Fitting an ATE Porsche 944 Brake Booster to an E30 BMW Part II

My ’91 BMW 318i e30 arrived with a partially completed 24 valve M50 swap. One of the problems with the 24 valve motors is that the intake manifold does not clear the factory brake booster which leaves about three common options:

  1. (Re)Drill or slot the firewall to move the factory brake booster over about 3/4″. This is not recommended as the linkage can bind.
  2. Fit an e30 325iX or e32 735i “double” brake booster (they’re basically the same) and use the appropriate master cylinder. This option is supposed to provide additional brake assist and the booster does not need to be modified. However, the booster is a good deal deeper which requires that the brake lines be bent to fit and it requires that a remote mount fluid reservoir be used so that the master cylinder will clear the throttle body.
  3. Fit a Porsche 944 brake booster and use the factory master cylinder. Of course, the booster needs to be modified to fit the e30 and the degree of modification varies depending on if the booster was made by Girling or ATE. I already covered the Girling booster here.

The previous owner of the ’91 opted for option two, and indeed an e32 brake booster & master cylinder were mounted in the engine bay.  I did not like this setup to begin with as the booster rubbed against my throttle cable & caused it to bind. Additionally, the master cylinder sat so close to the throttle body that it made it hard to work on the engine and the remote reservoir give the engine bay a clean, factory appearance in my opinion. The straw (or two ton weight as it were) that broke the camel’s back came when I fired the engine up; I had a massive vacuum leak that turned out to be the brake booster itself. Rather than replace the e32 booster with another e32 booster, I opted to buy a factory ’91 318i master cylinder and reservoir, and a “low miles” 944 booster.

E30 ATE 944 Booster

Rusty ATE Porsche 944 Brake Booster

 I was in a time crunch so I purchased this booster on an enthusiast forum rather than pull
it myself. The seller did not post pictures, but informed me that the booster held a good vacuum and actually came out of a 944 with “85k miles.” Sound good! Imagine my surprise when this rusty thing showed up, complete with a missing stud.

e30 Ate 944 booster

No matter, I’ll clean it up. First step – remove the barely attached sticker and protect the inside of the booster from debris. Next step – wire brush, then sand, all the loose rust off!

Prepped Porsche 944 booster

Prepped Porsche 944 booster

Then add a couple coats of primer. This is just cheap 97 cents-a-can primer from walmart.

944 booster wearing a fresh coat of primer

944 booster wearing a fresh coat of primer

Follow up with real paint. I used Rust-Oleum Satin Enamels Black that is supposed to help prevent rust. Truthfully, I used what was around the house but I’m very impressed with how the paint turned out!

First coat of black paint!

Second coat of black paint, still wet!

And here’s the finished product. I think it came out great! And I was even in a rush – it was getting late and rain was coming in the next day, meaning that this booster was slated to be installed before I’d have another chance to paint.

Final paint on the 944 booster

Final paint on the 944 booster

Now that the booster looked respectable, some more work had to be done before fitting it to the e30. On the ATE, the modifications are simple – extend the 10×1.5mm threads (using a die like this) and chop off the excess rod so that it matches the factory e30 booster. Also remove that bowl Porsche bolts on their boosters – it’s not needed on the e30.

Here’s the factory threads on the ATE booster. Note that the rod is smaller in diameter than the Girling version.

Factory ATE 944 booster threads

Factory ATE 944 booster threads

The threads need to be extended to near the boot.

Threading on the die

Threading on the die

I use a Craftsman die that requires a tap & die to use. If you already have the handle, you can save money buy buying this 10×1.5mm die instead. Make sure to use some sort of lubricant, like automatic transmission fluid (ATF), and run the die down the threads.

Threading the 944 booster

Threading the 944 booster

I use Vise Grips with the end in a vise to the hold the booster’s rod in place. You do not want to turn the rod on the booster a lot as it can rip the internal diaphragm and destroy the booster. After you are done threading, measure the factory e30 booster rod, transfer the measurement to the 944 booster rod, and cut the rod to length using a hacksaw or angle grinder. You are now ready to transfer over the e30 clevis & install the booster!

Installed 944 booster!

Installed 944 booster!

And there’s my installed 944 booster. Finally, I leave you with a shot of all three boosters.

e30, e32, and 944 boosters

e30, e32, and 944 boosters

From left to right – factory 1985 325e brake booster, e32 735i brake “double” brake booster, and the Porsche 944 ATE brake booster.

