|The MGA With An Attitude
Installing MGB KING PINS on the MGA - FS-205
On 12/18/04, rsperbeck wrote:
"I see where people are putting MGB king pin set up on MGA because the MGA part is weak. Is this possible to put this on a Twin Cam?"
On 12/19/04, Bill Spohn wrote:
"I've just been castigating people on the other group for doing this as it isn't legal for racing and in my view it is also unnecessary - just use strong enough king pins.
If I WERE going to convert, I'd probably use Volvo 4 pot calipers, which are even better than MGB .... but as the thing out-brakes just about everything else in class already, I expect I'll just leave well enough alone."
Then Roger Andreason wrote:
"I had a king pin failure many years ago when racing. As far as I could determine the cause of the failure was that the thread had been cut too far and the end of the thread had cut into the area where the section changes causing a stress raiser. I checked a few new kingpins and found two where the thread ended correctly and never had another problem.
I suggest looking closely at the original design parts and make sure that there is no undercut at the end of the threaded section. If necessary a small amount can be machined away - it will make the component stronger even though you have removed a small amount of metal. Obviously only do this on a new part, not an old one!"
On 12/20/04, Dave B. wrote:
"Should have told BMC this so they wouldn't have made that expensive tooling change from the MGA to the MGB suspension!
But for those of you who do wish to make the change, to end up with decent steering geometry is not a slam dunk. Some folks shorten the tie rod arms (even seen this recommended by a West Coast specialist who should know better!) which really messes things up. You really need to shorten the rack - about 1 1/2"+ - and move the rack forward (the Twin Cam chassis already has done this) and down a bit as well. I made this conversion on the Devin/MGA I sold a while back and the bump steer was at least as good as the original A suspension.
You also need to choose between the A or B shock. If you use the A shock, you can't simply throw some spacers between the arms and through bolt it. You need some rigidity in the assembly or it will get wobbly. I Helicoiled one of the shock arms and put a jamb nut outside to hold the 1/2" bolt rigid and solidify the unit."
Addendum, January 2013:
Dave B. has a couple of good points there. If the MGB steering arm is at the same height as the MGA arm, then shortening the MGA tie rod to accommodate MGB steering arms (toe'd in slightly more) should be okay. If the MGB steering arms are at different height from MGA, that would make the tie rods lie at a different angle, no longer parallel with travel of the suspension arms. For that situation the steering rack should be relocated to match height of the new arms. I haven't done this myself, so I don't have a fair comparison of the parts in hand. That said, I know a lot of people have done this swap without relocating the steering rack, and I haven't heard report of any problems. The Twin Cam steering rack is located farther forward to clear the front end of the Twin Cam engine, not because it does anything to improve steering geometry. I seem to recall the steering circle is slightly larger with the Twin Cam setup.
The issue with the top trunnion pin conversion is likely more significant. In the MGA there is a steel distance tube between the shock arms so the 1/2-inch bolt can be pulled up tight making everything rigid at the outer end of the shock arms. That steel tube also serves as the inner bearing race for the bronze bushing in the swivel link. The MGB has a narrower top trunnion with rubber bushings and no distance tube. The MGB top trunnion bolt is same 1/2-inch diameter, but it is a shoulder bolt with smaller thread and nut. The nut there pulls the bolt up tight against the shoulder to make the bolt rigid. The bolt also has a keyed head (one flat side) to prevent rotation of the bolt.
Dave B. has a good serviceable solution. You can install a Helicoil in one side of the shock arm so the MGA trunnion bolt will screw into the arm
(in addition to installing a pair of 3/8-inch wide 1/2-inch shaft spacers). You also need to run the thread farther down the bolt where it will screw into the new Helicoil. You then screw the bolt in until it is snug (not tight enough to bend the shock arms), then install the nut and tighten it as a jam nut to prevent movement of the trunnion bolt. You need to keep good notes on this conversion, and keep the notes with the car forever more so it will not confuse some future mechanic.
Mark Hester sent this picture of the MGA shock arms with shaft spacers and rubber bushings from the MGB top trunnion.
Addendum February 28, 2013:
Photos below are from Peter Caldwell at World Wide Auto Parts in Madison, Wisconsin.
Peter wrote: "2 MGB kingpins. We turned one on the upper bearing diameter so it would fit snugly into the brass MGA trunion, and we installed a MGB trunion on the other. Placed both in a vise ensuring they are square to each other. Inserted the sleeve and bolt into the MGA trunion and the OE rubber sleeved bushings and bolt into the MGB trunion. As you can see, there is a difference of roughly half a bolt diameter where the MGA is farther outboard than the MGB".
"Here is a picture showing, from left to right, standard MGA shock, standard MGB shock, and MGB shock body reversed with MGB arms flipped to fit an MGA.
"BTW, inside the shocks, only 1 piston has a rubber seal (the other has a steel ring), and on an A that piston is uppermost, while on a B the piston is lower. AND, the valving is the same on both shocks. Realize that compression and rebound are opposite between the two". -- Peter C.