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MG MGB Technical MGA Axle on an MGB? BBS discussion at MG-Cars.net
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MG MGB Technical - MGA Axle on an MGB?
| Will an MGA banjo rear axle interchange with that of an MGB? Are the lengths the same? I'm considering this swap so that I can have MG-authentic disk rear brakes. |
| Steve S. |
| Steve, I believe that they will, At least I have gone the other way around, MGB banjo axle into an MGA. There maybe some differances that I don't know of with the later MKII or Twin Cam type (rear disc brakes) I used the plain drum style. Rear end ratios are most likely different, and the knockoff threads and splines might be different as well. I do believe that very late MGA axles were the same splines and threads however. HTH Ron |
| Ron Smith |
| Ron- I'm aware that the eary (pre-February 1964) MGBs had fine threads on their hubs (12 threads per inch), and it's likely that the late MGAs used the same. MGA wire wheels will fit on the MGB, so the splines are the same. I'd be fitting a Quaife Engineering torque-biasing limited slip differential, so the drive ratio wouldn't be a problem. The real question is: are the lengths and other critical mounting details the same? Is it a bolt-in swap? |
| Steve S. |
| Steve, The lenght of the rear end itself measured from flange to flange (end of the tube to tube) is the same. The rear spring mounts are in the same position as well as the rebound strap mounts. As to what type of hubs are on the Twin Cam or MKII Deluxe axle I am not sure. My measurement was from in front of where the rear drum backing plates bolt to the rear end housing. The later MGA axles are the same as the MGB, fine spline, coarse thread. The hubs are somewhat different, the splines are the same, but the shoulder and lenght of the splines are different. I have both types of axles on the ground if you need any other measurements. Saftey Fast, Ron |
| Ron Smith |
| What about the final drive ratio? The MGB had a 3.909:1 ratio. I was under the impression that the MGA had a higher numerical ratio, such as 4.something:1. This is not really a problem, if its true, but it will cause your engine to rev higher than it would with a stock axle and it will throw your speedometer off. |
| Paul Noble |
| Ron- Thanks. The width is the critical dimension that I was looking for. At present I'm using the Salisbury tube-type) rear axle, so I'm already aware that I'll have to change to the earlier driveshaft, spring locating plates, damper link mounting brackets, pads, and U-Bolts. After consulting Clausager's "Original MGA", I've discovered that the Knock-on threads are coarser on the Twin Cam and Deluxe models, so a little research is necessary to find out if they're the same as what I have now. I know that MGB and MGA wire wheels are interchangeable for fit, but I wonder if the difference in the length of the splines that you mentioned is critical? Paul- Quaife America shows the same Part # (74.309.150) for their automatic torque biasing limited slip differential for both the MGA and The early MGB (Hardy-Spicer Banjo type) rear axles, so both that and the halfshaft issue is resolved. I wouldn't use an old crownwheel and pinion set with an uprated 18V engine, I'd use a new set for an MGB (3.909:1). |
| Steve S. |
| I don't have any MGA axles handy, but I'd bet that an A and B hub both have the same bolt spacing and "inside axle securing surfaces" (I should write technical manuals, ya think?) so you could just install a new B hub if need be. Anyone else know? Also, Steve, have you thought about finding a 3.7:1 gearset, if they exist for banjos? With the extra power your car may be able to work with it fairly well and you could drop a few rpms on the highway. BTW, I've seen legions of people write about how a B's rear brakes are huge and disk rear brakes are a waste unless you're running a V-8 or a 4 thats putting out substantially more than a roadgoing 2liter will do. I personally have a real hard time believing that, short of locking up the wheels and causing spinouts whenever you brake, brakes can be too good. Whatcha up to, Steve? Or is this just a fun winter tinkering project? Have fun with it, Wade |
| Wade Keene |
| Steve, though not too sure about the A, the B used both axle styles, the later housing being the stronger one, but there were a number of differences aside from the housing and I think the early B housing was also the one used on the A. Even though the later axle was a semi-floating while the early one was a three quarter floating axle and axle shafts would not interchange between them, both of them were pretty easy to change from wire to steel disk wheel and back, and had similar mounts for the brake backing plates. This leads me to think that the MGA rear discs and caliper brackets might bolt right up to either type axle. The later axles besides having a stronger housing, had a stronger axle shaft due to the way the hub was fixed to the outer end. Might want to see if you can use the newer one. I'd look at the spacing from the caliper mount/backing plate mount to the disc mount surface. I've not noticed any difference there, however I have noticed two types of brake drums one a bit concave and the other a bit convex. I think that was from whether they were for wire or disk wheels but could be mistaken. Jim |