How to fit an ATE or Girling Porsche 944 brake booster to an e30

One of the problems with swapping in a modern 24 valve power plant into the e30 chassis is that the factory brake booster will not clear the intake manifold on BMW M5x/S5x engines. Several options are available to remedy this, including slotting or redrilling the firewall to move the factory booster over, running no booster at all, and retrofitting a dual chamber BMW booster from an e32. However, the Porsche booster is among the most attractive options because it yields reasonable brake assist with no change of binding, and it works with the existing e30 master cylinder. The 944 boosters are easy to find in your local junkyard or scrapyard making this an economical option.

The first thing you need to know about this retrofit is that 944 boosters come in two flavors – one version is produced by ATE and the second by Girling. The BMW clevis must be threaded onto a 10×1.5mm threaded rod; unfortunately the Girling rod is 12mm, meaning the thread must be ground down to approximately 10mm before threading while the ATE comes from the factory in a 10mm diameter, making the ATE booster a more attractive option. Once this minor, and I do stress minor, issue is resolved, the Porsche booster is a direct swap onto the e30 chassis. Here’s my how-to guide!

Before you get started, you’ll need a 10×1.5mm die. I got mine in a set from sears for $14.99 and it worked great. Note the sears’ die will need the wrench too which is not included with the die set – you’ll either need to buy it seperately or get the whole tap & die kit. I already had a standard tap and die set with the wrench so I opted to get the metric die set only. (Interesting side note – the old tap & die set underneath was made in the USA while my new die set was made in china )Craftsman metric die set


A die like this should do the trick if you won’t use a set.

Champion ST-10×1.5mm Speedthreader M10 by 1.5mm HSS Die Permanently Integrated In Handle With Self-Centering Guide


Here’s a freshly pulled 944 booster with the funky extension installed. It’s held on by four 13mm nuts. Remove these and save them for later.Porsche 944 brake booster extension

I measured where the jam nut was at on the e30 and transferred the measurement to the 944 booster. This lets you know how far down the rod needs to be ground and threaded.

Porsche brake booster rod with clevis


Now you’ll need to remove the 944 clevis and jam nut. I found it impossible to remove by holding the rod with a 10mm wrench. Instead, I inserted a bolt through the clevis, held the bolt with the vice, and loosened the jam nut with an adjustable wrench. [no pics]

***If you have an ATE booster, you can skip the grinding section***

After this is done, it is time to grind down the rod. A bench grinder will do fine, but I clamped an angle grinder into a vice as I thought it was easier to work with. I clamped a pair of vice grips below where I was grinding and used them to turn the shaft while grinding.Angle grinder clamped in a vice

Ground down. It turns out this doesn’t need to be perfect as the die will clean it up. Just get it near 10mm and err on the larger side; the die will cut through a little excess material.Porsche brake booster rod ground down

***ATE Boosters skip to here***

Now it’s time to thread the rod. I again used a pair of vice grips clamped below the jam nut mark. These need to be clamped very securely to prevent the rod from spinning. It will take some ingenuity to prevent the vice grips from spinning, but I clamped the end of the vice grips in the vice which worked fine. Once the rod is secured, thread the 10×1.5mm die onto the existing threads, then continue to thread the die down the rod until it goes just past the jam nut mark. run it up and down the rod a few times until it moves freely, then run it down the rod until it sits at the mark and leave it there for the next step. Threading will require some effort.

cutting threads in Porsche brake booster

The result – I took the die off for the pic, but leave it on for the next step.

Finished threads Porsche brake booster

Compared to the stock booster (on left). Notice how much longer the rod is on the 944 booster. This will need to be cut down. Measure the stock booster’s rod length and transfer it to the 944’s rod. I used masking tape as sharpie was hard to see.

Porsche and BMW e30 boosters

the bottom of the masking tape is the cut line. Cut it with an angle grinder or hacksaw. After the cut is finished, take the die off to clean up the threads.

Cutting Porsche brake booster rod to length

that looks good!

Porsche and BMW e30 brake booster comparison

it’s all ready to go back on the car! Use the four nuts you saved earlier to bolt it up. [installed pic coming]

Wow, perfect fit the very first time!!! Seriously, that’s what I found when I went under the dash. It literally could not have been any better.

Under the e30 dash

After this swap is complete, you can expect to have power brakes for your 24 valve swap. Assist is less than the factory booster provides, but still adequate to stop the car well. I can lock the brakes up on my factory non-ABS equipped ’85 in a heart beat. I’ve heard some criticism to the effect of the brakes being too hard, but I’m a small guy and don’t have a problem 😀