New Day, New Problem, Rotor attachment Options

[kconway]

Well...today I'm trying to install the rear rotors, this is on a 5 lug changeout so I'm using rotor pn 901.352.401.18, this is described as a 914-6 rotor. First question, is this the appropriate rotor for my application? I'm trying to install them on PMB 5-lug hubs. I'm hoping I have the wrong rotor part number because as you can see looking at the attached photos, the holes in the rotor and the hub to attach the rotors don't line up, not even close.

Kev

[SirAndy]

I've seen this many times with re-drilled /4 hubs ... (IMG:style_emoticons/default/dry.gif)


The good news is, you can run the rotor without the little setscrews as long as you get it close to round when the wheels are torqued down.

Pain in the behind, but doable ...
(IMG:style_emoticons/default/popcorn[1].gif) Andy

[Eric_Shea]

Indeed...

For some reason the mounting holes on the 4-lug hubs and the 911 5-lug hubs are in different locations. 944's are again different altogether. Two solutions for people using these:

1. Use two open ended lug nuts to secure the rotor in place while you set the venting clearance.

2. Drill and tap those holes you marked.

This could be done at the time of machining but, it would add an additional $40-50 to the price. People already complain about the pricing due to the additional bosses so I felt it would simply put them out of the market.

Another solution is to use 914-4 rotors that are machined to 5x130. Seeing as how the 914-6 rotors need the edge milled anyway, the cost of the 914-4 rotors plus the machining would still be less than the 914-6 rotors and the machining.

Hope that helps.

E.

[messix]

ah.... but isn't the 914-6 rear rotor a thicker rotor that will not work with the 914-4 rear calipers?

[Eric_Shea]

ah.... but isn't the 914-6 rear rotor a thicker rotor that will not work with the 914-4 rear calipers?

No. It's not a thickness issue, it's a diameter/circumfrence issue. I'd say about 80% of the 914 calipers "can" bolt up to a 914-6 rotor however, it leaves no room for thermal expansion. It is too close in my book and there should be 4mm taken off the diameter to have everything happy in Happyland.

That's why more and more I'm starting to recommend machined 914-4 rotors vs. machined 914-6 rotors for this application.

[kconway]

Eric,
do you offer the redrilled/milled -4 rotors? How hard is it to drill and tap these hubs on the car?
Kev

[Root_Werks]

I borrowed a jig from a local 914 guy years back and just drilled my 914-4 rotors myself.

The little set screws ensure the rotor lines up perfectly.

[Eric_Shea]

do you offer the redrilled/milled -4 rotors?

I can. It would just be the hourly for the machine shop.

How hard is it to drill and tap these hubs on the car?

I don't think it would be tremendously difficult. I would:

1. Attach those rotors as mentioned (with the open nuts)
2. Use a #5 drill and drill through the hubs.
3. Use a 6x1 tap and finish them off

[kconway]

Wait a minute...anyone know what these holes are for?

Whats the root cause here, are the -6 rotors just drilled differently or are the studs in the hub located a little off to hit the holes for the rotor attachment?

[Eric_Shea]

Those holes allow you to insert an M8x1.25 bolt into the holes to break the bond of a rusted rotor and hub when replacing the rotor.

[Eric_Shea]

Whats the root cause here, are the -6 rotors just drilled differently or are the studs in the hub located a little off to hit the holes for the rotor attachment?

The root cause is the difference in manufacturing of the two hubs. Hence a difference in the two rotors.

The 914-4 hub has the mounting holes as you see them on you hubs.

The 914-6 uses a 911 hub and those same holes are indexed in a different location... the location you see on your 914-6 rotor.

[veltror]

Err,I have a question, Eric I have the 5 stud hubs etc from and 914-6 discs, does this mean I need to have 4mm taken off the discs diameter?

[Eric_Shea]

Eric I have the 5 stud hubs etc from and 914-6 discs, does this mean I need to have 4mm taken off the discs diameter?

If you are using 914-4 calipers, that would be advisable. 911 or 914-6 calipers and you should be fine.

[kconway]

Whats the root cause here, are the -6 rotors just drilled differently or are the studs in the hub located a little off to hit the holes for the rotor attachment?

The root cause is the difference in manufacturing of the two hubs. Hence a difference in the two rotors.

The 914-4 hub has the mounting holes as you see them on you hubs.

The 914-6 uses a 911 hub and those same holes are indexed in a different location... the location you see on your 914-6 rotor.

Crap, not only do I need to drill and tap the hub but I also need to mill the rotor down since I'm using 914-4 calipers. How much and what do I tell the machine shop about the -6 rotors?

I wish I had known this was a possibility up front, I might have gone a different direction. I don't dare remove those hubs now. (IMG:style_emoticons/default/chair.gif)

Kev

[Eric_Shea]

I wish I had known this was a possibility up front, I might have gone a different direction. I don't dare remove those hubs now.

