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Some NOS Hydrolastic Displacers with original colour coding on ebay a few years back...

Note: Minispares sell reconditioned 21A 2008 displacers on an exchange basis or can recondition your own S units.
https://www.minispares.com/catalogues/c ... 8/page/77/

Interesting. But as I and many others have said in the past, the only thing that hydro unit reconditioners can do is just that! They can bead blast, prime and paint the steel casing and replace the hose. You cannot get inside the unit to recondition the actual hydraulic or elastic (the hydro-elastic) components of the unit.

If you are of a delicate nature regarding the wonderous properties of Hydrolastic, look away now.

It seems as though Dunlop, BMC and BMC publications here at Cowley didn't understand the inner workings of the units either. Take the sectioned drawings illustrated in the previous page, taken from the workshop manual and kindly put up by Martin. Have a look at the part called the 'damper valve'. It should be called 'the MAGICAL damper valve'.
Why is it shaped differently in drawing 1 from drawings 2 and 3. And then...
In drawings 2 and 3, the same damper valve has suddenly, magically by dint of some of its mysterious properties, has been pulled through the metal seating (that it is operating against....., see it?), turned through 180 degrees vertically and re-appeared BELOW the support plate........, upside down!

Not even Paul Daniels (an English TV magician...) could do that!

If you ever chance to remove your units and flush them out, not difficult, have a look at the bits of rusty metal and chunks of rubber that come out with the foul smelling rusty sewage. Those bits that you have shaken out while flushing through are the remains of the rusted out damper valve

Peter Laidler wrote: ↑Tue Mar 11, 2025 10:05 am ....

If you are of a delicate nature regarding the wonderous properties of Hydrolastic, look away now.

It seems as though Dunlop, BMC and BMC publications here at Cowley didn't understand the inner workings of the units either. Take the sectioned drawings illustrated in the previous page, taken from the workshop manual and kindly put up by Martin. Have a look at the part called the 'damper valve'. It should be called 'the MAGICAL damper valve'.
Why is it shaped differently in drawing 1 from drawings 2 and 3. And then...
In drawings 2 and 3, the same damper valve has suddenly, magically by dint of some of its mysterious properties, has been pulled through the metal seating (that it is operating against....., see it?), turned through 180 degrees vertically and re-appeared BELOW the support plate........, upside down!


If you ever chance to remove your units and flush them out, not difficult, have a look at the bits of rusty metal and chunks of rubber that come out with the foul smelling rusty sewage. Those bits that you have shaken out while flushing through are the remains of the rusted out damper valve

The reason the damper valve appears to change position is because there are 4 of them: 2 above, and 2 below the "port plate" to control rebound and bump fluid flow. Their shape varies in the diagrams because they are not symmetrically round and have a different profile depending on viewing angle. See Mini Spares technical article on Hydrolastic units, which contains this photo of the valve (port) plate:

Peter Laidler wrote: ↑Tue Mar 11, 2025 10:05 amIf you ever chance to remove your units and flush them out, not difficult, have a look at the bits of rusty metal and chunks of rubber that come out with the foul smelling rusty sewage. Those bits that you have shaken out while flushing through are the remains of the rusted out damper valve.

Regardless of the state of the damper valves there must be thousands upon thousands of hydro sprung cars (Mini, 1100, 1800) worldwide that are happily cruising the highways today with owners very happy with the ride quality. Anecdotal statements of support for hydro versus rubber cone continue.

One conclusion from this is that the state of the damper valves is not a major factor in the workings of the hydro suspension.

winabbey wrote: ↑Thu Mar 13, 2025 1:44 am

Peter Laidler wrote: ↑Tue Mar 11, 2025 10:05 amIf you ever chance to remove your units and flush them out, not difficult, have a look at the bits of rusty metal and chunks of rubber that come out with the foul smelling rusty sewage. Those bits that you have shaken out while flushing through are the remains of the rusted out damper valve.

Regardless of the state of the damper valves there must be thousands upon thousands of hydro sprung cars (Mini, 1100, 1800) worldwide that are happily cruising the highways today with owners very happy with the ride quality. Anecdotal statements of support for hydro versus rubber cone continue.

