syzygetic

I have a 2015 981 2.7 / 6SP that I've been tinkering with since, well, about 2015. The rotors were up for replacement this fall, naturally worn out, and I went for an OEM parts bin upgrade while I was at it.

On front, I have installed:

• 2x front calipers, 6 piston, +pads
  • Left 991.351.423
  • Right 991.351.424
• Tarett Engineering Brake Caliper Studs
• 2x front rotors, 350mm
  • Left 9P1.615.301
  • Right 9P1.615.302

This all works great. No issues with function, noise, clearance, and it looks way nicer than stock.

On the rear however, I installed a pair of calipers from the 996 TT:

• 2x rear calipers, 4 piston, +pads
  • Left 996.352.425
  • Right 996.352.426

... but I cannot get the 330mm rotor which is compatible with them to fit the hub. 996.352.406 comes close, but doesn't seat all the way, it encounters a block approximately 2mm from completely flush with the hub surface.

The question for the group:
Is there a 330mm rotor anyone knows about which seats well on the 981 base rear hub?

TEF

It's impressive that you've found OE parts that improve braking and less expensive than big brake kits. Just curious about the 2 mm seating problem with rear rotors. Where does the contract problem occur: is the rotor's hub opening large enough to clear the hub flange? If not that, where does it hang up? Would machining any metal on the rear rotor correct the problem?

syzygetic

I was inspired by what Suncoast does with their "Brake Conversion", and by Flat6 Motorsports' PCCB caliper swap. I didn't want 350/350, though, because aside from driving up the cost, I have heard a lot of feedback about brake balance being messed up by too much rear brake.

The 996 330mm rotor hat appears to be a little too small for the 981 base hub, just by a tiny amount. Essentially, the hat in the 996 rotor tapers to a finer cap section by about 1-2mm, and so the hub outer edge intersects the side wall of the rotor cap before bottoming out against the flat cap face. Really, it's just barely, but it isn't ok. I confirmed with four different 330mm rotors of the same part (I bought them used as a test case for a song), and it's not just one deformed rotor cap, they're all like that.

So far, though, if I can make this work, this appears to be a very inexpensive and effective upgrade. The fronts were dead easy to do, and look beautiful.

TEF

Unfortunately, you live in the DC area where things like small machine shops don't exist because of real estate values. (I know because I lived twice in Northern VA and once in Maryland). Anyway, you might consider turning the hubs and shaving off about 2 mm from outer diameter. This would not affect the integrity of the hubs and provide the room needed for the rotors to seat flush to the hubs. Almost any machine shop would have a lathe large enough to handle the hubs (vice the larger rotors).

BTW, did the larger rear calipers mount to the rear knuckles without modification or spacers? I ask the question because I recently installed Girodisc two-piece rotors and 4 mm spacers were needed as standoffs from original caliper position.

The BMW forums offer similar threads where M-like brakes are installed on lesser sedans using a variety of OE rotors.and calipers.

syzygetic

Yes, clearly you know my pain. Especially during pandemic, when this whole city has been shut down and I've worked remotely for the better part of two years, the appeal of living in a place where nothing is actually produced, but at very high cost, is waning!

This is a good point, I might do that if I can't find a decent OEM part. I do know a shop in PA which could accomplish this, but... it's a hike.

Yes, they mounted with modification, and they operate just fine. Granted, as a temporary testing measure, I am running the stock tiny rotor in rear, way too small for the pads on the 996TT calipers, but the calipers mount, fit, and operate totally fine. I wasn't up for a spacer, which is why I went to the trouble here. Just something doesn't settle well with me putting spacers on a caliper vs having a longer caliper body.

The caliper bolts for the rears are standard Porsche 85mm torx bolts, torqued to 64ft-lbs.

Jawnathin

Have you looked into the rotors used on the 2016 Spyder? The rears are 330mm and since it’s used on an 981, shouldn’t have any issues with your hub. Not sure if it’ll fit your calipers though.

syzygetic

This is an interesting idea indeed. I may take a chance on those, especially if I can find a used set. This whole adventure has been fairly inexpensive so far, especially compared to a big brake kit on the market, since there's used parts aplenty, from others doing brake upgrades, and from wrecked racecars, etc. Especially on the rotor front, cheap rotors with tons of life on them, some nearly new, are easy to find, -if- you know the part number you're looking for... that's the hard part.

Jawnathin

Agree that an OEM-plus type big brake kit is a great way to go for many. I’ve done that with some of my other cars and they were a very cost effective upgrade and easier to find parts compared to aftermarket.

I like what you’ve done but if I were doing it I’d have also looked at calipers from the upper trim 981 or 718 models. I think by getting something already offered on the same generation/platform it can help avoid some of the compatibility quirks. Im sure you’ll figure yours out and it won’t be an issue once you do.

The 981 Spyder and 718 GTS 4.0 both come with larger 6pot front and 4pot rear calipers. The GT4 uses a different design so not sure if it’ll fit other 981s. But if it did I don’t think I’d recommend it. They’re good but they are overkill for this application and the large size won’t fit many wheels.

If it helps with your search, the 981 Spyder brakes are the same as the 991.1 S/GTS. I don’t know if the part numbers for rotors are exactly the same but worth checking into since the 991.1 was made in much larger numbers and should be easier to locate.

syzygetic

I got a pair of rotors to try tonight,

• 981.352.404
• 981.352.403

They measure at 330mm dia x 28mm thick, so they should fit the rears exactly if they rest flat on the hub face. I will be sure to measure the difference between these and the 996.x part which didn't fit, since they look darn similar. Really not distinguishable visually, but my guess is that the hat internal taper is just a mm or two off. Otherwise, these are the same casting as the 996.x rears from the 996TT.

Angryfist

Syzygetic, did you find a set of rotors to fit the 996TT rear caliper yet?

I was doing some research about the same topic and am surprised this info isn't available when Suncoast has a "kit" prepared for this upgrade. I wonder what rotor they are using...?