pieces out. During this task, I saw that the left and right upper sheet metal transitions function to trap/hold the ends of the weather-strip in place. To install the new weather-strip correctly, I needed to remove the left and right upper sheet metal transitions first.

Rivet removal:
The factory rivets wanted to spin when I drilled on the mushroom cap so I first used a 1/8” wide abrasive slicer disk in my 4-1/2” angle grinder to carefully cut away most of the factory aluminum mushroom caps- or enough of the cap that you can visualize the rivet mandrel hole in the pop rivet. Then I carefully drilled through the center of the rivet with a 3/16” drill bit.
Once the transitions are off, you can remove the remaining scraps of the cowling weather-strip trapped behind them

Rivet sources:
I did some research with pop rivet manufacturers and found that the pop rivets used by the Kaiser factory were relatively common 3/16” diameter all aluminum rivets. What’s less common was the aluminum mushroom cap they pressed into the rivet mandrel hole to make it look like a more finished like a solid bucked rivet. The specs for the mushroom caps made by Hanson Rivet in California are a perfect match for the ones used by the Kaiser factory. Hanson’s part number is PMPA6-1. Minimum order is 100. I also bought 100 all aluminum 3/16 diameter pop rivets with 1/8”-1/4” grip range. The staff at Hansen said that the mushroom caps will fit most but not all 3/16 aluminum standard pop rivets, but if I bought both parts from Hanson, the parts would be a perfect fit. I think it was like $25 for both items shipped to Portland.

Sheet metal transition repair:
The M725’s I’ve seen with some miles on them have had stress cracks in the lowest “valley” part of the transitions. Mine did too. I carefully stripped coatings and the heavy galvanized coating from this area and TIG welded the cracks up- using a 90 degree bent copper backer strip and I went easy on the heat. I blended the finished weld slightly.
I then removed rust and old failing paint and used phosphor rust converter on the rust I found on the cowling sheet metal and windshield frame under the transitions. Then I primed/painted both sides of the transitions and the area they covered on the body.

Cowling weather-strip modification and installation:
Since the clearance at each end is significantly tighter than in the middle of the windshield frame, the weather-strip trimming/sanding described below will help the ends to fit. Also, the height of the gap between the windshield frame and cowling may be different from M725 to M725, so your target height may be different than mine:

Use a dynafile to sand down the four glued-on rubber blocks on the ends of the weather-strip. Make the blocks 1/16” thick.

Also with the dynafile, sand down some/most of the rubber ribs from the top of the weather-strip on the last 3” of each end as seen in the pics. I also had to sand down a couple of the ribs on the under side of the ends as well.

Note: The three threaded windshield frame to dash studs are welded into the windshield frame, and have a flat base that extends below the windshield frame into this weather-strip area. These two stud bases on the ends are an additional impediment to inserting the weather-strip ends.

Cowling weather-strip modification and installation, continued:

Use aerosol window cleaner to lube up the weather-strip area and insert the ends to see if they will slide in with some persuasion. The ends of the weather-strip are a tight fit -so it took some wrestling to get the rubber to fit into this space. If that's all looking good, line up the weather-strip left to right on the defroster vent holes and start forcing the weather strip into the space with your fingers and gentle tapping with a mallet and smooth wood blocks working from the outside of the vehicle. I installed the weather-stripping without sealant and will evaluate if it needs a back-up bead of sealant once it gets a good dose of Portland rain.

Note: Before I installed the weather strip, I tried removing several other fasteners (the four rear most front fender bolts and the four bolts between each A pillar and the firewall cowling) to create a bigger gap to force the weather-strip into, but I’m not convinced that removal of any of these fasteners actually increased the gap to make installation easier. I also tried to flex the truck frame mid span with a floor jack with the same goal, and that seemed to create a small amount of additional space to shove the weather-strip into.

Prepare to reinstall sheet metal transitions:
Once the weather-strip is installed, you can line up the transition rivet holes with the rivet holes the windshield frame and mark where the transition overlaps the weather-strip. The weather-strip trimming you are going to do next will allow the sheet metal transitions to lay flat against the windshield frame. I fabbed up a custom cutter out of a razor blade and scrap metal to help make this cut straight and vertical. I smoothed this cut area of the weather-strip with the dynafile again.
Test fit the transitions to make sure they will fit- the part of the sheet metal transition that lays over the now trimmed weatherstrip should trap and compress/squeeze the trimmed weatherstrip ends and should come close to touching the windshield frame.

Reinstall sheet metal transitions:
Install a 3/16”-¼” bead of seam sealer on the back-side of the transitions adjacent to the line made by the rivet holes.
Place the transitions on the body and place a pop rivet in each hole. Working quickly before the seam sealer sets-up, work your way around the transition pushing the sheet metal flat while installing each rivet with your pop rivet gun.
Tool the seam sealer that bulges out from under the edges so there is tooled seam sealer around all edges except the small section that overlaps with the top of the lower longer transition. I see now that I could have done a better job tooling the seam sealer.

You are ready to install the mushroom caps. The aluminum mushroom cap tail is a little too long so I trimmed all of mine with side cutter dykes to about where the taper starts. I think you could install the caps without trimming, but I found that the longer tail will drive the popped-off end of the aluminum mandrel back out of the finished rivet, which might weaken the riveted connection somewhat. I used a small brass mallet to drive the mushroom cap down into the rivet mandrel holes.

You can paint over the shiny aluminum rivet caps and seam sealer if you want and that’s about it - it’s all set to go for another 50 years.

Thank you for posting, good job.

Great job!:)

Nice job, thanks for the write up.

Looks like quite the project, I'm glad mine's still in decent shape!

This is AMAZING!!! I want to make a tech page for the manuals section out of this if you dont mind....that is a great job on the work and the writeup.
THANK YOU Sir!!

That sounds great Jon. Happy to help out.
Bryce