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MG MGB Technical - RB to CB Crossmember

Hi

Ive got a Jubilee (1975) shell that im going to convert to CB spec. So far I have a full CB front suspension with Rack etc which imm hoping to fit. Will the engine fit ok with this as the engine mounts are different to CB. Also will the upper column fit ok as well? or will I need a CB column too. At the rear im either going to redrill the front spring mount or fit CB ones and then the same for the rear either fir CB repair sections or de-drill and weld an insert in for the shackle to locate on!! Is there anything bump stop related that I need to change??

Cheers

Mark
Mark Turner

Mark-
You'll need to switch the front engine plate to the earlier Chrome Bumper version so that you can install the motor mounts. You'll need a Chrome Bumper steering column as well because the pipion of the steering rack is at a different angle. At the rear, you'll need the Chrome Bumper axle straps. Don't drill the front mounts for the leaf springs. Instead, cut them off and weld in Chrome Bumper repair sections.
Steve S.

I have a ’75. I have converted it to a chrome bumper, lowered the ride, and put in a V8.
Drilling the rear spring hangers is quick and easy (if you drill small pilot holes first) and gets you an inch or so at the axle. I fitted one inch blocks as well. That way I did not have the change the shocker links but was able to ovoid having huge 2&1/2 inch blocks. Drilling the rear shackle holes and welding in spacers might be just possible with a right angle drill, but not at all easy to get up in there. I didn’t try.
Other possiblilities are reversed eye springs (springs made with the eye the other way up) or flatter uprated springs. These last are not so good, because you need the springs to be soft in back and strong in front to get the best roadholding in an MGB. The idea is that the rear live axle is free to follow and keep rubber on the road while ride level is controlled from the front.
The difficulty with changing the front cross member is that they changed the engine to body mounts on the ’75 to the V8 style. The steering shaft actually passes through the engine bracket. When you remove the rubber bumper one inch spacers (by either altering the existing cross member or using a chrome bumper one) the steering shaft comes up. Or would do so if the engine bracket wasn’t in the way. You could either cut and lower the steering rack supports (which will spoil the steering geometry) or weld chrome bumper engine brackets to the rails. The easiest and best option is to go and buy some uprated and lowered front springs. They are cheap and readily available. Along with a heavy front antisway bar, this significantly improves the cars handling. It introduces a small amount of negative camber, which is a good thing so I am told. Seems to work for me. It is also a bolt on change
Actually, if you haven’t got one, the first thing you will want to do is put a front antiswaybar on. The '75's didn't have one at the front (you don't want one at the back).This is easy to do because the captive nuts are still there so it is a bolt straight on job. You need to buy a heavy antisway bar and the links down to the lower arms. You can replace the front lower arms with the reinforced ones (easiest option), or drill holes and reinforce the existing (cheaper but harder to do). Get the thickest bar you can find. Probably around 7/8ths. The standard one is too thin.
Peter Sherman

As far as the steering column goes, the R/B column is longer at the lower column and shorter at the upper column than the chrome items. Also the under-dash mountings for the chrome and R/B upper columns are entirely different. So if you use a chrome rack and lower column, it won't be long enough to mate with the R/B upper column. If you put a chrome upper column in, you will have to fabricate the column supports and I'm not sure that the switch gear and shrouds will fit. Nothing is ever straightforward you know!

Mike
Mike Howlett

Can I carry on using the gearbox crossmember as it is and what about the rubber bumper engine mounts, do they have to be changed to the chrome bumper ones??

Nothing is ever simple is it!!!

Good job ive started this before I get into painting the shell!!

Ta

Mark
Mark Turner

You imply that you have quite a lot of rubber bumper bits. You can use your chromebumper front crossmember (since you've already got it) by fitting simple spacers. So that's easy. The factory chose to weld theirs in but they could have just as easily used removable spacers. Which would have made life much easier for all us chrome bumper retro'ist. I don't see a problem with the gear box cross member.
Peter Sherman

Look here

http://www.mgcars.org.uk/v8_conversions/files/body/enginemounts.html

and
http://www.mgcars.org.uk/v8_conversions/rogv8.html

this has lots of engine bay detail, mostly with a view to turning chromebumper engine bays & tranny tunnels into V8 or rubber bumper engine bays. The opposite to what you want to do. However, lot of details.
Of course you might consider "turning to the POWER of the darkside young skywalker". Since you are half way there.
Peter Sherman

An easy way of lowering the rear end which also improves the handling is to fit "reverse eye" springs. These effectively lower a rubber to chrome ride height. They are available from the BHive. Speak to Shane.
Allan Reeling

If I put the Chrome bumper engine mounts on I presume the engine is higher, what do I have to do to the gearbox crossmember (fit a chrome bumper one maybe??)

