There are several reasons why you may want to replace the window to body seals on your MGB. They may have deteriorated over the years, you may be painting the car and do not want to have dull gray old seals ruining the new job, or you may just be a glutton for punishment. What ever the reason the parts are cheap and with the following tips the job is relatively straight forward. The windshield seal is in the same league with replacing the "C" clips on the rear brake cylinders, for those that have done the job you understand what I mean, for those that have not , your day will come!
The tools needed to replace the windshield seal to body are a razor blade (to cut the seal to length), a screwdriver and a putty knife. The windshield must be removed from the car in order to replace the seal. I have not found it necessary to remove the dash in order to remove the windshield, and would suggest that you experiment with various wrenches before resorting to the dash board removal. *
Start by placing the windshield curved side up on a table at a comfortable height. If right handed start on the left side where you see the two screw holes in the bottom of the frame. Lubricate the "T" seal with liquid soap and slide as much of the seal as possible into where the two screw holes are (the area is hard to describe but you will know it when you see it). You will find that you will only be able to keep about three inches into the slot of the frame before it starts pulling out.Working with about three inches at a time place the lower portion of the "T" section of the seal into the slot of the windshield frame. While holding the bottom "T" of the seal in the frame with your hand, push on the top of the "T" section with the putty knife. Push hard, otherwise the "T" section will not go into the frame. Keep moving from left to right putting the bottom of the "T" into the frame slot then force the top of the "T" in with the putty knife. When you get about eight inches from the right end carefully cut the seal to length. Make a straight cut, like the factory cut, and not on an angle as you will be tempted to do. When you get to the very end use the screwdriver instead of the putty knife to finish the job. After the seal has been completely installed make sure that the rounded part of the rubber is fitting correctly into the grove on the windshield frame. If not push it into place with the putty knife.
Reinstalling the Windshield
Reinstalling the windshield is difficult and an extra set of hands will come in handy. What makes the job difficult is that the new seal is curled and acts like a spring, pushing the windshield off the body. Letting it sit a day or so will help compress the seal as will clamping the ends of the windshield to the body of the car. Do not put any sealer on the windshield until the job is done. If you do you will have a mess that is impossible to clean up. Wait until everything is installed then lift the seal and squirt the sealer in.
Contrary to what the shop manual says, the first thing to do is to attach the two bolts in the center of the windshield. This will provide the pressure to keep the seal from turning under itself. It helps to push out on the seal from the inside of the car with a thin object such as a wooden paint stirrer. Be careful not to scratch the paint.
The two bolts, on the right and left, that hold the windshield to the body can be extremely difficult to get in. It's necessary to push down on the top of the windshield while at the same time wiggling it to line up the bolt holes. At first it might seem as if the bolt holes do not line up with the holes in the body . If this is the case take a screwdriver and push the fiber washer inside the fender around a little. With luck you should be able to get the whole job done in about 2 1/2 hours. One half hour for the seal and two hours to reinstall the windshield. Remember it helps to let the windshield sit for a while to take some of the spring out of the new seal.
Side Vent and Roll Down Window Seals
The side window seals are straight forward. Roll down the window, drill out the old pop rivets, and install the new seals. The vent windows take about 45 minutes each from start to finish. It is not necessary to remove the side windows as stated in the manual. There are five bolts that hold the vent windows in. Two are at the top of the door right under the window, two more are reached from the outside edge of the door above the hinge. The access holes should be covered by plastic plugs. The last bolt is at the very bottom of the door. Once all the bolts are removed the vent window should pull straight up. To get it high enough to replace the seal, you will need to move the roll up window out of the way. You do not need to remove the window completely. Near the outside edge (the edge opposite the hinge) at the bottom, inside of the door, you will find a bolt that holds the window channel. Remove this bolt and slide the window over. Next take the vent window and twist it slightly, this will dislodge the roll up window (the roll up window uses the lower portion of the vent window frame as the channel which the window slides up and down in). You should now be able to raise the vent enough to remove he old seal. The new seal goes in just as the old one came out. When reinstalling the vent windows make sure to line everything so that the doors close without any stress on the vent. - Art Pfenninger
* Matt Wilson of Atlanta gives the following advice for getting the windshield bolts out without removing the dashboard (fascia) on his '76 MGB:
The lower bolt on both sides of the car can be easily removed with a wrench. To remove the top bolts without removing the facia requires a little more work. The trick on the driver side is to pop out the brake system warning light out of the dash (very easy). Simply use a socket wrench extension through the open hole and it comes right out. The passenger side is a bit harder. There is enough clearance on the back side of the facia for the screw to come out, but not enough room to get a socket wrench in there. Instead, use a 9/16 open face wrench (the extra space will help with the angle) and creatively start loosening the bolt. The further the bolt comes out, the more creative you have to get with the angle and attack of the wrench. Eventually you can finish it off with your fingers if they are small enough. And voila... no agonizing over removing the facia!
Another tip by Patrick O'Neill:
I have recently fitted a new windscreen to body seal on my MGB roadster and I found the best way of doing this is to remove one of the windscreen pillars to expose the end of the T channel. Apply plenty of lubricant by means of a small brush to the T channel and start feeding the seal into the T channel. This will take a little time to work along to the other end. When you reach the end come back to your start point and cut off any excess seal. Then pull out around 3 inches of the seal out of the channel so as to fit pillar and gain access to the pillar retaining screws. When the pillar is fixed in place use a large screwdriver with sharp edges rounded off to work in the 3 inches or so of seal you pulled out to refit the pillar.