One conclusion from this is that the state of the damper valves is not a major factor in the workings of the hydro suspension.

The late John Smidt of the Mini Kingdom fame often would say to me (and no doubt others) that he found within short time, the internal damping would fade and there after, all cars felt like they rode and handled the same. I'm very much inclined to agree with John's remarks.

Yes, Spider and John Smidt are right...... Regardless of the state of play INSIDE the rusted out, sewage filled, neglected hydro units, there will always be a damping effect. It's because of the not uncontrolled (?) fluid movement. But like gas (not absolutely correct but descriptive enough...), you can only pass a certain amount through a given diameter. So far as I can see, there is another valve in the system too. The 5mm (or is it 7mm?) restrictor in the bulkhead hydro pipe adaptor.

Mind you, in my experience, this will have become so rusty internally that the bore will have almost closed up. Make sure that you clear out the clag and shxx before you assemble the system up. Or better still, get the stainless versions from Gordon at Catmint.

Like thousands of others, as Win says, my car is still on hydrolastic and I know that the guts of the units have rusted out. But there is stil a semblance of damping for that reason

Hydrolastic unit Part Numbers & Colour Code Identification:-
https://www.minispares.com/blog/2022/07 ... t-numbers/

21A1477 Front only, 1 orange or green band, standard use pre December 1965 for Cooper and S, Hornet and Elf.
21A1703 Rear only, 1 orange or green band, standard use pre December 1965 for Cooper and S, Hornet and Elf.
21A1804 Front and Rear, one orange band, standard use December 1965 on for Cooper and S, Hornet and Elf.
21A2008 Front and Rear, one green band, standard use December 1965 on Cooper and all Mini models except S. Most common & most made.
C-21A1705 Front and Rear, one yellow band, Stiff, Rally use, pre December 1965 for Cooper S.
C-21A1819 Front only, one red band, Hard setting, Race use, pre December 1965 for Cooper S. Rally for 998/1275 0ctober 1969 on
C-21A1821 Rear only, two red bands, Hard setting, Race use, pre December 1965 for cooper S.
21A1811 Front and Rear, two orange bands, Stiff, Rally use December 1965 on for Cooper S.
21A1872 Front only, one blue band, Hard setting, Std and Race use December 1965 on for Cooper S.
21A1874 Rear only, two blue bands, Hard setting, Std and Race use December 1965 on for Cooper S.
21A2012 Front only, one silver band, standard harder unit, December 1965 on for Cooper S.
21A2014 Rear only, two silver bands, standard harder unit, December 1965 on for Cooper S .
21A2010 Front and Rear for 1275GT and UK Police, standard use, 1969 on.

Peter Laidler wrote: ↑Tue Mar 04, 2025 9:28 amAs for the different stifnesses, all I would ask is this. Has anyone ever seen any reference to ascertain exactly how the stiffness was quantiified by, say, fluid flow or a measure of resistance? Nope, me neither.

Hi Peter - these tables are on the BMC Australia displacer engineering drawing AYA4090. Is that the sort of data you are referring to?

YDO5 is the model code for the Mini Saloon, YDO6 for Cooper S.

The Spring Element Spec initials probably stand for Normal Rubber, Hard Rubber and Extra Hard Rubber - just a guess. There is more rubber in Cooper S displacers, the level being closer to the top of the displacer body where the hose exits when compared to a "standard" displacer. Maybe the rubber formula was the same in all displacers and the increased springing is due to the greater volume of rubber to be compressed in the S part.

I understand they were made here by Dunlop.

I would appreciate thoughts here in regard to the fitting of a replacement hose. I bought these hoses a few years back and believe they came from Canada. What is nice about them being that they have been made up using the original 'herringbone' style rubber.

My question is that after removing the old hose and ferrule I must then drill and tap (9/16 unf into the round metal hole. The available male thread on the hose is (after taking into account a aluminium or copper washer) 12mm in length. What I don't know by drilling into the hole is whether there is sufficient space to drill down say 15mm? (to take into account the non threaded end of the tap).

Before I drill in deeper - currently I'm at a depth of 13.65mm but noticed the drill was having to work harder as if there had become a change in the metal type.

