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MG MGF Technical - Ride Hight 280mm ... on only one side :(

Grrrr....... Now it hit me :(
Hydragas System in right side damadged (05/96, 61k km ) Probbably the right rear unit (see pictures)
That all results in a ride hight of only 280mm :( and a horrible ride comfort. The front mudflap has only a distance of 15mm to the road and scratched terribly at each little uneven point.
The wife got nearly seasick on app 60 km home ride.

Anyway, now its time to get the short knuckles ready turned (lathe) and do the last steps on my pump. (Carls suggestions)
But no enjoy with the F for the next weeks. :( huu...huuu :(

The only advantage ;-) I will soon own a damadged Hydragas unit to exlpore the last secrets of its inside design
(No warning required for me, I know about the high nitrogen pressure)

See some first pictures on:
(select 'MGF Defekte' Page)

It would be nice to hear about your experiance on repair costs.
Special the part number and material price of the unit. My F is out of warranty or goodwill and so I'll go to pay it myself.


Apart from the possibility of a leak of fluid somewhere in the system it could be a displacer

Front displacer unit part number RNK 100070
Rear displacer unit part number RPJ 100020

If there are other parts reuired that need identification then just ask.

Roger Parker

Roger thanks for the Numbers. :)

I checked the system this rainy afternoon. The car lifting was a little tricky because it had only 9 cm space to the floor. Andrew Philips stated already to build a little ramp and get then the jack under the car. I did so too.

It is indeed the rear unit.
A green tear is still at the rupper diaphragma and the whole rubber thing is wet from below. No drops or leaks at the hoses and joints.

I thought already, that the rear and front type is different.
Whats your opinion ?
Is it only outside different (fixture?)
or different inside (ball valves?)

Anyway, some more pictures will be enhenced tonight. I measured some more data too.

All around, I'll have a look at the manual .. lets see wether I try to change the rear one myself ... if I get the own pump ready next week.
After the selfmade change I'll go in to my dealer and let the system evacuate and refill. Should get quite cheap then.

BTW IMO only for engineers, that job !!!


Another member joins the club. Dieter, you got your broken hydrogas unit to play with after all, you should be careful what you wish for ;-)

Maybe you should talk to carl about designing a better Hydrogas system for us all :-)
Tony Smith

>Maybe you should talk to carl about designing a better Hydrogas system for us all :-)

Tony, I beleave that the system is Ok. But rubber is a very hard to produce stuff. a small failure in mixture, temperature etc can shorten the diaphragma live dramatic. No idea wether other mouldable materials could work better (i.e. Silicone ?)
Anyway, its damadged and will be repaired. I promise to deliver an unmasked failure analysis !! (forwarded to our special friends in Munich if I find the smallest evidence on manufacturing quality !!!) ;-)

Back to my 'story'
Things come together:
Carl sent an important number for the self pumping job <grin> Thanx !!

Hydralastic fluid 5 l in tin can GSZ1486 app. UKP 30 ....>;-(

I really think about mixing it myself 50/50 (coolant/water) ;-)
Any suggestions on what can be wrong on that thoughts ?

Find some new pictures 'tears of green stuff' etc in row 6 and row 6a @

PS. Don, where are you ? This is for Engineers <grin> :)


Front and rear units difer in settings - just think of the weight differences front to rear - so go for the correct unit to maintain balance.

I can buy Hydrolastic fluid in 5 litre containers at a number of normal motor shops for less than 10 UKP. It reflects the common nature of Metro and some earlier Hydrolastic/Hydrogas equipped vehicles, and the fact that even the smallest workshops often have a Hydro pump.

Roger Parker

Thanks again for your instructions.

>even the smallest workshops often have a Hydro pump.

I and IMO Carl too did not find any in our locations. There may be some workshops left for Minis, ('left' from the 'Rover/MG rollout' some years ago) but as sayed no one found recently.

Roger that all is not really vital, but fun for engineers :)
We decreased the risks to a minimum IMO. If it gets wrong, I'll hav to explain a little to my dealers workshop.