Cheers

Mark
Mark Turner

Why would you change the engine mounts? The engine mounts are welded to the chassis rails, not the cross member. The gearbox mounts are the same for both types of car.
Mike Howlett

The 1800 engine is in the same location, exactly, in both chrome and rubber Bumper cars. They changed the engine mount body brackets for the V8 application. The originals get in the way of the V8. They are the same as on the rover P5 and P6 I am told. This would make sense as that was where Costello got the V8 engine. Your engine compartment is exactly the same as the V8 engine compartment that they were making at that time. There was also an engine restraining bar on rubber bumpers that did not appear on some of the chrome bumper cars. This is a fairly pointless bar located under the gear box that bolted to the rear cross member. That is the only difference. This bar, by the way, makes for a perfect, and very necessary, engine restraining bar when fitted across the back of the V8 engine.
The steering in the rubber bumper is more advanced than the Chomebumpers. The chrome bumper one attaches to the dashboard and takes up more space in the engine compartment. The rubber bumper one attaches to the bulkhead and to an additional body bracket. It has a collapsible component as well. As you will have seen from those articles, most of the difficulty of fitting a V8 into a chrome bumper is changing the steering and bulkheads. Solutions to this have been to either do a complete steering bulkhead swap, or spend money getting the components of the steering shaft shortened and re splined with the addition of a second universal joint.
The Front crossmembers of the rubber bumpers are the same as the chrome bumpers except that the mounting points have one inch spacers welded on. These can be cut off easily. The reason why people don't do this is because the steering shaft and engine mount then interfere. They would then need to cut the engine mounting point and make up a special one.
Fitting shorter, uprated, springs is far easier and also improves the handeling of the car significantly. The small amount of negative camber is also supposed to improve grip in corners. It has not caused the slightest bit of additional tyre wear for me. Easier, cheaper, better.
This is why I suggest that you fit one inch spacers and shorter springs. Leave the steering, bulkhead and engine mount alone.
Actually I also suggest that you fit a V8, but I expect that you would have guessed that already ;)
Peter

Why not buy a C/B in the first place, it would save a lot of hassle.
K Harris

The possibility of fitting a V8 of course!
(nothing if not persistent)
Peter

The reason is that I have an excellent 39k miles shell that just needs a rh sill. The rest is absolutely perfect as it was ziebarted from new!!!

So hence wanting to convert to chrome bumpers by cutting solid metal rather than hacking rust out!!!(ive just done half my uncles Chrome bumper car and it was horrid and needed every panel touching!!) I dont want to lower via front lowing springs as it can ruin the suspension geometry and induce lots of bump steer if you go too far, hence wanting to use the chrome crossmember and rack which then leads to moding engine mounts and upper column etc, not too bad as at least its welding to solid steel and I have a nice worshop avail for the metal work!!

Cheers

Mark
Mark Turner

I have recently rebuilded my rb to a cb specs.

For the rear I used lowering blocks of 1 inch. The car seams to be fine at that hight.

The front was a bit of a hassle. Fitting the cb crossmember completely rebuild with new parts wasn't a problem, it took me half a day. The steering caused the big hassle. As pointed out above, a cb steering rack gets you in trouble with the rb engine mounting and it won't fit a rb steering column. Changing the column means changing your whole dashboard. So I made a custom lenght steering rod to fit the rb steering column. By making spacers to fit the steering rack to the crossmember I managed to get it nicely through the engine mount. Welding on a steering rod is strongely disadvised, but as a mechanical student I managed to do it in a way it satisfies me.
First I grinded both ends, that needs to be welded to each other, in a cone shape. I connected the two with a cut of bolt (I first drilled a hole in it with wiring on the inside). Then a certifite welder welded it full around the rod, making sure they were in a line. Then I slided a tube over the weld of aprox 10 inch with holes drilled in it. Then de ends were welded to the rod an the holes were welded to the rod. It looks very solid, and I have driven about 1000 miles with it without a problem. I'll post some photos below.

I suggest to think again about buying a cb body and sell this one, it makes life much easier!

Regards Rainier
Rainier

Here are the photos

http://picasaweb.google.com/RRPKarthaus/CbFrontSuspensionConversion

Regards
Rainier

The bump steer is a bit of a myth. Might happen in theory sort of thing, but in reality is swamped-disappears in a whole lot of other effects. Keep in mind that when you are going around a corner there is way more than one inch of suspension movement, even with the heaviest antisway bar. And allot of other things going on!
That reminds me. You must shorten the bump stops at the rear. Two ways of doing this. Cut an inch off the rubber which takes about a minute to do; or you can cut away the "add-on" crescent of metal that they fitted to the rubberbumpers and move the bumpstop base up. The original surface is still there under the added bit.
Initially I did not do this and found that about half way though a heavy corner the outside bump stop would contact and through leverage the inside wheel got very light and the most massive surges of over-steer would happen. Not good to put it mildly and caused a few near death experiences. This effect disappeared when I cut the rubber bump stop in half.

Anyway, your plan appears to be to fit entirely chrome bumper steering. You obviously can't mix and match without great difficulty
In that case you would do well to replace the engine-mount/body brackets with a chrome ones. The bulkheads should not be a problem. You will also need to fit a Chrome bumper dash for the steering to attach to.
Peter

This thread was discussed between 12/08/2007 and 22/08/2007

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