911 e brake conversion, 65-69 backing plates Options

[Shivers]

I'm sure you guys are sick of this subject but I am in the throws of it. I've been reading everything I could find here on the subject and it is pretty straight forward. But everywhere I read it says to use the 1970 through 1973 911 plates. Funny how those plates are near 250.00 - 300.00 now. For one. I couldn't do that. So I went to two different places and got two 1965 - 1969 e brake plates for 110.00 and that included shipping. In the picture it looks the same, maybe I would need to do a little work on it, maybe a lot but my labor is free to me.
I received one last night so at 4:00am I'm making coffee and measuring the hole size of the plate and then the bearing carrier on the trailing arm. Both were 4 1/4" so I tried to slip the plate on and it only went so far. Well it took me a second but I was trying to put a square peg in a round hole. So I dropped the step on the side of the screw bosses 1/2" - 5/8" and radiused the screw bosses. I also added a radius at the front of the boss because the plate in the hole has a radius where the carrier is to mate with the plate. Got that done and the plate slipped right on to the carrier. I checked that the plate would fit inside my disk correctly and that it would also fit right when it had a hub in place with the disk. It does.
So here is my question for you all, what kind of boogieman is going to jump out when I least expect it. So far it is smooth sailing but I really like a plan for everything, especially potential problems. Anyone know what I need to watch for? Thanks

[Jack Standz]

A few possibilities that come to mind.

You will need a "stop block" feature where the bottom end of the brake shoes push against. It needs to be fairly robust and in the right location. Some weld an approximately 45 mm(?) rectangle on the backing plate and some weld it on the trailing arm.

You also may have to reroute (rebend) the rear hard brake lines because your M calipers may have the brake line inlet in a location that prevents bleeding all the air from the brake lines when using a stock hard line.

You probably will need a spacer between your calipers and trailing arm ears (again "M" calipers and vented 911 rotors). IIRC it's a spacer around 5 mm. IIRC some rotors may be a little too large in diameter and will need machining.



On our '74 with this modification, the mounting holes in the backing plate had to be enlarged as they were slightly small and slightly off-center. Yours look ok.

[Superhawk996]

It needs to be fairly robust and in the right location. Some weld an approximately 45 mm(?) rectangle on the backing plate and some weld it on the trailing arm.

The reaction block MUST be welded to the trailing arm.

All the force from the parking brake reacts through that block. The sheet metal backing plate is not capable handling that force. The parking brake will fail if that reaction block is welded to sheetmetal.

I trust you’re already aware of the difference in offset and widths of the early backing plate and drum in hat section.
Drum Size and Dimensions:

Early (SWB - Short Wheelbase, 1965–1968): Features a parking brake drum that is larger and wider. The drum measures 180 mm in diameter and 32 mm in width.

Late (1969–1989): Utilizes a narrower drum. While the diameter remains 180 mm, the width is reduced to 25 mm. They are also positioned differently in the rotor in terms of axial offset.

If I were you I’d build up the parking brake assembly and make sure it fits the rotors and calipers you’re planning on using. With the difference in hat / parking brake offset, make sure you are going to be able to shim or space things to make this work without the early parts binding up side inside the rotor hat section due to the width difference.

[sixnotfour]

Don't forget the Bearing Lid... Not shown in parking brake section because it's a bearing retainer.. that goes on top of parking brake Hat... (IMG:style_emoticons/default/beer.gif)