.. Mr Hemmann here knows me already. It will be easy to buy only the required one litre hydrafluid of him, including a rear unit.


I did my dealer call this morning and got the prices: (incl VAT)
Front displacer unit part number RNK 100070 DM 352,28 (UKP 117)
Rear displacer unit part number RPJ 100020 DM 352,28 (UKP 117)
Hydralastic fluid GSZ1486 DM 100 (33 UKP)

I ordered the rear one and was promised to get one litre of the green stuff sold.

I think the whole job will be then about 285 UKP :))

- 'my pump' (cost 60 UKP)
- home made lowering knuckles (cost 18 UKP for a second hand standard set)
- rear hydragas spare part (UKP 117)
- evacuating and adjusting the ride hight at dealer (UKP 40 app.)
- four wheel alignement (UKP 50)

Thats OK .. IMO for a 20mm MGF lowering by knuckles & the replacing of the damadged unit.
I should reduce the price 'virtual' by a 'fun factor' which is app. UKP 100 worth <grin>

Hi all,

I think that in some future it will be very tempting for "Mr Aftermarket" to make a similar bracket as the Hydragas unit but with some type of "coilover" instead. Together with decent dampers that could be an easy-fit option with the big advantage of unlimited height adjustment One drawback is of course the lack of ride comfort that original system gives .

Regards , Carl.

I get all turned (lathe) parts this evening and can fit them to the pump and my 'Carls version' lowering knuckles.

A question on technical english:
Is 'backflow preventer valve' a common description for such a thing: --///o>--
-- = pipe
//// = spring (technical! not the spring which occurs in hopefully 5 month) ;-)
o = ball

> = hole, 90° cambered at the lower end (sealed by the ball)

A question on Hydragas evacuation pressure
I recently have no WS-Manual at hand.
Can someone please look up the evacuation pressure ?
Must something be in the area of:
xxx inch hg

Can someone have a look at this side:
Its worked out from Carls instructions and Tonys help on translation. I added a schedule yesterday which should describe the principals of our pressure pump, based on a Hydraulic workshop jack.
The related links are under construction, so not ready !!

A suggestion and new schedule for evacuating the system will follow. I must think a little on that.

Dieter - what an interesting project - couple of comments.

1) Knuckles - are you machining a standard set to be the same dimensions as the 'short' ones or to some other height.
I think something in between to two would be good.

2) Re the pump - can it be used to suck out the hydragas fluid as I was told that this is the way my garage replaced the knuckles. They sucked the fluid out causing the rubber diaphraghm to go up into the hydragas units and allowing the knuckles to "drop out"

3 English
Non-return valve = backflow preventer valve

4) Pressure
I may have misunderstood the question but when pumped up you are looking for pressures of 400 to 425 psi or 27.6 - 31 bar (assuming 14.5 psi = 1 bar)

When evacuated - I guess you just suck till no more fluid comes out.

Good luck!


John Thomas

> what an interesting project - couple of comments.
... belong at first to Carl! He sent me his results some weeks ago and I try to approve it be repeating all steps, making a webside etc. Tony was so kind to help us :)
So its a international english-'dutch'-swedish-german project. :) I think the next will be Don (Taiwan), if he realises our 'just-for-fun' works.

>1) Knuckles - are you machining a standard set to be the same dimensions as the 'short' ones or to some other height.
I think something in between to two would be good.

I found a lot of different length (+/-4mm)(remembering your and Torsten Ohms and Carls reports.)
So I made a drawing of all your input data. It covers both version of the knuckles bolt:
A. = Carls usage, shorten rear -7mm
B. = turned new, but length reduced -7mm at the front

I did'nt find recently a material for turning the 'new' version B bolts (only Titan from a friend, but who should turn that ?). I am under time pressure because my rear unit got broken and so I went for shortening the -7mm bolts.
That all calculated according to the lever relations of app. 4 to 1, which should hopefully result in -7mm x 4 = 28mm.
I try to setup a curve of pressure over resulting hight when its ready.

