Bolting a spline cup to a rear wheel....how I did it and why
FWIW there are a lot of 'right' ways to substitute the standard BMW
soft rivets, but more wrong ways I suspect. The way I've documented here works, but so do many other methods. Aircraft quality high strength rivets are plenty strong in shear for example, and could be used successfully...if you have a riveter that can handle them....:) Make sure your joint is tight in shear, and you are probably good to go.
Here is what I did, and why.
Thought you'd all be interested, and others I've sent this too thought
I should post it....so here goes. I've done a lot of work on this.
First of all, the rivets are mild steel. The steel rivets used in our
wheels work under shear, what I mean by that is they stop the hub from
rotating because of the tight fit they have to the holes in the hub
and female spline. These rivets are 'cold' upset, i.e. the heads are
peened over with a cold rivet. They don't really clamp the hub to the
spline as bolts are normally used to do, or as 'hot' rivets do on say
an old bridge.
Both the holes in the spline and the holes in the hub should be almost
exactly .250 inches. I have a rivet from my drive in front of me and
it measures 0.249" for the part that was in the aluminum to 0.247 for
the part that was in the steel hub.
If you are going to bolt the hub you can get the best of both worlds;
i.e. a tight shear load fit of bolt to hole, and a good clamping
force. Most people use socket head screws and 'clamp' the 2 parts
together. The friction between the 2 parts is what stops them
slipping, not the tight fit of the sheer load.
Now, some people just use Stainless Allen bolts instead of close
tolerance ones and they claim this is foolproof. I bet it does work
fine as they are adding a clamping force to compensate for a possible
(if the shank diameter is at the small end of the tolerance) lower
sheer loading ability. If the fit is tight they would work fine.
But I figure the original BMW designer used shear loading for their
design so I know it works, and using a shear loaded fastener of equal
or better strength and adding a clamping force only makes the joint
stronger. The problem with standard fasteners is that the shank
tolerance is 0.242-0.249 usually.....a big tolerance for a sheer fit
fastener and generally not acceptable when at the low range. We want
a very tight fit of the fastener to the hole for a proper sheer
fastener. If you are lucky you may find 0.249 standard
fasteners...but I couldn't find any. So the fit was loose. I went to
MIL spec...see below.
Basically you want a 'close tolerance' bolt, or one that has an
unthreaded portion of the shank at 0.2485-0.2495 in diameter. You
don't want any of the threaded portion of the bolt in the holes. If
threads are in contact with the aluminum it will act like a saw and
machine its way through your hub....ouch.
- Use hardened washers under the nut.
- machine(I needed to use a carbide cutter in a lathe, any machine
shop can do this for you)/file the new spline hub so that the surface
where the nut is going is parallel to the mating surface of the hub.
There should be enough clearance for the nut to rotate on the bolt.
Failure to do this will cause the bolt to fatigue and loosen as the
surface is curved right now. Also the carbide cutters leave a radiused
'corner' so as to minimize stress concentrations. Don't remove any
more material than you have to.
- degrease/clean the mating surfaces, remove all oils etc.
- bolt the hubs together. You may have to ream a few holes out with a
0.25" hand reamer to get everything lined up, but I did not. I did
have to use a soft faced mallet to tap some bolts in place as they
were a tight fit. This is good and ensures a good sheer fit. The
bolts come from the outside of the hub. I had to very lightly chamfer
the hole on the outside of the aluminum hub so as to give clearance
for the radiused portion of the bolt head where it joins the shank.
Failure to do so will leave a gap between the head and hub potentially
and could cause the hub to become loose eventually. Make sure there
are no gaps.
- torque and I would also use loctite red or blue...your choice (The
nuts are self locking though). I torqued to 13ft/lbs. I would have
used 15 ft/lbs but as the threads are wet from loctite I reduced it a
- I then used a small diamond wheel in a Dremel tool to cut off the
protruding threads of the bolt so they did not interfere with the rear
Now, if your holes are elongated, all is not lost. You could most
likely move up to the next size fastener and drill and ream the holes
in the hub and splines to match. A lot cheaper than a new hub.
I had to very lightly chamfer the hole on the outside of the aluminum
hub so as to give clearance for the radiused portion of the bolt head
where it joins the shank. Failure to do so will leave a gap between
the head and hub potentially and could cause the hub to become loose
eventually. Make sure there are no gaps.
As mentioned earlier, for the bolt you need unthreaded shank in the
holes, + the thickness of a washer under the nut. The unthreaded
portion or shank length should be ~.675" + 1x the washer thickness
(I'd measure your own hub and driven spline cup to compare). You can
always stack washers as spacers if you want. Basically you want no
threads in the hub or spline cup.
I used MIL spec bolts, nuts and washers.
Here are the part numbers I used:
MS21250-04012 (12 point close tolerance bolts, cad plated)
MS20002C4 (washers, countersunk on one side as the bolts are
radiused under the heads, you will also need to grind one side so as
to fit this under the nut on the spline cup.)
MS21042-04 (self locking nuts)
All of this stuff is for 1/4-28 thread size.
I have my hub together, so if you want I can take some pics for you.
I haven't used it though, but it is properly designed and I believe
this is better than original. BMW couldn't afford to do this as these
fasteners cost me ~$3 each.
Just so you know, there are other options. Aircraft AN-174-12 will
probably work as will NAS 6204-13 for bolts. If you have an aircraft
supply place they can help you out. Check head diameters and
clearances on the hub though.....
Also, if you know anyone that manufactures aircraft, there are some
VERY strong blind rivets out there, that equal or exceed that of the
steel rivet (it's about ~2,000lbs shear I think, my data is at home).
BTW, I can back all of this up with engineering books. Carroll Smith
has a great one on fasteners for racing. I'm a techno geek so I love
books like this...(my wife thinks I'm crazy)
NOTE: ALL FASTENERS ARE NOT EQUAL. I hate using bold typeface, but
if you substitute full thread fasteners, loose fasteners, ungraded
fasteners, etc. there is a good chance that your hub will fail! Clean
the mating surfaces, torque everything properly, machine the hub
surfaces and everything will be done in a proper way. Fasteners
differ in many regards, not only in strength or corrosion resistance.