[1.6 FSI] - Thermostat Housing Issue O-ring

E

Evripidis

Member
Hi all,

I am after technical opinions on the thermostat housing. The housing is fairly new having being replaces a couple of years back. I have since then taken it out to replace some other bits and put it back with the same o-ring. Now I have taken it back out and because it was leaking. When brought against a straight edge ruler the gap seems excessive. I used a marker and a gave it a slight rub with the file and got the following picture. There is supposed to be some gap for the o-ring to squish but I believe that it is excessive on mine hence the leak.


IMG_20220627_064751_964.jpg
 
Not very technical, but try holding the housing, with the "O" ring, against a piece of glass, to get an idea of how compressible the "O" ring is and how much of it is accommodated by the recess in the housing. This will help estimate how it fits when re-installed on the head.
Mac.
 
I think that the housing is distorted somehow, can't be otherwise. Unless the brand new o-ring which I do not have is extremely thicker than the used one that I have.
 
Can you measure the width, and depth of the slot in both parts?
If they are both the same, you'll know that the new part is the same as the one removed.
There are online "O" ring calculator that will give you an idea of the size you want.
Mac.
 
The trouble is that it is an oval-shaped o-ring. I noticed that on the later FSI models this has been revised to a thicker and round version o-ring.
 
If the revised thicker round O ring is long enough to fit into the oval seal groove then that could be the answer provided it does not pull out of the groove.
 
Evripidis said:
The trouble is that it is an oval-shaped o-ring. I noticed that on the later FSI models this has been revised to a thicker and round version o-ring.
Click to expand...
If the slots for the "O" ring are of equal width and total depth (depth of each added together), then I think that suits a round "O" ring.
Mac.
 
The revised o-ring comes with a revised thermostat housing for the A3 (or is it the A1?). I have taken the housing to the local o-ring supplier that I use and the closest one we could find gets seriously distorted once in place. Because of the oval groove that is.
 
Yes, I know thank you. I have already contacted Andy who will be forwarding me one by post. Along with some other niggly bits!
 
I am getting lost with the vocabulary.

The shape of the aperture is NOT oval in my mind but unfortunately half the internet think it is, I agree with the other half who think not. The only problem is what to call if not oval, there are lots of contenders. Being a simple soul I like 'running track' but others are 'pill shaped' (not bad until you think about it and realise many pills are round), 'discorectangle' (ugh), 'squectangle' (getting silly), 'obround' (like), 'sausage body' (I don't want to eat it), an exotic 'sphenoellipsoid', 'rounded rectangle' (sensible) and many others but wait for it, the official winner is STADIUM.

Enough of this nonsense so on to what matters the cross-sectional shape. The groove is near enough SQUARE and the seal OVAL(ish), not round. (oval makes a comeback!).

It's late back tomorrow with some pictures.

Andy
 
Andrew said:
I am getting lost with the vocabulary.

The shape of the aperture is NOT oval in my mind but unfortunately half the internet think it is, I agree with the other half who think not. The only problem is what to call if not oval, there are lots of contenders. Being a simple soul I like 'running track' but others are 'pill shaped' (not bad until you think about it and realise many pills are round), 'discorectangle' (ugh), 'squectangle' (getting silly), 'obround' (like), 'sausage body' (I don't want to eat it), an exotic 'sphenoellipsoid', 'rounded rectangle' (sensible) and many others but wait for it, the official winner is STADIUM.

Enough of this nonsense so on to what matters the cross-sectional shape. The groove is near enough SQUARE and the seal OVAL(ish), not round. (oval makes a comeback!).

It's late back tomorrow with some pictures.

Andy
Click to expand...
In my mind "O" rings are round in section, and the radial slot into which they fit are rectangular in section.
There has to be space for the "O" ring to occupy when the two halves of the joint are brought together. An "O" ring does not form a seal by being compressed, it seals by changing the shape of it's cross section, and needs the space to do so. It is the cross sectional area that is important, so can be circular, or oval, (but not square).

Mac.
 
Andrew said:
I am getting lost with the vocabulary.

The shape of the aperture is NOT oval in my mind but unfortunately half the internet think it is, I agree with the other half who think not. The only problem is what to call if not oval, there are lots of contenders. Being a simple soul I like 'running track' but others are 'pill shaped' (not bad until you think about it and realise many pills are round), 'discorectangle' (ugh), 'squectangle' (getting silly), 'obround' (like), 'sausage body' (I don't want to eat it), an exotic 'sphenoellipsoid', 'rounded rectangle' (sensible) and many others but wait for it, the official winner is STADIUM.

