Replacement de dion tube?

K

kwacka

New Member
Hi all, my recently acquired '73 3500S has a really annoying suspension knock coming from the rear. I've narrowed it down to a badly worn de dion tube. I guess it's original at 86k miles, so perhaps not surprising. What is surprising is that doesn't appear to be any replacements available, so any ideas as to how I might repair or replace the tube and cure the knock?

Thanks
 
I would take it out, clean it up and check it was the tube causing the knock first. If it is, then keep your eye on ebay, put a wanted ad on here, and phone around the normal suppliers, like MGBD and Wins.
 
As Quattro said.
I would clean it out.
The tube may be devoid of oil. Grease it up and reassemble without the oil. Just grease.
Ive gone the no oil route -no problems so far. Just a rubber gator that keeps on keeping on.
 
You think it's badly worn at 86k? They're designed for several times that.

Strip and service it, before you assume it's buggered.
 
chrisw said:
You think it's badly worn at 86k? They're designed for several times that.

Strip and service it, before you assume it's buggered.
Click to expand...

By servicing, do you mean clean out, replace seals and refill with oil or grease? I can’t find anywhere where the synthetic bushes are available, or the inner tube.
 
Wins International, JR Wadhams, Mark Gray (MGBD Spares) and others should have the two inner seals, the tube gaskets and the outer bellows. If the bronze bushes in which the inner tube slides are badly worn, it might be a job for an engineering shop which could machine new ones.
 
Thanks all for replies, I’m coming to the conclusion that this isn’t going to be a quick fix. I had already contemplated having the inner tube surfaces hard faced and reground, and new bushes made (I read on this forum that they were synthetic material), but there is no info on the internal dimensions that I can find. I’ll order a repair kit for seals etc and strip down to investigate. I’m a little surprised that there are no spares for an item that will wear over time.
 
Normally the bushes and inner tube seem to wear hardly at all. I've replaced seals and bellows in all three of the P6s I have owned, coincidentally at about 66,000, 77,000 and 88,000 miles respectively, and there was no sign of unwanted movement in any of them.
 
kwacka said:
Hi all, my recently acquired '73 3500S has a really annoying suspension knock coming from the rear. I've narrowed it down to a badly worn de dion tube. I guess it's original at 86k miles, so perhaps not surprising. What is surprising is that doesn't appear to be any replacements available, so any ideas as to how I might repair or replace the tube and cure the knock?

Thanks
Click to expand...

As others have commented, 86k is very small for propogating wear in the de dion tube. My Rover has over 400,000 miles and the de dion is in as new condition. If it it knocking, then something else has caused the damage, possibly someone jacking the car up under the de dion resulting in damage. Bushes and shock absorber rubbers that are worn are much more likely to precipitate knocking. The pinion extension case mount and the mounts that support the diff plate, should they be lose or, in the case of the former, oil damaged, can allow unwanted movement when either breaking or accelerating which will generate an audible noise.

The only way really that the de dion could be truly worn would be if the gaiter had split and aggregate had entered allowing the sand paper affect. When you remove it, you will be able ascertain whether this has been the case or indeed, something else is amiss.

Ron.
 
Thanks Ron,

I think I’m going to have to get underneath again if collective thoughts are that it probably isn’t the de dion. The noise is a knock that is in time with every slight movement of the suspension. To try to isolate it I made a heavy lever from 3x3 timber and had it in contact with the diff, so that wifey could jump up and down on the free end whilst I crawled around trying to isolate the noise. At first I thought it was the front pinion extension mount, but there was no movement there. I could stop the noise by hanging off the de dion tube, so therefore came to the conclusion that I had taken out free play at the bushes. I’m a biker so very familiar with the practice or renewing fork internals because of wear, the design of fork tubes is similar to a de dion tube.
All shock absorber, trailing arms and link rod bushes have recently been changed and there appears to be no play in driveshafts or wheel bearings, and the tube appears to be full of oil. It’s difficult to pin it down when I don’t have ramps, I feel sure that a P6 expert would know what it is within seconds of sitting in the car :D.
 
I can think of one such item that will cause a knock as you describe, as I have had experience of this........
The shock mount end stems have a rubber or poly bush that goes either side of the body or axle mount, through the centre of the bush goes a hard nylon sleeve to stop the stem contacting the metalwork, the stem is @ 3/8" and the hole is @ 5/8", the sleeve takes up the lack.
When the sleeves wear they get 'sliced' by the metalwork and then the shock knocks back and forth.
There is also the possibility they have been omitted by the unwary.
You can easily check to see if the uppers are there by removing a rear seat squab, lifting the felt and removing the shock retaining nut and washer. You should see the nylon in the centre of the bush.
J R Wadhams Ltd | Shock absorber sleeve: 3500
 
Beware when buying those rear shock rubbers and sleeves, most suppliers that sell both together have had them made so that the sleeves wont fit through the rubbers. As said above that is the most common cause of a knock. In my experience the de dion tubes don't knock, they just grunt and groan.
 
I have seen pitting on the tube causing oil to migrate into the rubber boot, which then degrades. The solution is to use grease. The grease filled tubes had felt seals which are no longer available. You can get the oil seals still. I use Castrol rubber grease which will not eat rubber and this works well. Do not mess with oil in the tube.
 
Thanks again for replies. The noise is spookily like a rattling shock absorber mount, but the car had new gaz shocks and rubbers less than 2000 miles ago, so didn’t bother to check other than a quick visual. The mounts are easily accessed so will take them apart. I was aware of the poly bushes, and also that some mounts claim not to need them....

onwards and upwards :D
 
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