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MG MGA - MGB Front Suspension on MGAs

There seems to be a lot of fuss about converting MGAs to MGB front suspension. I do not normally advocate doing this as I believe that the MGA king pin is adequately strong and have been racing with them for years. Updating to MGB is technically illegal in most racing groups in any case, although enforcement seems to be lacking with some organisations. While there have been some reported king pin failures over the years, it is best to remember that we are talking about old components that may have reached the end of their usable life, and in the case of new parts, about a certain incidence of faulty design, prone to failure. With new properly designed king pins, I would not hesitate to run and race an MGA with original suspension.

Having said that, I found myself with a problem in my current project, transplanting a GM V6 into my rebodied MGA 1500. http://www.rhodo.citymax.com/Jamaican.html

The body of the shock on the MGA sticks into the engine compartment and occupies some valuable territory required for an alternator bracket, among other things. I therefore decided that as part of my scheduled upgrade from original drum to disc brakes, I would switch to MGB front end components. You can go the route of swapping shock arms from MGB to MGA shock, or of packing the extra spacing in the upper outer joint fork if you keep original MGA shocks, but as my goal was to end up with MGB bodies, which point outward rather than inward, the simplest thing to do was to just use the whole shock.

There are only two (major) problems with using the entire MGB front end assembly – the bolt pattern of the shocks are different, and the steering arm of the MGB is located a bit inboard of the MGA one.

Problem #1 – bolt spacing – easy solution – move two holes. You will find that two diagonally opposed bolt holes line up exactly with the MGA shocks. Take a high speed air grinder with a small cutting bit and move the back-side (toward engine) hole in the direction of the other existing hole on that side of the shock. The outer (away from engine bay ) holes have to be treated differently – move the diagonally opposed hole to the one you just modified away from the other hole. So your pattern ends up with the inner holes being closer together and the outer ones further apart, and you have 2 untouched holes (diagonally opposite each other) and two slotted holes (diagonally opposite each other). The untouched holes are .375” in diameter, and the modified slotted holes measure .545” along their length – I did the first shock by hand fitting, used a micrometer on it and then did the second to suit that measurement and it just slid on perfectly. You can use the original studs but I prefer replacing them with grade 8 bolts (5/16” x 1.25” - coarse thread on MGAs, fine on MGBs) with lock and flat washers.
Bear in mind that one hole will have moved toward the shock body and you will need to make up a ‘D’ shaped flat washer to suit, by snipping the side off a regular one with shears. The modification does not take the bolt head so close to the shock body that you will have any clearance problem with a thinwall socket. I believe that there is no potential for problems with slotting the holes this way, although I have seen discussion about such concerns.

Problem #2 – shortening the steering rack in the middle to suit the relocated steering arm ends is expensive, but is the proper way to do this. Hacking a bit off each one is a pain and doesn’t solve the potential for bump steer. Or you can take the easy way out and live with any bump steer one might have introduced – fit the longer negative camber A arms that are readily available for the MGB, and the tie rods ends need not be shortened.

There is a picture at http://www.rhodo.citymax.com/f/front_suspension.pdf
Bill Spohn

Bill,
This is a bad procedure. Using two existing holes as you state, you will cause the shock and arms to be at an angle to the original position. Depending on which holes you use, the shock arm ends will be either forward or rearward of the correct position. This will cause it to either have negative or positive castor, possibly different on the two sides. Since the suspension assembly is fairly rigid in the fore/aft direction, the induced castor changes may not show up as such on a tight unit, but will induce serious binds and stresses at the trunnion pivots and kingpin threads.
When I did this years ago, I moved the centers of all 4 holes 1/8 in the appropriate directions by offset boring in a mill, then pressed in 1/2ODx3/8ID bushes. This results in the holes being centered on the shock body and in the same position as the A shock. Use regular flat washers. Seems to me that the studs/bolts are 3/8".
Cutting the requisite small amount off the tie rods makes very little difference in noticeable bump steer, and is certainly less of a problem than the castor/misalignment problem you have introduced.
FRM
FR Millmore

To correct myself: As the inner and outer pairs of bolt holes differ in center distance by 1/8, it is necessary to move the center of each hole only 1/16, not 1/8 as I stated above.
FRM
FR Millmore

You are correct about the bolt size - 3/8"

As for the misalignment of the shock, I was prepared to move the other bolt holes as well, but when I test fitted a core shock I had been playing with to determine the amount I needed to slot the holes (and that had only one pair slotted), the upper trunion slotted exactly into the fork in the shock. Bolted it down and same thing - no misalignment to either side.

I agree that what you say is theoretically correct - I'm just not sure that it makes a practical difference. I'll let you know when I get it on the alignment rack.

The misalignment will be in the order of .085" someone with a better calculator than I can tell us what theoretical caster that would give as a result.
Bill Spohn

We routinely do this at the shop.

We put MGB arms on MGA shock bodies. The MGB arms need to be fitted upside down and look a bit silly, but it allows using the stock mounting bolts on the MGA shock. I don't like seeing shock mounting holes egged out.

You can also just shim the MGA shocks to fit the upper trunion on the MGB kingpin assembly, but I'm not a fan of that either.

Either way, you lose the upper bump stop as it's incorporated into the upper trunion of the MGA kingpin.

As for bump steer, we've measured it and it isn't much of a change--I've never noticed it in driving. Sometimes we have to shorten the tie rods, sometimes we don't.

Here's a picture of the display we use to show this to customers:
http://www.eclecticmotorworks.com/mgbsusp.html

--Carl
carlheideman

Very timely thread.

I will be fitting MGB front suspension to my MGA 1500 coupe in the future, as well as doing a LHD to RHD conversion. Problem I have is lack of a RHD steering box, (or lack of the $$$ to get one).