>2) Re the pump - can it be used to suck out the hydragas fluid as I was told that this is the way my garage replaced the knuckles. They sucked the fluid out causing the rubber diaphraghm to go up into the hydragas units and allowing the knuckles to "drop out"

No, not possible recently. I developed a strategie with Carls help to reduce the non wanted air to a minimum when fitting the new rear unit. The rest (evacuation) must be done at my dealer.

> 3 English
> Non-return valve = backflow preventer valve
Jep, sounds much shorter. I need to built two non-return valves for the evacuation pump enhencement.

>4) Pressure
>when pumped up you are looking for pressures of 400 to 425 psi or 27.6 - 31 bar (assuming 14.5 psi = 1 bar)

Jep, but that are maximum values. I got some more infos on actual values of typical 360psi.

Anyway, I go for a sequence of different setups to find out more about the curve of pressure related to the way that the 'sprin' does. IMO only the 'hight values' do not meet the requirements of each single F.
IMO the hight values are only 'help values' for 'dummys' in workshops.
It will not matter me if my F will have at last a different ride hight (side to side) of about 5 mm, but I want to have at first equal pressures on both sides.
360psi is the target on both sides. Then lets see in what hight this results. If to low, or to high... I turn new shorter or longer knuckles. :) until it results in about 345 mm.

> When evacuated - I guess you just suck till no more fluid comes out.

So at maximum 1 Bar ? ;-), no, no, to unprecise for engineers. I'll look up this evening. I want to calculate wether a water, or pressure air based injector provides enough vacuum
... :-)

> Good luck!
Thanks and I hope that we all may again learn a little more about our little F. The drawings and picture sides will be reworked on the fly with all new or revised results.

Thanks for your help !!



Hm, first there was a "Hall Of Shame" (head gasket failures). Looks like it's about time for a "Hall of Down" (suspension failures).


FAO Roger,

I found in the workshopmanual an advise for the grease wich ist to fill into the knuckle, between rod and the cambered metal plate.

'Dextragrease Super GP'

The old one in my second hand purchased parts was black. I've recently no information of this grease but tink it could be one with included 'graphit'.

I know such a grease here in Germany under the brand
it us black with 'graphit particles' and used for greasing movements of metal-to metal parts.

What do you mean, could that be OK ? Any other knowledge from your side on that Dextragrease Super GP ? :-/

FAO John
the under pressure for evacuation should be 27 inch Hg. :-/ = 685mm Hg (for my actual knowledge is that about 65 percent vacuum) A lot of stuff !!

PS. Webside updated, all mashined parts ready, for the knuckles and for the pump.

a little sad report today.

I got my new rear HG-Cylinder today and found by comparing all parts 'on the work desk' that the shortening of the knuckles bolt at the rear side does not work. !!!

I'll have some mails with Carl to find out whats wrong or changed or whatelse.

Remark for your plans:
Only new machined bolts according to the just updated drawing is OK.

BTW the grease problem is fixed. I take the graphit filled grease.

Sorry, the new to turn bolts will delay my lowering some days.
A brief description follows then in a new thread.


To keep the thread 'warm'
Front displacer unit part number RNK 100070 DM 352,28 (UKP 117)
Rear displacer unit part number RPJ 100020 DM 352,28 (UKP 117)
Hydralastic fluid GSZ1486 DM 100 (33 UKP)

That was as to expensive reported
the true costs incl. tax is:
Front displacer unit part number RNK 100070 DM 303,51(UKP 101)
Rear displacer unit part number RPJ 100020 DM 303,51(UKP 101)
Hydralastic fluid GSZ1486 DM 100 (33 UKP)

... and on the technical 'confusion'. Some english-german-swedish multilingual misunderstandings come more and more clear now, under 'heavy email traffic' between John an me and Carl <g>

I note that you have sorted out the grease problem and the graphite grease is what I would use.

I am not quite clear what you are saying relating to the lowered knuckles.

Roger Parker

>I am not quite clear what you are saying relating to the lowered knuckles.

