Rear Disc Issues - 8 3/4” Rear Disc Conversion Kit

-
Cal Tonsley

Cal Tonsley

Well-Known Member
Joined
Mar 11, 2017
Messages
354
Reaction score
739
Location
Australia
Looking for some advice!
I purchased an MBM rear disc conversion kit from the US and am experiencing some issues with regards to the rotor fit on my axles. I am using aftermarket 1 piece large bolt pattern (4.5” PCD) axles with .500” screw in wheel studs. The axle register on these axles is 2.835” and given that the hole size on the rotors is 2.845” there is an issue in keeping the rotors concentric with the axles. Of greater concern and compounding this issue is that the rotor stud holes for BBP on these discs is approx .652” and poorly finished at that.
I’m guessing that they feature this size to suit std Mopar press in wheel studs? and that these studs must have an extended shoulder beyond the knurl which exceeds the .400” thick axle flange and then locates the disc rotor in place?
I’m wondering what options I have to correctly locate the disc rotor bearing in mind that I’ll need to have a full engineering inspection performed by our licensing authority?
  • Do I look to replace the screw in studs with some press in and if so, what part should I use bearing in mind that I’m looking to run alloy wheels so some extra length is probably needed? I looked at ARP 100-7705 and these don’t have an extended shoulder.
  • Do I look for an alternative rotor with a correctly sized register and .500” stud holes and if so, who should I contact?
  • Do I use an alternative conversion kit, least desirable given the extra cost and risk in going through a similar experience again?
Images below, thanks in advance for any help.

8D0FBBC8-9F07-4178-92CB-8FF8C2E9ACE0.jpeg


B0233515-543B-48F8-9A01-E0E802386C38.jpeg


3D0FB4E5-61D0-4425-8D57-00AE294E92E6.jpeg


281390ED-F772-4DA5-9BAF-DC18C3964A65.jpeg


DC8DD945-5460-476F-BA43-1A21D4A5B553.jpeg
 
Have you contacted the people you got this kit from?

If so what did they have to say?
 
I think I would either fab bushings that slide tightly (crimp?) over the studs and into the studholes in the rotor, or,
I would make internal threaded bushings/colars that thread onto the studs all the way down to the flange with some Loctite so the rotor at least centers itself on the studs. But I think the resulting 0.075" wallthickness of the colar will not be sufficient to make threads in.
 
Looks to me that the rotor were meat to be mounted with the shoulder pressed in wheel studs. The easiest may be looking for different rotors that have the correct bolt holes, seems like a lot of work making spacers that you now have a special application use. Just my thoughts.
 
A second thought, can the rotors be re-drilled with the correct bolt pattern?
Is there enough room?
 
Okay you're going to really think I'm out there in left field on this but hear me out.
The hole in your leaf spring pads on an a body are 1/2 inch diameter. The new pads that you buy to weld on when you change your spring location or when you shorten up a rear end have a 5/8 hole in them.
I have found that the pickup tube for the Mopar engines are half inch inside diameter and 5/8 outside diameter. So I just cut a little 3/4 in piece, stick it in the hole of the pad and Tack welded on the backside to close the whole up to half inch diameter.
So what I'm trying to say here is if you were to drill those holes to where they are 5/8 in which would only be a few more thousands rhan what they already are. You could tap those little bushings in to your rotors. Then if you wanted to you could tack weld them on the backside or just leave them pressed in.
Isn't that just a crazy idea.
 
So, I really think we're over complicating this ... the holes in the rotors will never be the same diameter as the studs that go through them. Have fun getting that rotor off ! As long as the center hole of the rotor is not super loose when it's over the center hub of the axle then it will be fine. That is what will keep the rotor spinning "true" with the axle. according to your measurements there is .010" difference between ID and OD, sounds about right. The stud holes are not a big deal, once the wheels are torqued there will be no movement there. I personally say you'll be fine. I've never installed a rotor that fit tight over the wheel studs and I do this everyday.
 
radial run out on a disk is not really important. I see the register is pretty close but your looking for a more centric mount? How about using 2 of the SBP holes as mounts for the rotor on the axle? Use a countersunk screw and tap the axle for the 2, or get a .005 centering bushing. .005 is not alot of meat, almost a piece of wire.
or there are these little guys....nylon inserts!
s-l500.jpg
 
You can...

1. Install Dorman # 610-287 press-in studs

2. Clock the rotor and re-drill the bolt pattern

3. Run it as-is

Although the center bore of those rotors is 0.005" larger than I prefer, most people choose option # 3. The stud hole size is not critical.
 
If I read that right, the customer service consists of email only?
Sorry if this appears rude, but **** that. What a shitty business model that is...It is far too easy to be ignored or put off through email. Also, sometimes a verbal conversation is the only way to express an opinion or condition. I"ve seen an increase in these impersonal type dealings and it sucks *** from the top of my crack to the hair on my sack.
(I actually have no hair on my sack)
 
DoctorDiff said:
You can...

1. Install Dorman # 610-287 press-in studs

2. Clock the rotor and re-drill the bolt pattern

3. Run it as-is

Although the center bore of those rotors is 0.005" larger than I prefer, most people choose option # 3. The stud hole size is not critical.
Click to expand...
Thanks Cass, you may remember that we were discussing hoses, I was going to buy the kit off you but I think you had to travel somewhere and I was pushed for time - wish I'd waited for you.
 
Appreciate the great advice everyone, I might have to think about it some more as I'm keen to try and get it right.
Best options that I currently like are countersunk screw through SBP holes, dorman studs or find some correctly sized discs that fit the axle register properly-still not so sure about just using as is though. Thanks again.
 
Last edited:
You must log in or register to reply here.
-

Back
Top