| Jim Blackwood |
| Wade- Nope, this is a very real possibility in a dead serious project. For months I've been intending to replace my heavy Salisbury tube-type rear axle with a Hardy-Spicer banjo rear axle to save 75 lbs of worse-than-useless unsprung weight. This would normally mean that I'll need to find new rear brake shoes lined with the Carbon Kevlar material to go with the earlier drum brakes and match up with the same material that I'm using on the front disks, and I don't think that they're out there. So, disk rear brakes seem a logical move. I like to go driving on certain winding mountain roads in nearby West Virginia and the brakes do get quite a workout. Standard brake lining material heats up and fades under those circumstances. With disk brakes this is less of a problem, so a disk brake rear axle from an mga twin cam seems to fill the bill. With this approach, I can kill two birds with one stone and build the car that BMC wouldn't let MG build, even though they could've and would've if only the parent corporation would've let them! |
| Steve S. |
| Not sure if the dimisions are critical, but as I recall (don't have the axle shafts loose at the moment) the A hubs were about 1/2" shorter in spline lenght, and longer in the shoulder area. This may make sense as the A wire wheels are narrower than a B's. I have late MKII axles in my B rear end, and the splines matched the B differental and the knock offs are coarse thread, which caused some trouble as I had to find MG crested two eared knockoffs in 8tpi instead of the A 12tpi knockoffs. |
| Ron Smith |
| Steve, Actually, I've been thinking a while about installing disc brakes on the rear wheels of my B too, but wouldn't there be an easier way to do that? Greetings, Alexander |
| Alexander M |
| Ron- Are you saying that the hubs of the wheels themselves are narrower? Alexander- You could use another disk brake system, but it would require the services of a machine shop to assist in the design and fabrication of custom brackets to mount the calipers, plus you would have to have the shop modify a set of rotors and quite possibly the rear axle itself. This would be an expen$ive approach. Would this be "easier"? I think that my approach is the easier one, and in the end the car has the plus of remaining "MG", i.e., making use of MG components. As I said in my previous posting, I want to build the car into what MG could've and would've built, if only the Parent corporation would've let them. The MGA Deluxe sold out fast, and a parallel MGB model would've certainly done the same. Being more expensive and appealing to a more performance-oriented customer, I think that such a model would've created more sales in that market sector than it would've stolen from standard MGB sales. In fact, I suspect that the resultant enhancement of the sporting image probably would've spurred overall sales of the standard model. Unfortunately, the Big Boss Leonard Lord was always a businessman first and had little sporting vision where MG was concerned, and his sucessor Lord Stokes' vision was that of The Grave for MG. Good thing he failed (sort of). |
| Steve S. |
| I thought of something else that may be bad news; hopefully not. With an A having 15" wheels might you have clearance issues, or are you already running 15" wheels instead of 14s? |
| Wade |
| Wade- Yup, Dayton wire spoke 15 inchers with P 195/60 tires. If the installed width is the same, I'll be OK with the clearance problem because I've already got both a Panhard rod and a rear stabilizer bar (I love winding mountain roads!) |
| Steve S. |
| Steve,the MGA rear disc brake caliper is the same unit used on a Jag E-Type.I am building a B with this rear set-up so I have a few calipers handy.Have you ever weighed one of these set-ups?They're heavy.They are big cast iron units with the parking brake caliper attached.If you are concerned about unsprung weight I would suggest looking somewhere else.Besides putting those calipers on the back with stock fronts will throw off the brake balence without an adjustable balence valve.I'm not sure how complicated you want to get,but I know of a few racers that get their shoes relined in racing compound(kevlar,carbon/kevlar,etc..) and that sounds alot easier than converting to a totally different back axle just to minimally improve the rear brakes.I don't know how wide your rear tires are,but the B is so light in the rear that putting big brakes in the rear will do nothing but lock the back end up.Then you will dial down the balence to what was there originally.At least that is my experience anyway.If you really want rear discs,then there are a few outfits that use modern caliper/paking brake assemblies or lightweight alluminum units in a ready made kit. |
| Dave |
| Dave- Gottcha! You didn't read this thread before you posted. The problems are unsprung weight and brake fade. I wish at this point to correct what I said in my earlier posting: the Hardy Spicer banjo axle is 60lbs lighter than the Salisbury tube-type axle, not 75 lbs. I'm aware that the disk rear brakes are heavier than the drum units and will reduce the savings in unsprung weight accomplished by switching to the banjo axle. I also know that I'll have to install a proportioning valve to prevent premature lock-up of the rear brakes and that the more modern aluminum calipers available today would be lighter. But they wouldn't be "MG" and that's important to me. I'm out to show what could've bene done by MG using MG parts. My car is a true-blue MG, not a Hot Rod. No Pontiac Fiero seats, Ford transmissions, Chevy engines, or Toyota brakes need apply. I appreciate your efforts, though, and take them in the spirit in which they were given. If I wanted a racer for the track your ideas would be spot-on. Thanks. |
