1960 356B brake drum inside diameter dimensions & tolerance

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Donald Pirch
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1960 356B brake drum inside diameter dimensions & tolerance

#1 Post by Donald Pirch »

In a previous discussion topic I requested a response to the problem that my brake pedal is extremely hard to depress and that the brakes don't grab as they should. I bought the car in 2004, & had no record of brake repairs. I have removed the front drums & noted that the slave cylinder exterior surfaces look like new, were dry with no leaking of brake fluid, and that the backing plate looked like it had relatively new paint with no indication of accumulation of grease, etc. So I'm assuming at this point slave cylinders are ok, & that the problem lies in the surface condition of the drum contacting brake shoes. Shoes look glazed with large areas of spider web cracks as if the shoes & drum became very hot during braking. Drum surface have continuous scratches, some relatively deep.

Question #1: Where can I find a source to tell me the expected diameter of the brake drum inside diameter, and recommended tolerances for this?
Question #2: What repair shop in the SoCal area of Huntington Beach can turn 356 Porsche brake drums? I'm talking to a VW shop who could do an inspection.

Research on line tells me that brake shoes should be 'arced' to the same radius as the newly turned brake drum. I've found online videos that show how to do this and may try doing this myself vs. having a shop arc them.

Please respond with suggestions & comments. KTF, Don Pirch, PE coupe VIN 1133341
Donald E. Pirch

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Martin Benade
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Re: 1960 356B brake drum inside diameter dimensions & tolera

#2 Post by Martin Benade »

You should make sure that the front wheel cylinders are facing the right way as it is easy to install them backwards, which will stop very poorly. I have seen brake shoes with lots of tiny cracks and a shiny surface that stopped very well, to me that is not necessarily a problem. Also get a helper to slightly press the brake pedal with one drum off at a time, while you watch to verify that none of the pistons are stuck.
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Mike Wilson
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Re: 1960 356B brake drum inside diameter dimensions & tolera

#3 Post by Mike Wilson »

Don: the dimensions should be in the workshop manual. As far as turning the drums, check with C.H. Topping in Long Beach or Johnson's alignment in Torrance.

Mike
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Richard Shilling
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Re: 1960 356B brake drum inside diameter dimensions & tolera

#4 Post by Richard Shilling »

First, it's possible the flexible brake hoses are swollen on the inside and need replacement. The hoses on my 1965C, which had a rock-hard pedal were so swollen I was surprised any fluid got through to make the car stop. Also if there is a project for a do-it-yourselfer, in my opinion, it's not brakes. Take it to a Porsche shop that's familiar with the car.

That said...

From the Porsche Spec's Booklet: 280.0 +/- 0.1
Maximum with oversized brake shoes: 281.0 +/- 0.1

Any brake shop can arc shoes to a drum. When you find one to turn the brake drums they will likely have a drum arcer as well.
Richard Shilling
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Martin Benade
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Re: 1960 356B brake drum inside diameter dimensions & tolera

#5 Post by Martin Benade »

If you are getting drums turned, you want a real expert. A standard brake drum lathe is a pretty poor piece of equipment but the right guy can make it work well. Also the first few thousandths of surface metal gets pretty hard so it is not that easy to make a very shallow cut on a drum, but a deep cut makes it out of spec. Grinding is nice but not commonly done.
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Dick Weiss
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Re: 1960 356B brake drum inside diameter dimensions & tolera

#6 Post by Dick Weiss »

Yes, grinding brake drums is not common, but I do it on my special universal tool grinder; The fronts, using wheel bearings on a centered stud for concentricity during rotation. The rear drum is fitted on a splined axle stub; Not by the drum's chamfered bore for its concentricity during rotation. While grinding the drum, I hand-roll it while lightly contacting the grinding wheel due to its 11"+diameter.

I also grind disc rotors (per above operation) and it'll take a little longer than using a lathe/cutting machine which may cut both sides
at the same time(?).

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