Thats the right description according to the confusion that happened the last hours and days ;-)

But now its everything sorted. I am sure !
I got a fax from Carl and will go tomorow for machining the bolts down to the final measures following his now agreed suggestion.
The pictures are updated. The photos of the bolts are reworked by a graphics Software, but I think its enough now.

John and Carl may check that again.
Thanks again for all.


everything is fixed.
The pump worked good :)
But I join my dealer tomorrow for a complete evacuation and refilling.

I had lots of mails and helpful hints from Carl and John.
Thanks fly to both.

The short knuckles are recently not fitted. That is another story.

Anyway, see loads of pictures of this nice work ;-)

It would be interesting to get some comments of experts on the reason why such a unit blows. IMO my car was driven to long in to low ride hight !! This 'stressed' the material round the cone piston to much !! (wrong work angle of the piston and piston to deap located in the diaphragm.
The structure of the diaphragm design seams to be equal to a 'tyre' with included fabric.

PS. really not an easy job !! Now that part waits on some freetime to cut it in two halfs ... LOL

Interesting series of photos and I would concur that the most likely cause of failure was a continued period of use at the wrong ride height for the set up.

Roger Parker

>at 02:26:44 UK time ?
Roger, I ever thought I am the last one :)

According to the diaphragm wear.
I think equal. It looks this way!
I do not make anybody responible for this !! I bought the car second hand and have no information of the pre-history of its first 30.000 km. There must have been tyre wear, because the front tyres where rather good conditioned when I bought the car one year ago (now 61000 km !). The prowners where anones (the last one a 'half-expert' .. fuel-station owner :-/...) so I'm nearly sure that the car was driven a long time below the specs. of 368 mm !!

Anyway, I need time to carefully cut that thing into two halfs...
More pictures will follow.

;-) hope to get no problems with 'Moulton patents LTD' <g>

when my first suspension breakdown occured, the car wasn't too low for sure.
For the second one, well, the ride height had just been set up to 368 mm, but before, it had been around 340 or less for a while ...



Thanks for the information about ride height and suspension breakdown risk. My car is at 340mm without too much font tyre wear, so I was hesitating about having it pumped up or not. Now I'm sure that I should have it done.

Peter and volcano 1.8i
681 APZ 38

I beleave we will never find a general explanation on why they fail and why not.
(Mr Gauss had some nice mathematic explanation on such diversifications.)
Some fail very early, some later and hopefully the rest will live 'endless'

Shows that fabric looks already out of the rubber, see the white stainrow, left of the circle.

No idea wether it is worn by stress of the piston or never was covered by rubber. (IMO the last one, never was covered or covered to thin.)

PS I do not really worry about this failed unit at my already 4 years old F. It happend and I repaired it .... [dot] :-))

Earlier posting tonight with more brain cells functioning!!

Leading on from my last comment which was perhaps dulled with the time of day! I should have added that all of the units would have been subjected to the same stresses so if it were just the lower ride height then this would have also shown up some degree of 'wear' or 'marking' in the other units. The fact that there is no obvious signs points clearly at the failed unit as having a weakness, probably present since manufacture.

Roger Parker

>also shown up some degree of 'wear' or 'marking' in the other units.

Yup, thats important Rog. Sorry on brain functions :)
I'm with you. <g> But for such brain refreshing is this BBS the best what I know.

My dealer had such problems too on monday ;-)
I wanted to have the right side evacuated and repumped... and what did they to my dear little F :(
THEY PUMPED IT UP TO 395 mm >;-(

I'll check the other units when I fit the 'other' knuckles. I got some good instructions by Email of another friend here. It's to cold here recently for creaping around under the car. I will only put in back to 360mm for a weekend ride and 'win' some fluid for the next steps.

Any weakness failure is IMO impossible to valuate at only one failed unit. I keep the unit a little may be Carl has some additional ideas on what to check! He is absent until monday.
There are some more terms which need time for me to explain it.

Thanks for writing your indeas and instructions.