| Steve S. |
| Steve, Jag/MGA Twin Cams will fit using an MGA or early MGB axle tube but only with some hub and disc machining and your limited to mags or bolt ons. How do i know? I've done it on my V8 MGA project. Can send you photos if your interested. By the way Mk II Jag calipers are the same part number as Twin Cam and the caliper brackets fit right up to the MG axle tube. regards mark |
| mark mathiesen |
| Dave, Out of curiosity, could you tell more about the ready made rear disk kits? Thanks |
| Wade |
| Mark- Yes, to mount the disk brakes of an MGA Twin Cam with a run-of-the-mill MGA rear axle would require modification of the axle to Twin Cam axle specifications. However, the inabilitiy of the Twin Cam rear axle to take wire wheels is news to me. Clausager's "Original MGA" offers no direct information, other than "The final type of wheel that may be found on an MGA is the center-lock disk wheel, also made by Dunlop, used for the Twin Cam and De Luxe models.... Neither the wheel hubs nor the knock-ons of the Twin Cam and De Luxe models were interchangable with those of an ordinary MGA of any model." On every page where there is a photograph of a Twin Cam or De Luxe model they invariably show the same disk wheel secured by a knock-on. However, there's no information one way or the other mentioning the subject of wire wheels on these models. So, this brings up the question of the possibility of mounting a wire wheel hub onto the half shaft of a Twin Cam. Would the BMC "parts bin" approach work? The knock on threads are different and Clausager says that the hubs aren't interchangable, but not the actual reason why. How I wish that I had an MGA Service Parts List Manual! Hmmm....... |
| Steve S. |
| Steve S., As the brakes on the M.G. are Lockheed, not M.G., you should be able to use any Lockheed brakes of the period and still remain true to your objective. You could save some additional weight by using the early Porsche 911 aluminum calipers as supplied by Lockheed, no? |
| George B. |
| George- Are the Porsche 911 aluminum calipers that you mention interchangable with those of the Lockheed models used on the front of the MGB? Reducing unsprung weight in the front suspension would be a great idea! I know that both the front and rear disk brakes on the MGA Twin Cam and De Luxe models were made by Dunlop. |
| Steve S. |
| Steve, Werner van der Clap in Lynchburg is running 911 eleven inch vented disks on his conversion, but he went with after market 4 pot calipers. The 911 has an aluminum hub which mounts directly on the MGB spindle for added weight savings, that's the good part. The bad part is that 911's are 5 lug wheels and I don't know of a good way to adapt the hubs to four lug without machining off the wheel lug mounting surface and heliarcing on a four lug mounting surface. It might make more sense just to machine new hubs from solid billets. I don't think that the calipers have the same hole to hole spacing as the MG, but adapters to handle that are easily made. |
| George B. |
| This is for the Archives in case anybody ever contemplates this conversion: On page 94 of Clausager's "Original MGA" in the OPTIONS, EXTRAS, AND ACCESSORIES section there is a listing: "All-wheel Dunlop disc brakes in conjunction with center-lock disk wheels and Road Speed tyres Standard on Twin Cam, optional on 1600 and 1600 Mark II models from car/chassis 91240 in conjunction with numerous other detail changes. Pushrod-engined cars with this option are known as De Luxe models. Originally built to individual special orders only, but in the last few months of production larger batches were built for the USA." From both this and research previously mentioned in this thread I conclude that wire spoke wheels were never available for the disc brake rear axle and that only the special center-lock disk wheels were compatable with the disc brake rear axle. Unfortunately, Clausager's "Original MGB" does not mention these special wheels as having ever been compatable with any MGB. Without special conversion work an MGB equipped with this rear axle would have different style wheels on the front and rear of the car. Oh, well. It was worth investigating as a bolt-on conversion, anyway. At least now we all know. |
| Steve S. |
| Steve, I can remember reading that the disc wheels were used because there were doubts that the A wire wheel could stand the stresses. Ok, wire wheels were never produced for this A, but does this necessarily mean that later wire wheels will not fit? Is it possible that the hubs for this car became the B wire hubs? Has anybody out there ever tried to slip on a B wire on to an A with the steel knock off wheels? Just wondering. Ken R. |
| Ken Rich |
