xschop

Hey Doc, Where do the 86T spindles break at?
The sad fact is that the cast hubs have a service life. Cast alloy has a stress load rating of @17KSI.... whereas 6061-T6 has 42.5KSI couple that with the fact that I'm going to machine the area around the inner bearing seat 2.7x that of an ABS hub puts me right @ 7x the strength of the cast hub.

86 951 Driver

Make it forged. All problems go away.

86 951 Driver

BTW do these fail during street drivnig?

PorscheDoc

Most of the breaks I have seen have been right past (to the inside) the threads for outer nut. I thought I had some pictures of the various failures I have seen over the years, but must have lost that folder during my last computer crash.

And i fully agree with the service life. Just a fact of life for those hubs. They will all fail eventually.

PorscheDoc

First one I ever broke was shortly after I bought my car. All street at that point.

924turbo2

87 disc is 74mm high not 75.

xschop

Thanks, I have another Centric tech drawing for the 87T front rotor that states 75mm. I have some on the way for the mock-up. I know that the 87 NA tech drawing from Brembo states that the height is 74.25 and the Centric drawing rounds that one up to 75mm as well.

****And i fully agree with the service life. Just a fact of life for those hubs. They will all fail eventually****

I'm glad someone else recognizes the dilemma.The stress cycling of this grade of cast is what fatigues the alloy, and that is not taking into account the heat cycling that makes it worse. As for the spindles, unfortunately they are also cast but stronger steel and should be Magnafluxed regularly if used for racing/track duty.

Olli Snellman

Are you planning something like this? Gives the possibility to use late style disk on old ET cars. There is a 911 version in the picture.

xschop

Yes, exactly the idea. I will machine the O.D. to accept the 87-up turbo rotor and match the 87-up HUB offset so none of the other parts,calipers need to be changed as well. I need to ask this critical question early on if one of you Guru's know.... Are the 86T front calipers the exact dimensions/changeout to the 87-up calipers? This would confirm the spindle ear dimensions to rear rotor sweep were transfered from the 86T spindles to the 87-up Turbo (although bearings changed). You guys know your $hit

And before anyone goes spouting off about 7075 being stronger than 6061. Yes it is, but it is also more brittle and susceptible to stress cracking and it also has a higher tendency to undergo galvanic corrosion against steel (aka bearing races) than 6061.

PorscheDoc

Yes, the non S turbo calipers are the same and interchangable. The only difference was the later ones used a different piston seal system, but the mounting dimensions are the same. I run 86 calipers on 87 hubs/spindles/rotors in the front.

Olli Snellman

'86 & '87up calipers are the same what comes external dimensions.Slight difference on internal parts, which does not play any role on your plans. My friend fabricated hubs in the way which allows to use new ET disk but still have same ET as earlier, so there won't be any difficulties what comes to wheel ET.

PorscheDoc

BTW, i pulled some 86 spindles out and look at them, and the breaks that I typically see are right where the step up is from the thread area to the main spindle area. It is about 1/4-1/2" to the inside of the threads.

xschop

Thank you Pdoc and ollie!
I've been offered an 86T spindle and rotor, but looks like I will not need it to confirm the changeover. I do have the 86 hub on the way as I will CMM it just like I'm doing for the 928 hubs project. So I am very curious about the thread pitch of the 86T spindle at the lock-nut? I have the 87-up spindles on hand to confirm. I have never seen the info posted if Porsche went to a bigger thread there on the new spindles. I mic'ed the 928's at M18 x 1.0 and that spindle is the brute.

What is interesting about this scenario, is that sand-cast parts, whether steel or alloy are made weaker by machining them. I've seen way more pics of failed hubs that have the ABS rings machined on them which decreases the wall structure at their bases around the bearing seats, where they are the weakest link in the system.

PorscheDoc

Same threads between the 86t and 87t spindles. They all use the same locking nut and washer mechanism. I don't know what the threads are off hand though, lol. I literally just backdated my track car from 87 to 86 hubs/spindles for parting purposes and reused the locking nut and washer as I didn't have a spare.

xschop

I will confirm the threading on the 87 spindle. If it's the same, then you could conclude that the smaller 86T outer bearing is less of a heat-sink than that of 87-up larger outer bearing and causing greater heat cycling of the spindle..... interesting. I also want to confirm the 86T spindle dim as it relates to this LATE spindle dim of 2.75"....