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Heater Flapper Box Replacement
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Pelican Technical Article:

Heater Flapper Box Replacement

Tom Sharpes


3-5 hours






Metric socket set, Phillips and Flathead screwdrivers, hacksaw, welder, Needle nose pliers.

Applicable Models:

Porsche 911 (1965-73)
Porsche 912 (1965-69)

Parts Required:

Heater Control Box (left and right), heater cable wire (left and right), heater cable guide tube.

Performance Gain:

Restores your heater box functionality.

Complementary Modification:

This is also a good time to check your handbrake condition.

The heater flapper boxes on the older 911's will rust out over time and need replacing.  This would not be a difficult project if it were not for the fact that the nuts are probably rusted onto the studs that attach to the frame of the car. In removing the rusted heater boxes I snapped off 5 of the 6 studs on the boxes.

I looked into the different ways to fix the studs, but with the engine in place the options are limited. I tried to remove the studs from the chassis, but they were too rusted  for that. I settled on  welding them into place by using tack welds on the exposed studs that were left. Now I guess my only worry is how to get them off in 25 years to replace them when they rust out again.

This is not the purists solution - it is the solution that works best for me.  I can not justify spending more on removing and replacing the studs than the replacement cost of the heater boxes alone.

OK, enough of the justification of my actions. I went to Pelican Parts for the heater flapper boxes, control cable and  ducting  to tackle this job. The heater boxes I received were a better design than the existing ones so I was excited to get underway with this project and find out how well they would work.

[Click on Image to Enlarge]

As you can see these boxes have seen better days. The boxes have a flapper valve inside directing the airflow that is heated as it passes over the headers through the heat exchangers and up the ducting to the valves.

In the position shown above the flapper has closed off the passage to the cabin and is venting hot air out through the holes in the valve body. With the spring in place they will fail in the closed position - no heat going to the cabin area.

Removing the passengers side box was made more difficult by the presence of the A/C hoses .
As I applied a little torque to the wrench the nuts snapped  off. So begins the process of figuring out how to get the new ones installed.

I thought I might dodge a bullet and not have ALL of the studs snap off , but that's not how life is when your working on  a 25 year old car. Five of the six studs snapped off. Once the nuts were gone I has to pry the boxes off - great gaskets.

The gaskets were holding the boxes on very tight. I had to use a pry bar to get then off.  Once I had them off I loosened the clamps around the heat exchanger and pulled the box out

After you get the box clear of all of the stuff under the car you will have to cut the wire cable because of the rusted nuts. You will be replacing the cable and barrel nuts anyway

Here is a good look at the passenger side opening leading to the cabin. The boxes seal against the frame and the tube extends into the box stopping just short of the flapper.

You can see that the studs were not in very good shape to begin with and when I applied the torque to them to loosen them it didn't take much to snap them off completely.

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Comments and Suggestions:
rriveralebron Comments: 911 targa 75
May 17, 2016
rriveralebron Comments: Hi,

how can I remove blower base.. see diagram

Thank you
May 12, 2016
  Followup from the Pelican Staff: What vehicle are you working on? - Nick at Pelican Parts  

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