Chapter 9 Braking system

1 - Specification (wheels and tires)

2 - Brakes general description

3 - Brake Conversion

4 - Wheel Refinishing


1 - Specification - Wheels and tires

The 4-bolt 924 is a '4x108' meaning it has 4 lugs ( duh ) and the lugs measure 108mm
diagonally across from center to center. However, if you have many Audi
4000 quattros or VW Quantums around ( must be AWD hence Quattro/Quantum ) they are 4x108. All other (2WD) VW's and Audis are 4x100. Toyota, Nissan, Honda all pretty much use the 4x4.25" 4x108mm)  lug pattern as the 924.

From: body shop rob
Date: 15 Apr 2000
Time: 02:13:32


I've owned several 924 Porsches and I've never liked the factory wheels. The bolt pattern is a common one: 4 x 4.25 but the offset is almost impossible to find with this bolt pattern. The solution is to use 1/4 or 3/8 inch spacers with the front wheels, so the rim will clear the strut and the disc brake caliper. Also you have to purchase studs for each wheel and secure them with Loctite when installing. Any tire store will sell you spacers and studs to make the rims fit. Don't forget to have the rims retorqued after a few days of driving.

body shop rob



"928", "Trash Lid", "Flat Dish"

477601026B, ET53, 6JX16

225/50 tires work front and rear with these wheels retaining the rear 21mm spacer (pn 477 501 701). But; its tight in the front (rubs at full lock) and may be tire specific.

The 928  version: 928362  52.3  7JX16

Phone Dial, 4 bolt, 6JX15H2, ATS KBA 40391
Spiderweb, cast aluminum, 6X15, ATS, 477-601-025B. Offset 2 1/16", backspacing 5 5/8 ".
spiderweb.jpg (4636 bytes) Spiderweb, cast aluminum, ATS, 6Jx15H2
PN 477 601 031 H; this one appears to have been made 7/79. Wheel bolts M14x1.5; 70mm long, 28mm diameter.
8spoke.jpg (3566 bytes) Your basic 924 wheel; VW and the Genuine Porsche Parts logo on them. This particular wheel was manufactured 4/77. Cast Aluminum, use wheel bolts M14X1.5; 25mm long, 28mm diameter.
6Jx14H2, 477 601 031 B
C2C4wheel.jpg (9013 bytes) Ed Colicos uses Porsche C2/C4 wheels on his 1980 931. They are 7Jx16. The standard 19mm spacer is used on the rear. Tires are 225x55/16 rear and 205x55/16 front. Rubbing (or not) on the front is tire brand specific. Toyo Proxes work, Bridgestones did not.

wheelc2c4wilhem924s.jpg (20194 bytes)Installed on Joe Wilhems 924S.

Ed operates K-W Collision Center in Waterloo, Ontario, Canada, and continues to do much work with the 931. You can reach him at:  (519)-888-7711

  944-362-102-00, 6JX15 ET52.3 This one is marked '87. Any of you chemists know the significance of all these wheels marked: GKAlSi12Mg?

Wheel and Tire Size and Offset Information

 All wheels which we are positive fit and for which we have dimensions  are in this table or pictured above. To browse all the 924/944 wheels you can go to the 944faq Wheel Offsets  Also check out the Porsche wheels and pictures at Wheel Enhancements:

Original Tire Size
Front - Rear

Wheel Size
mm / inch
mm / inch
924  4 bolt
108mm diameter (4.25")
165 HR 14
185/70 HR 14

205/60 HR 15
165 HR 14
185/70 HR 14
205/60 HR 15
5 1/2 J x 14*
6 J x 14
6 J x 15 
110/4.33 33.8/1.33
924 (79 -84) 5 lug 185/70 VR 15
205/55 VR 16
185/70 VR 15
205/55 VR 16
6 J x 15
6 J x 16
924S 1986-1988
Sport/Club Sport

Special M030 Rears
205/65 VR 15
205/55 VR 16
205/65 VR 15
205/55 VR 16
205/65 VR 15
6 J x 15
6 J x 16
7 J x 15


From: Mathew Davies 
Just thought someone might want to know... 16 inch Fuchs rims fir the 924 Turbo. I have just fitted a set to my car and they look great. Take a look at



2 - Brakes - general description

There are three varieties of braking system available on the 924, 931 and the 924S.