The thread has got long enough :)


Dieter, quick question. When I set my ride height last time, I notice on letting the fluid out that it seems to be at a much lower pressure on the right hand side compared to the left even though the height was still the same. Green stuff came out fast on the left (some eye protection required) and but sort of fizzed out on the right (this is the side I had a collapsed unit on the other week). It was also like this before the hydrogas failed the previous time I had to lower it (from 385+).

Do you think this is normal or would you suggest another possibility, any ideas? You seem to have had yourself quite deeply into the subject recently.

I hope you are having fun
Tony Smith

everything I would say on this phenomen would be more then I know.
Only questions:
- can be the valve itself ?
- can be a lack of flow, because any valve inside any unit was or has recently closed ?

I'll have fun again when I get some results of 'pressure related to hight change' measures ;-)
and when thing ugly thing is back from 395mm (at 7.5°!) to 360mm.


I complained the bill for that stupid evacuation just this afternoon at my dealer. LOL
They cannot explain why the car did this. ;-)
that rising on both sides from 350 to 395mm, because they promised that only and really only the left side was pumped up.

It got a little loud in the know what I mean.

ANyway the wnated to show me that everything wents like at experts and took the car in the workshop.
both sides had about 420 psi. Again no explanation on how that could appear ... ;-)

Anyway, now its on 360mm and its OK.

BTW the pressure gauge at their pump is poor. It has a low precision/resolution (only 40mm diameter) but a large hysteresis..
A Acetylen meter is much better

>A Acetylen meter is much better

Plus acetylen - adds a new meaning to flaming MGFs.

Ted Newman

Hi Dieter!

Your car shown at "Ugly in darkness " is Roverīs revenge for making a pump of Your own!!
At least there is a benefit of a few dL of "free" fluid when itīs taken down to correct height....

Very interesting series of pictures regarding Hydra-unit. Do You think there could have been some object - small stone etc. inside of dust cover that have made the damage ?? Hopefully no unit leaves factory with such damages....( When doing first spring cleaning on my "F" there was a varity of small parts found everywhere under seats , carpets , in boot etc.)

Regards, Carl.

Dieter, when I had my recent look into the matter. I saw my car being pumped up to 365mm, I even did the measuring. We did not follow procedure, by the time I got the car home it was 380mm.

Before this I watched my dealer lower the car to 365mm without following procedure, by the time I got home the car was at 340mm. Not sure about 30mm, but this this is 15mm difference from when set if the two hour procedure is not followed.
Tony Smith

you and I and Carl and Ted and Rob and and and some other too know about this. A small settle or a small raise depending on whatever it is. Tempereture, lack of gauge precision, etc.

If all the instructions would be followed then our friends would have less confuion and problems.
I.e. this afternoon at my dealer. They forgot to bounce the suspension to open the hydragas inner valves. They where not able to roll the car, because it stood too short over the workshop jack (the large one with 4 lifters).
I bounced the car maself and got the answer 'does not matter it will settle' when I asked for the forward rolling.

Anyway, I think this was the last time that my 4 years old MGF saw any Rover garage.
(Without on any organized 'meeting' :))

FAO Carl,
I make a brief description of what I found at the unit later. It was no stone or anything to see. IMO it was a kind of bended stress in 'one direction'.
I have a first schedule of the vacuum pump on base of the same type of hydraulic jack ready. Will be more easy then the pump. Only some lathe works to the piston to do.
But the largest problem:
Any idea how to measure minus 1Bar :-/.
I'm not the one who aims to handle with HG (Quecksilber) :-/
(the main working sealing at the piston must be turned round and needs a washer at lower side)
Additional we need a combination of taps (2-way & 1-way) to switch between vacuum and pressure. I thing the 40Bar meter will not like vacuum :)
I did not send it directly to you because you wrote of absence until weekend.

I'll be back on Sunday, only some hours online tomorrow.

According to low knuckles and stainless steel wire: I found some 0.3mm brass wire. This will work. But installation delays because I found no date at a near tyre store for the four wheel alignement. All all booked with installation of winter tyres.

PS sorry for this swedish-german smalltalk

This thread was discussed between 06/11/1999 and 18/11/1999

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