Collecting the bits for my project, (4 to 6) rear brake question...
Are the rear 4 rotors the same thickness as 6?
If so, can the 4 rear calipers (with their parking brake) be used?
I am doing the 5-lug.

Thanks in advance

Yes, but the rotors need to be cut down by 3 mm.

The real reason to go to /6 calipers is due to the /6 weighing more - it got bigger pistons and a slightly larger efffective radius due to the larger OD rotor. Translation - more rear brake.

The /6 pressure regulator was also calibrated differently to account for the fact that the /6 could tolerate more rear braking due to the added rear weight.

/6 calipers are pricey and hard to find but are the way to go if you can swing it.

First, read this thread. It's the best 5 lug conversion thread written. http://www.914world.com/bbs2/index.php?showtopic=105728
Basically, If you just want 5 lug on the back, have your hubs and rotors drilled for 5 lug or buy 911 solid rotors (used on the 912) and shave 2-3 mm off the diameter. Use your 914-4 calipers.

You have some options. If you want a bigger rear caliper, you can use a 911 M caliper and 911T rotors. Add the 911 e-brake with this thread.
http://www.914world.com/bbs2/index.php?showtopic=4909

Or buy 914-6 calipers, 911T rotors and bolt them on. Still need 5 lug hubs made from 914-4 hubs.

PMB Performance has all the parts and know how.

You will need new bearings with any hub removal and replacement and always change your brake rubber soft lines if you don't know when they were last replaced. Again, PMB is the source.

Or if you are not in need of the parking brake, your front calipers can move to the rear with a 6mm spacer. Works great with SC front calipers.

Thanks all for quick replies...

Have done some reading before asking, but just wasn't sure about re-using the 4-cyl calipers, which I have.

Thus far have had trailing arms powdered, new bearings, etc, and have 5-lug hubs (non converted ones).

Definitely want the parking brake feature.

Don't think I need the full-on 911-type 'drum' parking brakes.

If I can find reasonably priced 6 calipers, I'll do that. If I can't, I can use the 4 calipers with trimmed rotors.

Also doing full-911 front conversion, new lines and hoses, and 19mm M/C. 155 h.p 2.2E motor, no track use.

Thanks for all the tips.

Hope to start a build thread after making fender flare decision.

Be aware that the 911 brakes on the front have a piston area about 30% larger, so your brake bias will be off. You’ll loose some ultimate braking potential, and the fronts will wear out faster than the rears.

It’s a system, changing just the fronts throws off a number of things.

What are 5-lug hubs (non converted ones) ?
It's hard to find 6 calipers for anything under $1K. Not they aren't worth spending the money on.

Ohhh....I probably got the nomenclature wrong, I mean the splined flange that the wheel bolts to.

You can re-drill a 914-4 hub for the 5 lug pattern but unfortunately a 914-6 or 911 part won’t just bolt on.

Hoping these would work...

https://www.pelicanparts.com/More_Info/UR91...ASABEgKWuPD_BwE

They should. I didn’t realize that conversion part was being made but looks great. If you are using 914-4 calipers, it’s easy to buy solid 911 rear rotors at flaps and have your local mechanic turn them down. I bought my mechanic lunch in exchange for the service. Took him 5 minutes. Cheaper than buying 914-4 rotors and having them drilled.

IIIII...like it!

That is my hope. I thought better than drilling new holes in the 4-cyl stuff.


My local tech prefers Peroni...

You want to make sure that the holes to locate the brake disk and hub line up, if not the disk can cause an out of balance situation.

I don’t believe that’s true. The two holes hold the rotor to the hub but have nothing to do with balance. If you don’t bolt the rotor to the hub, bolting the wheel on will keep the rotor in place.

The screws center the disk to the hub, several mm play between the lugs and the disk.

Those flanges allow the 914/6 rear rotors to use both of the rotor retention screws.
I just bought a pair of flanges (rock auto as everyone else was out of stock) and a pair of rear 914/6 rotors from PMB and I was worried they would not fit.
They fit perfectly!