The standard/base brakes for the 924 are easily identified by the 4-lug bolt pattern, and the use of wheel bolts.This braking system consists of solid (non-vented) front disks, and rear drums. The upgraded brakes for the 924 are part of the M-471 option, and use vented disks all around with the more common five-lug pattern (with studs and lug nuts).

The very early 931 also used the standard/base brakes as per the 924 above. But by the 80 model year the brakes with M-471 option became standard on the 931.

Finally, late 924's and 931's (924 VIN 92 CN450832 onward and 931 VIN 93 CN150332 onward) with the M-471 option may not be correctly specced from some parts suppliers with respect to the front calipers. The late 924's and 931's use a lot of early 944 components, but this was one area where they differed. The 944 used different brake biasing than the 924/931 (front/rear vs. x-split), with the result that caliper sizes are slightly different. For this reason, if ever servicing the front brake calipers, ensure that the correct parts are used, even on these late cars.

The M-471 brake system is essentially the standard braking system on the 924S (and early pre- '85.5 944) with a major exception: the 924S uses a stepped master cylinder (in common with the 944). The 924 and 931 master cylinders have the same diameters front/rear. The 924S/944 is stepped 23/19mm. So the 924S master cylinder can NOT be used on the 924 or 931 unless you also change the plumbing from a "X" split to the front/rear split used in the 924S and 944.

To convert a 4-lug car to the M-471 suspension, the simplest way is to buy a parts car with the option; failing this, get the complete front and rear suspension from a parted-out car including the brake master cylinder. It's usually easier to find a 924S or 944 as a donor than an M-471 924 or 931, due to higher production numbers and better market saturation. (The widths and offsets of the wheels from a 944 are different, and while they will fit, they will not fit under the stock fenders.) Options are to use the wheels from an M-471 924 or 931, buy aftermaket wheels to fit, or to use the 944 wheels and flare the fenders like the Carrera GT (fiberglass or carbon fiber fenders and flares to do this are available from a number of sources, such as GT Racing and AIR). The latter will, in addition allowing you to use 7" wheels and correspondingly wider tires, allow you ability to use wider Fuchs and other wheels from the early 911's, with lighter weights and widths up to 9".

However, as mentioned in the above note regarding brake biasing, the brake line routing will not be correct if parts from a 944 or 924S are used; due to the different components, the brake system must be converted from x-split to front/rear if using parts from a 944 or 924S. This means new hard lines must be bent, flared, and installed, and that the correct 944/924S brake master cylinder must be used. Again, the simplest conversion will be with the complete system from a single car to ensure the correct parts are used.

Brake circuit plumbing review: For braking systems in modern cars, federal law requires that every car have two independents braking systems. On passenger cars this is achieved by having two hydraulic circuits, each unconnected to the other and capable of operating independently should a failure/leak occur in the other. The different brake circuits used in the 924/931 vs. the 944 are, respectively, referred to as X-split and TT-split or front/rear split. They are a visual reference to the layout of the connected brake components. The X-split means that wheel-end brake components at opposite corners of the car are on the same circuit, so that the front right and the left rear share a brake circuit, and the front left and right rear share a completely independent circuit. These circuits are both operated by the same master cylinder and feed from the same reservoir, but there the commonality ends. The TT-split or front/rear split indicates that the front wheels are on one circuit, and the rears on the other.

Without going into extensive detail on brake system design, the reasons for selecting one circuit layout vs. another are intimately connected with suspension geometry and the resulting stability or instability that occurs when one circuit is disconnected. This is the reason the 944 has a different layout - the x-split would not be stable in a failure with the different geometry of the wider wheel offsets and fenders. The 924/931 has different geometry and is more inherently stable. However, a TT-split will work in a 924 or 931.

The final note that is relevant here is on brake biasing or proportioning. The front wheels must develop more brake torque (braking power, if you will) than the rears, due to weight transfer under braking, in order to prevent the rear wheels from locking first and spinning the car. This can be achieved many different ways; in modern cars it is now being handled electronically. However, in the 924's, 931's, and 944's, it's handled by hydraulic component sizing (to get the correct ratio front-to-rear). In the 924 and 931, the master cylinder has the same bore size for each circuit, and the front and rear calipers use different sizes to achieve the correct proportioning. The 944 uses a master cylinder with different bore sizes front and rear - since the fronts and rears are on distinct circuits. Therefore, for a given amount of pressure on the brake pedal, a 924/931 will develop identical pressure on all four corners, whereas the 944 will develop more pressure up front than at the rear. It then becomes clear why the brake corner components must be matched with the master cylinder when doing a brake conversion - if they are mixed, it becomes very easy to either lock a rear wheel or one of the front wheels could end up with insufficient braking ability - depending on how the system becomes plumbed.