| Ken- Not having an MGA Service Parts List, I can only speculate on the issue. It would seem logical to assume that apart from the final drive ratios (e.g., the crown wheel and pinion gears) the Hardy Spicer banjo axle used on the MGA was simply carried over during the production shift and was thus the same as that used on the MGB. However, I suspect that the wheel hub mounting splines of the half shafts used on the MGA Twin Cam were unique due to Clausager's reference to the hubs not being interchangeable. I guess that the only way to be sure would be to cross-reference the part numbers for the halfshafts, so I'd better go over to the MGA BBS. Of course, custom halfshafts with the appropriate hub mounting splines could be made. Hmmm...... |
| Steve S. |
| EUREKA!! According to Chris Betson of Octarine Services over in the UK, the 25 spline halfshaft of the MGA Twin Cam is MG part number BTB153, which also just happens to be the same part number for the wire wheel halfshaft used on the early MGBs fitted with the Hardy-Spicer banjo Axle! Now, if the banjo axle casting is the same part #, then with the right hubs (a possible sticking point, maybe Jaguar items?), maybe...... |
| Steve S. |
| Is the weight savings really 60 pounds? The only real difference in the axles is the differential carrier the early one being aluminum and separate and the later tube axle being cast iron and integral. Is there 60 pounds difference between the two carriers?? |
| James Johanski |
| James- Yup. The Hardy-Spicer banjo axle of the early MGBs weighed in at 115lbs, while the Salisbury tube-type axle of the later MGBs weighed in at 175 lbs. A 60 lbs. difference! The Salisbury axle is almost bulletproof. It will absorb the output of a 200hp Rover V8. |
| Steve S. |
| Steve (and others), if you look at the early model MGB hub you will see it's outer surface is left in its original rough cast form. Now measure its outer diameter and the inner diameter of a new undrilled MK II Jag disc. Very slight machining of both will allow for a nice press fit of the disc up to the inner face of the hub. Next having removed the 4 bolts on the hub that normally either retain disc wheels or retain the splines to the hub, get the machine shop to drill the Jag disc to match your original hub bolts. The sequence for the bolts will now be: through the disc (which is press fitted up to the inner face of the hub), then through the hub. Now you will see that the hub bolts were originally knurled to fit into the hubs and now they won't fit! So just use some longer high tensile bolts (mine have been modified so they can't rotate when being tightened). After the hub the bolts go through the splines (for wires) or become the wheel retaining bolts for disc wheeled cars. The original drum setup was the bolt through the hub, then the splines, then the drum. My way the track is about 10mm narrower each side (no drums now). The hubs carriers and their retaining bolts on the Mk II Jag are a perfect fit up to the axle casing on an MGA/MGB. You should trial fit as you may need to get about 3mm machined off the hub carrier face (or just use the Jag spacer shims from the MkII). So you see using this method above centre locks are not needed (where you would be able to get them I would not know!). Machine work is a total of about 2 hours all up (my brother in law is a machinist and said it was dead easy to do). Parts for this conversion 2 years ago were about $300 Aust (which included 2 new undrilled Mk II discs) or about $150 US. I would question wires on a V8 unless triple lace though. But knock on or bolt on minilites look cool!! regards mark |
| mark mathiesen |
| Mark- Very interesting and useful. I take it that you went through all of this because you couldn't find MGA Twin Cam rear disc rotors? |
| Steve S. |
| Steve, Do you think the banjo axle can take 200 HP? |
| James Johanski |
| James- I'm not so sure about that. It suspect that it might be a "stretch" in terms of long-term durability. I do know that if I was going that high up the power scale I'd rather go with the stronger but heavier Salisbury tube-type axle and put up with the greater unsprung weight. You don't get something for nothing. |
| Steve S. |
| Steve, Thanks for your input. I am finishing off a V8 conversion and love the looks of the banjo axle. The weight savings while not critical would be nice. If the differential gear set could take the HP, it might be a nice axle in a V8. I'll poll the V8 BBS to see if anyone has used it. |
| James Johanski |
| The durability issue with the banjo axle is housing flex, a problem which has plagued Ford drag racers for many years. Not having the rigidity of the center section of the tubed axle, the housing flexes under hard accelleration and causes rapid wear of the ring and pinion. I'm not sure of the exact mechanism of how that happens. Apparently the flexing of the housing forces the carrier flange out of round and moves the pinion relative to the ring gear. Anyway the tried and true fix is to weld internal reinforcements between the carrier flange and the rear of the housing. Sometines you will also see braces welded across the back side of the housing as well. I think before I used an MG banjo axle or an 8" Ford for that matter in a V-8 car I'd add the proper reinforcements. |
| Jim Blackwood |
This thread was discussed between 04/01/2002 and 13/01/2002
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