Enough of this nonsense so on to what matters the cross-sectional shape. The groove is near enough SQUARE and the seal OVAL(ish), not round. (oval makes a comeback!).

It's late back tomorrow with some pictures.

Andy
Click to expand...
What about a Superellipse?
https://en.wikipedia.org/wiki/Superellipse
 
Although the O-ring was originally so named because of its circular cross section, there are now variations in cross-section design. The shape can have different profiles, such as an x-shaped profile, commonly called the X-ring, Q-ring, or by the trademarked name Quad Ring. When squeezed upon installation, they seal with 4 contact surfaces—2 small contact surfaces on the top and bottom. This contrasts with the standard O-ring's comparatively larger single contact surfaces top and bottom. X-rings are most commonly used in reciprocating applications, where they provide reduced running and breakout friction and reduced risk of spiraling when compared to O-rings.

There are also rings with a square profile, commonly called square-cuts, lathe cuts, tabular cut or Square rings. When O-rings were selling at a premium because of the novelty, lack of efficient manufacturing processes and high labour content, Square rings were introduced as an economical substitution for O-rings. The square ring is typically manufactured by molding an elastomer sleeve which is then lathe-cut. This style of seal is sometimes less expensive to manufacture with certain materials and molding technologies, especially in low volumes. The physical sealing performance of square rings in static applications is superior to that of O-rings, however in dynamic applications it is inferior to that of O-rings. Square rings are usually used only in dynamic applications as energizers in cap seal assemblies. Square rings can also be more difficult to install than O-rings



Excerpt taken from the O-ring Wiki.
 
I have two A2 FSI thermostat housings here.

The 7zap diagram linked above does not match perfectly reality, the block/housing aperture seal is shown as round yet it is stadium shaped, okay it is only schematic so it has license to deviate but you would not expect major functional differences, for example the large radiator flow leaves the housing in the wrong direction on the diagram, it should be at right angles, you can see it on Evros's diagram and mine below (if you know what you are looking at). I wonder if the diagram is for some other 1.6 FSI housing, eg GOLF FSI even though it is linked from the A2 FSI coolant page.

The first housing I have is new made by Mahle who I have good reason to think is Audi's supplier for this housing. The seal I described yesterday.

Thermostat Block _Housing Seal - 1.jpg


Thermostat Block_Housing Seal - 2.jpg


Thermostat Block_Housing Seal -3.jpg


Very difficult to measure accurately but the seal's 'oval' cross section approximately measures 2 x 4 mm.

The second housing I have is marked Audi and used. The seal on this one is squashed flat from its working life but worth commenting there is very little standing proud to seal the joint, by eye probably 0.5mm.

Used Audi Thermostat Housing - 1.jpg
Used Audi Thermostat Housing - 2.jpg
Used Audi Thermostat Housing - 3.jpg


A trivial point, the seal has 'ears', 4 little tabs, visible in the first photo and more so here

Block_Thermostat Housing Seal.jpg


Can't see the ears have any functional use, must be a leftover from the manufacturing process but useful for getting a fingernail on to pull out the seal, maybe that is their function?!

Andy
 
Last edited:
PlasticMac said:
Hi, Andrew,
Is the radial slot rectangular in both housing, and of similar dimensions?
Mac.
Click to expand...
I am struggling with "both housing" in the question, what other housing? As I understand it the other side of the joint is the block face to which the housing is bolted which I assumed is machined flat, no groove. You will have to confirm this with Evros.

Andy
 
The opposite side is entirely flush aside from some corrosion (admittedly, maybe severe pitting in my case) but well inside the o-ring internal perimeter.

Mahle is the OEM supplier for this one I can confirm.

Andy, is there any chance for you to check the new housing for flatness with a straight edge maybe?

Evros
 
Andrew said:
I am struggling with "both housing" in the question, what other housing? As I understand it the other side of the joint is the block face to which the housing is bolted which I assumed is machined flat, no groove. You will have to confirm this with Evros.

Andy
Click to expand...
I meant the Audi used part, and the new Mahle. Since they're probably separated by quite a few years in production.
Just curious if the possible change in "O" ring cross section was matched with a change in the radial slot.
Mac.
 
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