I am minded to suggest that you turn to page 1 of this thread and have a look at MAB/Martins photo of a sectionalised hydro unit.

Cutting the bonded-in ferrule was a bad move in my opinion. Because you don't have a decent thickness of metal to hold the new thread (and the pressure that the thread is under). That said, it LOOKS like the thread on the new hose is a taper thread that will lock itself as it is tightened. But that is yet another problem because the rubber will want to rotate with the tap as you attempt to tap the thread. And it'll CERTAINLY want to rotate when you try to tighten up the taper thread.

As for drilling/tapping deeper, I think that you are now coming hear to head with the internal valving - that is probably shot-out in any case.

Let us know your further thoughts Richard

I take your points Peter, the replacement hoses are nicely finished but in effect I'm in the hands of the knowledge (or otherwise) of the guy who made them. (Unfortunately I don't know who it is). But I'm hoping that this was thought about at some point. The diameter of the original ferrule was smaller so hopefully the manufacturer has done due diligence in that respect!?! so the original ferrule would have to go (fingers crossed). The way it was told to me that you made a drilling and then tap nothing more complicated than that...

I'll check whether they have a taper thread (hadn't thought of that).

I think we agree best not to drill down further and if it does have a taper thread that would anyway avoid the need (will check this out later this morning). In so far has holding the outlet tube - I'll take a look to see if it might be possible to create some kind of holding device either by flats either side to suit a spanner or clamping device such as mole grips or possibly the casing of an old style expansion bolt...

If it's found to be a tapered thread It doesn't allow much of a contact point - but the material of the new hose ends does appear to be a soft material that might pinch well but somehow a decent section of straight thread with either loctite or ptfe tape would seem preferable with as you say to withstand the pressure.

Fortunately these are a spare set of displacers (in a decent external condition) but wishing to use these i.e. meaning that I have to replace those on the car are showing signs of seepage when looking down the (front end tubes) and noticing a gentle lowering of the car since pressurising 4 months ago - but crucially the car has yet to be put through any road test and so my hope is that I can fit the newly hosed one's and they will be good!!!

It may help to read this article > https://www.qchydraulics.com/npt-vs-bspt-thread.html
For tapered threads you usually use tapered taps, and is yours imperial BSPT or American NPT

Got all of that Rich. Correction in my last.... Para 3 should read HEAD to head instead of HEAR to head!!!!

To tighten the new hose in the old hydro unit, because it is recessed an inch-plus a bit deep into the 'bowl' and inaccessible you'll need a 90 degree angled crows foot spanner* of the correct AF size.
(* similar to spanner needed to unscrew the hidden 1/2" AF nut on the left side of the clutch master cylinder - or 13mm AF if you've got a later car)

I suggest that you tap down until you get a complete thread through the nut part

I had a slight seepage problem too, probably identical to yours, from the very pliable bag/diaphragm at the opposite end of the unit. It would allow the car to lower about an inch on one side over the laid-up winter months.

(Off subject for a few minutes now. I moved the weepy unit from the front to the rear for easier access and eventual removal. But before I replaced it on the real, totally flushed it out - it had fairly clean fluid inside anyway - and vacuumed it out and totally dried it out. Then poured in some rubber/latex based cycle inner tube sealant. Fitted a valve to open end of hose, pumped it up to approx 70psi and left it hanging there for a week, hoping that the pressure would press the latex into the passage that was the cause of the leak/weep.

Worked a treat..... Held its pressure for a week. Put 'fixed' unit into the rear where it's easier to get to and it's been good and leak free for about 8 years now)

When fitting hoses and you are using ptfe I've found the string to be a better product than the tape .seems to be very popular with plumbers
Hope this is helpful Alan

Thank you Alan re the tape - now ordered..

Thank you Ronnie re thread types - my thread gauge seems accurate for a 9/16 unf 18tpi and this is born out by a compatibility between the actual male thread aligning with the taper lead tap.

Peter re crows foot spanner - I've got a couple of bent ones! There is very little upstand however to create a couple of flats without damaging the rubber and so might just take it very slowly and see if the rubber is strong enough...

