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I ordered some Metalastik Spitfire bushings for my upper A-arms, as described in the Champion book.

The dimensions are:
length outside to outside 1.433" / 36.5mm
outside diameter: .829" / 21mm
bolt hole: .379 (.375+.004")

note: these were New Old Stock bushings. Many of the reproductions use 10mm ID tubing, .394". While these apparently work okay, it's a very sloppy fit and you should probably drill your brackets to use 10mm bolts.

I wanted bolts with the threads (grip length) outside the load-bearing area, which meant AN bolts with the proper grip length. AN bolt sizing is somewhat strange; allowing for bracket thickness and washers, it looks like AN6-22 is the correct size. I'll find out when my order arrives and report back.

I'm using a Kinetic-style lower control arm; a perpendicular arm and a drag link. Since I'm putting my caster adjustment on the bottom arm I wanted a bushing that would take more angularity than the Spitfire bushings. After much searching I decided on an inner control arm bushing listed for an Audi TT, New Beetle, and a host of other VAG cars going back decades. The bushing is relieved on the ends to allow the control arm to run at an angle if necessary.

Dimensions are:
outside width: 2.123" / 54mm
width of the control arm loop pocket: 1.1" / 28mm
diameter for control arm loop: 1.28" / 33mm
bolt hole .482" / 12.25mm (12+.25mm)

The bolt for that should be a 12mm bolt 75mm long with a 15mm grip length. That is, of course, an oddball size. The obvious choice is to use a New Beetle or TT lower control arm bolt, except from the pictures I've found on the 'net VAG chose to start the threads midway through the bushing and cross the shear plane. While this could be condemned as a design failure, probably half of the control arm bolts I've ever taken out of various cars have done the same thing. On general principles I'd like to avoid having threads through the shear plane, so I looked for an ordinary metric bolt with a long enough shank and cut off the excess threaded section... except online sellers are remarkably chary about telling you how long the threaded section is. Tomorrow I'm going to hit the hardware store and see if they have anything that can be cut down to work.

If your bushings have a steel tube in the centre which fits snugly withing your bracket (which it should) then you can make a proper clamped joint. In a well designed joint the bolt only provides a clamping force and should never actually see any of the shear load because it's taken up by friction between the bracket and bushing inner tube. That means that if you have a properly clamped joint then having a threaded portion of the bold withing the load bearing area is actually not a problem and bolt shank length becomes less critical. Some high tension bolts will actually have a shank smaller in diameter than the threads.

this is all a matter of load over cross sectional area, if you establish the shear in a fully threaded bolt of the thickness you intend to use, then any area of the bolt that is shank will be higher so if you work on the threaded number you are good to go, or just use a bigger bolt.

being as precise as you are to me is bad engineering from a practicality point of view just like most of Colin Chapman designs, paper fireproof bulkheads and all, using one bolt to do three jobs and making that bolt as small as you can just means that three things will fail when a stress rises above the design limit, ask Jim Clark, oh! you can't.

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OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

To add to what I was saying in my previous post, this may be interesting reading for some of you:
http://www.boltscience.com/pages/basics1.htm

A bolt should only act as a clamping element in a properly designed joint, and not as a locating element unless the joint is specifically designed that way. In order to use a bolt as a locating element, the bolt should be a shoulder bolt with a precisely dimensioned shank, and the hole has to be precisely drilled to be a tight fit (class 3) or transitional fit (class 4). With a standard drill bit what and regular SAE or metric automotive bolts what you usually have is a clearance hole with a loose fit.

For example:
Say you are using 1/2" grade 8 bolts to hold your suspension arms. You drill a clearance hole with a diameter of 0.500" - 0.505", a small tolerance of 5 thou. A grade 8 bolt has a shank dimension of 0.500" to 0.493". This means you could end up with a clearance of 0.012" between the bolt and the hole. If you are seeing reversing loads (as you do on suspension arms) then you can't rely on that as a locating element because the bolt would be banging back and forth within that hole. While the shear strength of the bolt (~17000 lb) would be enough to withstand suspension loads, the impact forces would eventually destroy the hole. So what's to be done? Use the bolt as a clamping element only. The clamping force of a properly torqued 1/2-20 gr 8 bolt is around 14000 lbs. With a friction coefficient of 0.20 between dry steel parts, your joint would take 2800 lb force before the jointed parts slid on each other an allowed the bolt to see any shear force at all.

I found and bought some "12x100mm" bolts that have about the right thread; I might have to scrounge a thicker washer for one side. There's an inch of thread sticking out past the nut that I'll have to cut off.

3/8" AN bolts and 3/8" bracket holes worked for me, although only 15K miles so it might be too early to say for sure. I originally got some of those 10mm bushings, but didn't like the loose fit. They're actually sold as replacement bushings for Spitfires, so I guess they'll work with a 3/8" bolt.

This is the Audi "front control arm bushing."

Posthumane, thanks for the nice explanation and also pointer!

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as post humane eluded to,it is important to note that the shanks of (example) grade 8 1/2" bolts are not necessarily 0.500" The fine thread is a lot closer than course, though. I have measured course shanks as small as .490. The fine are usually .498-.499. I suspect this is due to the course thread having a smaller minor diameter, so smaller stock is used during the manufacturing process(?) This is also important when using rod-ends.

Another useful link for a summary of bolt specs and selection is the Fastenal Technical Reference Guide:

http://www.fastenal.com/content/documen ... eGuide.pdf

The 12mm bolts I bought are almost .020" smaller than the ID of the bushings.

I'm half-tempted to make some sleeves for smaller bolts with a tighter fit, but I think I'll just see how things work out.

TRX, is your br@cket made to fit the bushing inner sleeve fairly closely, length-wise? If so then the clamping force will keep it from moving and the smaller shank of the bolt won't be a problem. If you want to have the bolt locate it perfectly with very little clearance then it would be quite difficult to assemble. Just make sure you torque the bolt to spec.

Out of curiosity, how do the bushings fit into your control arms? Is there anything to keep it from moving axially?

The fit should be tight in the brackets. I have plenty of clamping force available and no threads across the joint; I'm sure it'll be fine, but I'm just a bit irked by the sloppy fit.

The big rubber ridges are support to keep the loop on the arm from sliding around. That's how it works on VW and Audi cars, anyway. Since it's a drag link style lower arm, any axial forces on the bushing will also be very small.

The AN bolts came in today. The AN6-22s are the perfect length; the shanks will go through the bracket and a flat washer on each side, with the threads going slightly into the washer on the nut side.

The Grumman bolts are .373". The bushing IDs are .379". Clearance is .006".