I'm guessing the 911/912 solid rotors are the same as the 914/6 rotors as I will have to have these shaved down on the OD to fit the 914-4 rear calipers.

But the wheel is lug centric. It’s going to make a bigger difference than the rotor.

Not having one of the screws installed doesn't throw the balance off that bad but what is does do is it turns the rotor in a non-concentric path which can cause weird ass brake judder. It doesn't take much offset to create serious judder.

This is far more noticeable on something like a Honda (front rotors) which uses these little bolts everywhere. Most late model rotors are hub centric anyway but the ones that aren't judder like a bitch if both centering screws are not present.

The Honda centering bolts are direct copies of the Porsche bolts and can be used if real Porsche hardware is not available.

Rear rotor centers from the backside on the outer edge of the hub. There is a bit of rotational play but once the wheel is torqued down nothing moves. Not an issue.

The -6 vs -4 rotor diameter is 3mm diameter, or 1.5mm radius. Someone anal made the claim that it needs machined and it's been parroted since. Plenty of clearance on the -4 caliper for a -6 rotor.

-6 rotor securing bolts (available at Lowes etc) don't line up with -4 hubs.

Compare the pad size of a 914-4 rear caliper to a 914-6 rear caliper. Not much of a difference. For me the 914-6 rear caliper is not worth the money. I used to have 911S aluminum front calipers and 914-6 rear calipers. I am now running Boxster brakes front and rear with a 911 parking brake. Much better braking for the money.

Thanks for that bit of info. Best to try fitting it FIRST before machining it.
You might have saved me some time here.

Also, good to know the rotor centers on the hub flange. I was not aware of this and this negates the need for the centering screws.

Sorry If I posted mis-information. I was going off other vehicles and not 914s for that bit of knowledge.

Chris - I wish it was true that it always fit but that wasn’t the case for mine. It locked up until I asked my local mechanic to take off a couple mm. One he did, the rotor spun fine. So now I recommend people turn it down right off the bat.

Steve - you are really buying the six calipers for the bigger pistons to match the fronts. M calipers have small pads so they don’t last as long as later, larger calipers, but it’s the pistons that are doing the stopping. Once upon a time, six calipers weren’t so much money and since they were a bolt on upgrade, the swap made sense. Boxster calipers are great but by the time you buy the adaptors, calipers, 911 ebrakes and all the hardware, it’s not an inexpensive upgrade either.

But it’s good we have options.

The /6 rear calipers have about 30% less more piston area than the /4, which results in a similar reduction increase in clamping force and braking effort. Most braking is done by the fronts, so it may not be noticeable in most situations. Porsche did a pretty good job balancing brake components, care should be taken when changing out individual bits. The 911 could be a good match for the /6 rears, and the Boxster stuff should balance appropriately since they were designed for a similar weight distribution. Big honking front calipers and stock /4 rears will be somewhat unbalanced, particularly on cold, wet pavement.

And then there’s the pressure regulator, but that’s another story.

Not sure what you’re comparing to.

914/6 has larger rear pistons - 38mm vs 33mm on /4

914/6 has more rear lining area 106 cm^2 vs 80 cm^2 for /4

Sorry, I misspoke and got that backwards. The /6 has roughly 32% more piston area in the rear than the /4 (14.4 cm sq vs 10.9 cm sq)

Yes. 914-6 rear calipers are basically an M caliper with a handbrake.

I believe the M caliper has a 48mm bore and was used on the front of the /6, rather than the 38mm bore of the stock /6 rear. A previous owner of my car had changed out the rears for M calipers, and then added an aftermarket brake 'proportioning' valve to keep it from swapping ends.