Did you buy the hubs from me? I "always" try to have a more thought out and thorough discussion with people buying my hubs. I searched my records and only show the Elephant stuff on the books. Those are excellent hubs for a 914 to 140hp. That said, I've actually talked more people out of buying them than I've sold. I can think of a couple V8 guys in the last month alone that I've pointed to a heavier CV solution, one that I don't even sell. Again, there's nothing wrong with the hubs but, you'll be limited to 914 CV's. What engine is in that car? Check the 5-lug thread. Now's the time to make a switch before you go deeper:

http://www.914world.com/bbs2/index.php?showtopic=105728

I have a couple sets of 911 hubs coming in if you want them. Then you'll need about $500-700 worth of CV's, axles and flanges (those are coming as well) and you'll still have a problem with these rotors if you're using the -4 calipers.

4mm off the diameter if you decide to go with these hubs or the 911 hubs. Again, it's the 914-4 caliper that will be a tight fit on those -6 rotors.

If you bought the rotors from me you can send them back. If not, sell them and buy the -4 rotors... I can take those to the machine shop and you're done. I don't think you got the rotors from me though. I usually would have talked you through all of this.

I see a simple solution. People will always be in the market for a new pair of -6 rotors. Sell them in the classifieds. Spring is a good time. Use the money to buy drilled 4 rotors.

You're almost there. It's a rotor issue.

[kconway]

Eric,
I did buy these hubs from you but it's been at least 3 maybe 4 years ago. I didn't buy the calipers or rotors from you though. By the way, my comments weren't directed towards you or your products. Maybe I've been asleep at the wheel but I never read these details on the procedure, my bad for not researching this more throughly.

That said, I drilled and tapped the new hole locations in the hubs. That was simple 15 minute job. I was a little worried about edge-to-edge hole clearance on one hole but it seems okay. I'm running a 2056 motor in this car, not a whole lot of HP. But this whole scenario got me thinking. It looks to me like that at braking there's a heck of alot of shear load imparted into those two 6mm fasteners. (IMG:style_emoticons/default/blink.gif)

The calipers do clear the rotors on the side I finished tonight but as you mentioned, there isn't alot of room to spare.

[Eric_Shea]

Glad you got it to work... you're right it really isn't that big of a job.

No worries... it's just people weighing in making (IMG:style_emoticons/default/dry.gif) faces about drilled -4 hubs and then you mentioning you would have gone a different direction... I'm just concerned it could give people the wrong impression. There is a right time to use a drilled -4 hub and your set-up is it. With your 2056 I think this is the absolute proper and most cost effective solution for you. Be it these rotors or the drilled -4 rotors this will be a solid set-up. The -4 rotors probably would have been a less expensive option and less work on your end but... that's behind you now.

I would really have those turned though... we had a member sign in here from The Netherlands (I think) a few years back that had this issue and as he drove his rear calipers locked up. We all had a hard time figuring it out but, that's what happened... thermal expansion caused that rotor to swell into his caliper. Same thing happened to one of our NW teeners. Take an afternoon and get that done right.

[kconway]

I've never used a machine shop, anything in particular I should look for or ask a machinist to decide if they're capable? I'm just taking off the diameter correct?

Kev

[MrKona]

ah.... but isn't the 914-6 rear rotor a thicker rotor that will not work with the 914-4 rear calipers?

No. It's not a thickness issue, it's a diameter/circumfrence issue. I'd say about 80% of the 914 calipers "can" bolt up to a 914-6 rotor however, it leaves no room for thermal expansion. It is too close in my book and there should be 4mm taken off the diameter to have everything happy in Happyland.

That's why more and more I'm starting to recommend machined 914-4 rotors vs. machined 914-6 rotors for this application.

Wow I'm glad I read this thread when I did. I'm also using 914-6 disks and 914 calipers on a car I haven't yet driven since doing the five lug conversion.

Eric - I'll have 4mm taken off the diameter, but my question is about thickness. I was checking Haynes and a Pelican tech article that indicates the 914-4 new rotor thickness is 9.5mm and the -6 rotor thickness is 15mm. Along with the 5-lug conversion, I installed new parking brake cables to both the driver and passenger wheels, calipers are rebuilt as well. I set the venting clearance to both calipers with the cables detached. After I reattached the cable to the passenger side caliper, I can't eliminate the brake pads from dragging a lot. I have the cable adjustment nuts at the firewall backed off as much as possible and it still seemed during installation that I could use more cable. Could this be due to the added 5.5mm of disk thickness? That sounds pretty significant. If so, how much thickness should I have removed?

[Eric_Shea]

Kevin, the only thing you'll need to check is if their lathe can handle something that wide. It is 4mm off the diameter, or 2mm off the radius. Some lathes are small and don't handle that.

When calling, make it sound like the really simple job it is.

Bryan, as mentioned before the thickness has never been a problem. That drag should go away in a few miles. I'd rather have that there than reduce the overall life of the rotor. If it concerns you them have them turned to the 914 size but if it were me, I'd drive it around the neighborhood and recheck.