Fred (Nippy) suggested using a dowty washer which I have from repair of the Churchill pump...

Time to be brave and see what happens...

richardACS wrote: ↑Fri Mar 21, 2025 2:06 pm Thank you Alan re the tape - now ordered..

Thank you Ronnie re thread types - my thread gauge seems accurate for a 9/16 unf 18tpi and this is born out by a compatibility between the actual male thread aligning with the taper lead tap.

Peter re crows foot spanner - I've got a couple of bent ones! There is very little upstand however to create a couple of flats without damaging the rubber and so might just take it very slowly and see if the rubber is strong enough...

Fred (Nippy) suggested using a dowty washer which I have from repair of the Churchill pump...

Time to be brave and see what happens...

If you Google injector socket > https://www.bing.com/images/search?q=in ... C3&first=1 make one out of an old deep socket or even a piece of pipe with a grinder, don't worry to much as 300-400 psi is not really high pressure or you could use an anaerobic ( Loctite ) type of sealer.

Peter Laidler wrote: ↑Thu Mar 20, 2025 7:33 pm I am minded to suggest that you turn to page 1 of this thread and have a look at MAB/Martins photo of a sectionalised hydro unit.

Cutting the bonded-in ferrule was a bad move in my opinion. Because you don't have a decent thickness of metal to hold the new thread (and the pressure that the thread is under). That said, it LOOKS like the thread on the new hose is a taper thread that will lock itself as it is tightened. But that is yet another problem because the rubber will want to rotate with the tap as you attempt to tap the thread. And it'll CERTAINLY want to rotate when you try to tighten up the taper thread.

As for drilling/tapping deeper, I think that you are now coming hear to head with the internal valving - that is probably shot-out in any case.

Let us know your further thoughts Richard

Your comment re "decent thickness of metal to hold the new thread" has resonated. The outer dia of the metal tube encased in rubber is 16mm dia, the OD of the male thread is 13.3mm and so to your point a bit of a risk especially if the drill prior to tap is slightly off centre.

I'm thinking therefore to make up a short adaptor on the lathe - lets say with a 7/16 unf male and a 9/16 unf female but I wonder what internal hole size I can get away with (within those O/D dimensions) to avoid a pinch point flow restriction. The standard pipe has an I/D of 6.5mm - would appreciate your thoughts...

Interesting....... Your Q is like a problem that the tutor would throw in to the uni group of students to ponder.

You mention the pinch point dia of the fluid transit point. Without going into the intricacies of the fluid movement, don't forget that the REAL pinch point in the F to R fluid line is the 5mm (?) measure this though) dia hole through the F to R adaptor fixed to the front bulkhead cross member.

So in my opinion, your 6mm guestimate will be fine.

It has been said that the works bushed this adaptor hole to a smaller dia to stiffen the suspension. How true it is I don't know!

Just re-read and digested your thread....... If the i/d of the hydro unit in/out let tube is 6.5mm......... I'm not so sure........ If it were me, in a fix, I'd machine up a slave/dummy 'tube' and see what I could get away with. As Ronnie has said earlier. In the world of hydraulics, 300psi is but nothing and easily sustainable with our thread systems

Peter Laidler wrote: ↑Sat Mar 22, 2025 12:28 pm Interesting....... Your Q is like a problem that the tutor would throw in to the uni group of students to ponder.

You mention the pinch point dia of the fluid transit point. Without going into the intricacies of the fluid movement, don't forget that the REAL pinch point in the F to R fluid line is the 5mm (?) measure this though) dia hole through the F to R adaptor fixed to the front bulkhead cross member.

So in my opinion, your 6mm guestimate will be fine.

It has been said that the works bushed this adaptor hole to a smaller dia to stiffen the suspension. How true it is I don't know!

Just re-read and digested your thread....... If the i/d of the hydro unit in/out let tube is 6.5mm......... I'm not so sure........ If it were me, in a fix, I'd machine up a slave/dummy 'tube' and see what I could get away with. As Ronnie has said earlier. In the world of hydraulics, 300psi is but nothing and easily sustainable with our thread systems

Thanks Peter. I'll get on the lathe and complete the tap and die work and get a feel for what I can get away with...