One note for those thinking about doing this. I also upgraded to 108mm CVs and stubs and also 911 ebrakes. This has been one of the more challenging jobs that I have done recently as absolutely nothing is straightforward. Be careful going overboard. For my 6 there was no real reason to upgrade rear brakes and in hindsight I don’t think I would do it again. Just would do the CVs and hubs and still use 4 cylinder calipers

I agree with Ben and admittedly haven't read all of the posts. Here is a simple solution for your consideration. I have set of front struts with hubs drilled for the favored 5 x 130 mm bolt pattern by PMB Performance. Use your stock front calipers. The struts need shock inserts but otherwise they are complete with dust guards.

I also have a set of new rear wheel hubs from URO with the correct 5 x 130 bolt pattern. They are perfect for your stock stub axles, half shafts and rear calipers.

All for less than $750.00 not including shipping and handling fees. If you are interested send me a PM with your email address for details.

Thanks in advance for your interest

Henry

Actually, rear rotors are different in thickness.

/4 rear rotors thickness: 9.4mm
/6 rear rotors thickness: 10.4mm

I didn’t realize this.
I wonder if it matters - since we can adjust venting clearance. I have 914-4 calipers with 911 solid rotors on my 3.2 conversion car. Everything works fine.

Thinking about it more, I’m sure we gain or loose more than one mm when we switch out old brake pads for new.

we can use stock six rotors with the aftermarket 914-4 five lug hub (five lug with splines that fit the four lug stub axle) and the four rear caliper, bolts right in and works just fine

Here is how I made it work

911 rear brakes - e brake was shoes pulled by lever


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Extremely bad idea.

"Rear" M-Caliper is what Mark's referring to. 914-6 calipers actually share the same nose section casting and they are basically the same 38mm piston size. A proportioning valve would not be necessary (and would only limit rear engagement) with a 48mm front and 38mm rear. I would keep a stock pressure regulator with that setup and ditch the P-Valve.

Eric, is it possible to space out a stock 914/4 rear caliper to use vented rear disks?

Correct

Yes, but "why?" is the question.

A stock 914 rear caliper will never heat up a solid rotor enough to require a vented rear. We had them made in the past but when you look into it, it just doesn't make sense.

BMW 3.0CSi rear calipers have the spacers and the fasteners required for this conversion. Hens teeth

See photo
<———-

I’m using the vented rear disk cut by 3 mm with the /4 caliper using the V kit, the spacer for the brake shield has to be placed on the outer side of the caliper instead of between the caliper and trailing arm for the correct spacing.

Good to know, I have solids and will relay this into to me brother. Easier to go back to 914/4 rears and solid disks than fab up a 911 style e-brake. I'm sure he'll write you about while waiting for his 2300i

Does anyone have experience with the URO hub for 5 bolt conversion? Just wondering about the uro part and their reputation.

Aa's going on the engine run-in stand today. Zach should be out the door...

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We probably have more experience with this than anyone. I'm in direct contact with them regarding these hubs and helped with the initial issue of pressing in studs. I'll tackle the "part" first; These hubs are a game changer for 5-lug conversion. They will still end up a little more expensive than a re-drill because of the cost of the Verbus press in studs. The only issue we've had is a box label mix-up where we had one of their 911 hubs in a box labeled with the 914 part number. This probably happened when they were actively machining the edges on an early production run (stud issue). The fact that they addressed the concern speaks volumes about your second concern... their reputation.

We've had other vendors ship us parts that had issues and they would dance around the issue making false claims of "you must have an early vs. late" (of which there was no difference with the maligned part) and claimed we were wrong and nobody else was complaining. We came to find out later that this simply wasn't true.

Bottom line, URO appears to us to be a solid vendor with sane business practices. They address issues as they arise (which they always will when remanufacturing a part for this small incestuous industry) and they understand the value of their relationship with both the end consumer and their retailers. Have they had issues in the past? Yep. Have they addressed them? Yep.

Thanks Eric for the encouraging words
Jule

If you want 914-6 rear calipers, send me a PM. I have a set that’s been restored to better than new condition by PMB.