I had a rough look at setting up RHD MGB box (which I have) New mounts don't seem to be a problem, and perhaps the rack and housing need to shortened as well, but that does not seem to be too much of a problem either. Pinion angle seems to be the same.

Any majors that I have not thought of?

Cheers
ian F
Ian Fraser

I'm confused. IWhen I acquired my 61 1600, it had been fitted with B front end (shocks and king pins). It was also in a very sad state all round. I looked at the front end and it had negative camber instead of positive per the book, and decided the PO was a bodger and ripped it all off and fitted A parts. The A and B shocks appeared to fit the studs exactly the same. The camber is now correct. Does that mean the PO used B arms on A bodies ?
Art

Bill,
I ran into the same space issues too. I slotted the four B holes 1/16" each, two inwards, two outwards and used Grade 8 bolts. I cut 1/4" from each tie rod end and chased the threads. It bolts up great and I used a set of rebuilt Lockheed calipers with Limetime warranty for $88 at the local Autozone. Use the V8 rubber bushings too.
Scott

Carl said "Either way, you lose the upper bump stop as it's incorporated into the upper trunion of the MGA kingpin." I'm wondering what is the practical result of losing the upper bump stop. with the car up on stands and the suspension in the full 'droop' down position, the steering is binding because there is no upper bump stop. in this position, the steering ball joints are at their limints of swivel, and are binding. i realize this will not happen in typical driving conditions with the weight of the engine in the car, but still i'm a little concerned about binding the steering.
Joe Beck

Bill,
For the Supercharged 1.8G motor in the '57 MGA, I took Carls advise and used the MGA shock body with the MGB upsidedown arm.

There is also the slotting of the MGB shock, which if i remember Doug Jackson at http://www.mgbmga.com has tech advice on how to install the MGB shock with slotting.

There are also tube shock conversion kits which you could install and build your own bracket for the upper shock and only use the MGA or MGB shock arm.

Then there are full coil over conversion kits for the front of the MGB which with little retrofit, should work fine with the MGA.

-BMC.
BMC Brian McCullough

My solution is a bit more radical than the above but no more expensive and really makes future spares quite cheap...I am simply getting a hot rod shop to cut off the complete cront x member and weld on a MGB one, then using full MGB suspension and a new MGB steering rack which can be bought off the shelf for about 10% of a used r/hand MGA, one that probably would have needed a rebuild anyway. The rod shop say no problem, they do this type of thing every week and assure their welds will be as strong (weld certified for State transport authorities) as well. I plan to reduce camber to about 5-6% though
regards
mark
mark

Hummm not keen on the cut off the front idea.
What's wrong with the original cross member anyway ?

I just fitted everything MGB I could and used 2 * 10mm spacers in the MGA shocks.
I had to cut about 10mm of the end of each MGA steering rack arm though, but there was still plenty of thread left to get the correct alignment.
Raced like that for years.
http://freespace.virgin.net/marka.hester/MKII_April2005.jpg

Cheers <MARK>
Mark Hester

I am not keen on 'flipping' the arms from an MGB - I made up and welded proper mounts for the MGB snubbers - very simple to do, so it now uses stock MGB items, including shocks and all you need to do is oval the holes a small amount (2 or all 4 holes, take your choice).

The pattern for the rebound mount is at http://www.mgbmga.com/tech/mga5.htm

Very easy mod to do and you are set to use all MGB stuff without concern for not having proper rebound-ability!
Bill Spohn

We have done quite a few "arm flippings" fitting B arms to A shocks. The internals of an A shock and a B shock are identical. The valves and pistons are identical. BUT, the two shocks operate backwards from each other! Makes you wonder about rebound v. compression valving, eh? Anyway, I am not a fan of slotting the bodies. We see some pretty badly worn shock cores where the holes have gone oblong and the mounting breaks. Be careful doing that. Make sure you keep the nuts or bolts tightened. The Healey guys (same shock body) generally tighten the bolts every oil change. Another concern about the rebound mounts is brake hose travel. Check the hose is long enough at full turn and full drop. Be careful. Peter C.
Peter Caldwell

By my reckoning the MGB shock will move 1/16" forward or rearward of the original MGA shock position, depending on which diagonal was used, but they will not be rotated. This will result in a castor change of approximately 0.4 degs. The sides should be mirror imaged so that the change is the same on both sides. Since MGBs have 7 deg+ castor and MGAs 4 deg+, I doubt that it will make much difference, but you would probably want to move the shocks rearward for 4.4 deg+ castor rather than 3.6 deg+.

For reasons others have pointed out, I am still not convinced this is a good thing to do.
John DeWolf

Mark, if you can't get an MBA rack (and you can't in Australia without first winning Lotto) then an MGB is your only option. Beside all suspension geometry is pure MGB then
regards
mark
mark mathiesen

How do you distinguish a B shock, from an A shock with B arms on it ?
Art

MGA shock (inboeard c=ylinders) with adjustable valve (special): http://mgaguru.com/mgtech/suspensn/fs120.htm


MGB Shocks complete (outboard cylinders): http://mgaguru.com/mgtech/suspensn/pics/fs_mgb_shocks.jpg

MGA shocks (inboard cylinders) with inverted MGB arms: http://mgaguru.com/mgtech/suspensn/pics/b_kingpins1.jpg
Barney Gaylord

I have been looking at the cataloge pix, if you use MGB uprights with spacers and a arms,why couldnt you use MGB inner tie rods connected to the MGA steering rack.
wc wolcott

Art, Barney's pictures are worth at least a 1000 words, but also, Armstrong stamped their part number on the underside of their shocks. MGB are 8177, MGA are 6172.
Peter Caldwell

This thread was discussed between 29/12/2005 and 03/01/2006

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