Martin's Soapbox

It's a Porsche for Goodness Sake

Beware of businesses that take shortcuts to re-iron line your drums. The Porsche factory lined their drums in two ways. The first was a cross-hatch pattern and a lip as seen to the right. They then did away with the crosshatch and used a smooth turned finish on the outside of the liner still with the tiny lip to stop the liner working its way out. The Machine Shop uses a wire to simulate this tiny lip. 


Of course, the factory or factory supplier fitted the liner during the (very hot) forging process. While cooling the aluminum drum shrunk in size, securely gripping the liner. Now, when replacing a liner, the drum is machined out to a size and the liner is made to that same size plus an amount of interference fit. If there is too little interference, the liner will come loose at operating temperature. Remember that aluminum expands about 90% faster than iron. So, if you have too much interference the elastic limit of the aluminum will be exceeded. Some guy named Young had a bit to say on this. I perfected this amount of shrink by 1) calculation, and then 2) by some trial and quite a few errors on the racetrack.

The aluminum in your Porsche drums is a very closely grained structure, and has been heat treated to a hard state. Cast drums cannot offer this strength, and as such the shrink fit is not adequate, meaning those pretty Italian knock offs sold by NLA will fail when worked hard.

I tried to accommodate the tiny lip on my liners, but this meant that the drum had to be heated to a temperature (to expand enough to accept the cold iron liner) that annealed and softened the drum. The solution was the stainless wire. Half a groove is machined in the drum, and a half groove is machined in the liner. Once the drum has shrunk onto the liner, 3 foot of wire is inserted into these two half grooves, now a 1/16″ whole hole. The entry opening for this key wire is then closed off. By using this method, drums are heated to the threshold well below an annealing temperature.

Old corroded factory iron liner

Old corroded factory iron liner

 One other shop uses glue and a stud to affix their liners. (I wonder if JG stands for “Just Glued”?) Why would one depart so radically from the factory’s tried and true method? This is a cheap, nasty, and frankly cringe-worthy fix that should be avoided at all costs. Because there is glue between the iron and aluminum, there is no heat transfer, something that a performance minded owner is looking for. Also, by pinning the liner at 12 and 6 o’clock, the drum is only ever round at room temperature, see sketch to the right. This out of round will make your car have a lumpy pedal, and could lock a wheel sending your pride and joy into a ditch. 


The spline is no place to skimp, either. The factory fitted these steel spline inserts in the same manner as the crosshatched knurl used on the very early drums. I use a very course straight knurl which is cut onto the steel insert and into the aluminum drum. The insert is then heat shrunk and pressed into the drum with a 50 ton press. This method has proved itself on the race track. Other shops have installed splines with a couple of studs inserted between the spline and drum. This does not work.

To make matters worse, these inappropriately restored drums are being sold at a premium price by two well known and trusted vendors that I know of: Sierra Madre and NLA/Stoddard. Hot off the press! As of August 2015, Stoddard has dropped this suplier. I now supply to Stoddard. Just ask if the drum you are getting is warranted for life and for RACING. If the answer is no, then you better get my drums, which are.

Another issue I’ve come across is people having to grind the ends of their brake shoes to fit into aftermarket cylinder slots. If there is a fitment issue, the aftermarket cylinder slots should be ground…not the original shoes. Or buy real OE cylinders…they are worth the extra money.

As these cars because more rare and valuable, it pains me to see people take shortcuts and spend money miserly as if they were dealing with an older British car. It’s a Porsche for goodness’ sake! Get your work done properly!

-End Soapbox-

Hand-drawn diagram showing the heat expansion of aluminum is greater than that of iron.

Hand-drawn diagram showing the heat expansion of aluminum is greater than that of iron.