The most important thing to remember from this - use the correct brake master cylinder from the car from which the calipers came from when converting, and ensure that the brake lines are plumbed correctly. If using a 944 donor, realize that the brake line routing must change to have both rears connected to each other and feeding from the smaller bore of the master cylinder.

From: Jim Pasha <>
Subject: 924 4-Wheel Disc brake option

The 1979 Sebring M471 option was for four-wheel disc brakes. The wheels
were 6" X 15" with 205/60 H15 tires, usually Pirelli P6s. The option
added $1400 to the price of a 924. The option was offered MY1979 to
MY1982 in the US. My 1981 924 has this option.

The same 5-bolt wheels were made standard for the MY 1981-82 US 924
Turbo, all ROW markets had these wheels as standard. 16" X 6" forged
wheels were optional in all markets with the M471 option, front and rear
swaybars and mandantory four-wheel disc brakes. The standard 924 M471
wheels were the cast spider-web type ATS cast wheels, 5-bolt versions of
the 4-bolt normally offered.

The rear wheels also used a 19mm spacer to improve handling, this
induced understeer. The 4-bolt application did not use a spacer, though
the factory offered one and recommended it to improve handling.

Jim Pasha



Must have been a long winter for someone to paint the brakes. 931 rear brakes installed on a 924. The dust shield is not installed (yet).

brakediff.jpg (11834 bytes)If you do want to convert from rear drum to disc. Also required, in addition to the obvious parts: backing plate, dust shield and the brakeline. On the left 931 disc pieces and on the right 924 drum setup. Early 944 parts work also. Notice the brakeline on the drums goes from the cylinder at the top of the backing plate whereas the brakeline runs under the axle for the disc setup.

3 - Brake conversion - Disc/Drum to Disc/Disc

spindles from John Brown's conversion strut from John Brown's conversion Sr. Webmeister changing the brakes


I swapped the brake system from an 84 944 onto first my '77.5, then my '79 924.

The rear brakes swapped in as described, by switching the backing plates, stub axles (probably not necessary), hubs, etc., plus the hard line from the caliper to the hose. The fronts I swapped in the strut assembly and upright. The balljoints of the early 924's (including my '77.5) are of a smaller shaft diameter, and will not fit the larger bore on the M471/944 upright. This can be fixed by either using the M471/944 a-arm, or just swapping in the newer balljoint onto the old a-arm - they are compatible.

One particular note, arising from some confusion on terminology. The new backing plates for the rears include the caliper mount. They replace the backing plates from the drums. However, the dust shield is a separate piece from the backing plate. Use of the dust shield is somewhat optional, and is usually left out on a track/race car. See picture below for clarification:

brakediff.jpg (11834 bytes)

The MC is the biggest PITA, as the lines have to be re-routed due to different layout of the MC hydraulic circuits. Basically, they're mirror-imaged front to rear and side to side. I'll have to take a picture or two of what I did. The reason for the change is to clear the CIS fuel distributor - with the 944 booster (yes, it does fit) the old layout would hit the metering unit. Amusingly enough, this was carried over into the 944 (from which I got my MC).

If converting a 4-bolt 924 using a proper M471 924 or 931 as a donor car, the plumbing of the brake master cylinder is correct, and all that is necessary is to plug in the appropriate lines where they fall naturally on the MC.

4 - Wheel RefinishingHi Vince!

Vince Ponzo has completed an excellent article on refinishing the 16" flat-dish forged wheels found on some 924's. Most of the article is about removing the clearcoat, which had yellowed severely on his rims (which should be bare polished, not painted). The same procedure can be used to refinish other alloy wheels, if desired. Vince used oven cleaner in his case to help strip an old finish off of the wheels, but you may not wish to do the same. Read the entire procedure, attached here in a separate page due to it's length, for more detail. In any event, Vince gives a very good description of how to polish the wheels up properly. Thanks Vince!