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Chris Miller
Prolific User
Username: cjm51213

Post Number: 193
Registered: 5-2013
Posted on Saturday, 16 August, 2014 - 10:50 am:   Edit PostDelete PostView Post/Check IP

Hi Folks,

Is there a comprehensive list of o-ring sizes anywhere? The manuals talk about replacing the "sealing ring", which is apparently Crewe for "o-ring", but they don't tell me what size it is, and we know from experience that measuring a worn o-ring can, and probably will, lead to errors. So, it seems like there would be a comprehensive chart of o-ring sizes of various applications.

If this doesn't already exist, then this seems like something Brian Vogel would have squirreled away somewhere... Wishful thinking?

Thanks for the help,

Chris.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 976
Registered: 6-2009
Posted on Saturday, 16 August, 2014 - 12:03 pm:   Edit PostDelete PostView Post/Check IP

Chris,

To my knowledge there exists no list of O-rings used in all the various locations they're used in on these cars. The seals tend to come as kits with a Crewe Original part number.

That being said, it has been my mission to document each and every O-ring in each and every kit I've either purchased or that's been kindly loaned to me by someone about to rebuild something. The cost of the seal kits, most of which are nothing more than a collection of a few O-rings, is obscene for what they are.

I have created a document that gives the O-ring sizes and, where I've done the rebuilding, the location of use, for the RR/Bentley SY Hydraulic Kits. The O-ring for the header tank coolant probe is an AS568A-208.

Every O-ring I have measured, and I've done most of them more than once, is an AS568 standard O-ring. In the hydraulic system and cooling system they're made of EPDM.

If you end up buying kits please either send them to me to measure or send me the measurements so I can add more kits to the document.

By the way, measuring a worn/used O-ring is still a good way to figure out what you need. It may be a bit compressed, but even in that state it's probably going to very closely match only one of the AS568 dash numbers.

Brian, who just sent out another two ACV rebuild seal sets out to someone in NC
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 455
Registered: 5-2012
Posted on Saturday, 16 August, 2014 - 02:03 pm:   Edit PostDelete PostView Post/Check IP

Chris

If you do buy a kit make sure you measure the O-rings to check that they conform to the AS568A standard. The actual sizes are different from the nominal sizes by a difference significant enough to cause a fluid leak. For example, the accumulator to ACV O-ring is, from Brians chart, an AS568A-218. This has the nominal dimensions of ID= 1 1/4" (1.250) and CS= 1/8" (0.125). However, if you check the AS568A charts, which can be found online, the actual dimensions are ID= 1.234" and CS= 0.139". Note that the actual dimensions are slightly greater than the nominal dimensions. If you fit an O-ring where the actual dimensions are the same as the nominal dimensions i.e. not to the AS568A standard, I guarantee that after a few hundred miles they will start to leak. I know this as it happened to me, even though the kit I bought was from a reputable supplier. The O-rings Brian sent me, that conformed to the AS568A standard have worked perfectly.

Geoff.
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Chris Miller
Prolific User
Username: cjm51213

Post Number: 194
Registered: 5-2013
Posted on Sunday, 17 August, 2014 - 03:24 am:   Edit PostDelete PostView Post/Check IP

Hi Geoff,

So, this raises an interesting question...

If I get an o-ring that is AS568A-X, does this mean it may come in either the nominal dimension or the nominal dimension plus tolerances, and I can't be sure which because both are covered by the specification?

Or does this mean that the AS568A-X dash designation will uniquely and correctly define an actual dimension, which will be close to but probably greater than the nominal dimension?

Meaning, if a design calls for an AS568A-X o-ring, and I buy this, will I still be in jeopardy of having an undersized o-ring because the manufacturer used nominal dimensions? I guess the more precise question is, how were the unsatisfactory o-rings you used specified -- nominally or with an AS568A-X designation?

I've read much of this Parker-Hannifin O-Ring Handbook, and I suspect I'm not the only one.

Thanks for the help,

Chris.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 977
Registered: 6-2009
Posted on Sunday, 17 August, 2014 - 03:42 am:   Edit PostDelete PostView Post/Check IP

Chris,

I'm not Geoff, but I'll chime in here.

All O-ring specs are for nominal dimensions with a +/- tolerance. I don't know of a sizing specification for any sort of seal that doesn't work this way. Since seals are "a bit large" and get squished, anything that fits the nominal +/- tolerance is accounted for in the design of the thing the seal is used on.

Geoff posted here when he had that seal failure, and the seal supplied in the kits he purchased were, quite simply, of the incorrect size.

The figures in my document for the ACV rebuild kit, and the accumulator rebuild kit seals are now running around on both of my cars and numerous others that I've supplied with the seals for periods of years now. They work for the stated applications in the stated AS568 dash sizes.

Brian, who now knows that things that use them are engineered around standard seal sizes, not the other way around
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 457
Registered: 5-2012
Posted on Sunday, 17 August, 2014 - 07:45 am:   Edit PostDelete PostView Post/Check IP

Chris

Brian is right - I simply used O-rings of the incorrect size. However, please don't think I was trying to cram a 1 3/4" O-ring into a 1 1/2" housing - they were the wrong size by just a few thou.

From your entry:

"Or does this mean that the AS568A-X dash designation will uniquely and correctly define an actual dimension, which will be close to but probably greater than the nominal dimension?"

I believe this assertion to be correct. Taking the cross sectional diameter as an example, where the nominal size is 1/8, all the charts I have looked give an actual size not of 0.125" as would be expected, but as 0.139". My question would be, why would the charts list an actual size so much larger than the nominal size? Most charts do not specify a tolerance although some give a tolerance of +- 0.004". So even taking the tolerance into consideration, the minimum value for a 1/8" nominal cross sectional diameter would be 0.135", 10 thou greater than the nominal value. This is why I believe the actual CS value for an AS568A-218 O-ring is 0.139" and not 0.125".

I carefully measured the depth of the channel that the sphere to ACV O-ring fits in. It is 0.120" deep. An O-ring of 0.125" CS diameter would therefore only be proud of the housing by 0.005". This, to resist 2500 psi of pressure. The 0.139" diameter means that nearly 0.020" of O-ring is proud of the housing - much more substantial.

Geoff
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Chris Miller
Prolific User
Username: cjm51213

Post Number: 196
Registered: 5-2013
Posted on Sunday, 17 August, 2014 - 08:11 am:   Edit PostDelete PostView Post/Check IP

Hi Geoff,

I think I see. Had you known the AS568-X designation and ordered an o-ring based on that, then none of this would have happened. So, if we know the AS568-X designation, then we can rely on that. Right?

Thanks for the help,

Chris.
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Bob uk
Unregistered guest
Posted From: 94.197.122.81
Posted on Sunday, 17 August, 2014 - 11:14 am:   Edit PostDelete PostView Post/Check IP

Geoff,
You said after a few hundred miles they start to leak.
About 20 years ago I fitted a valve to a trucks tipping gear. 3 months later the O ring is leaking.
So I put a new one in. Same thing happened again. So I got another from a different supplier. No leaks. The new o ring was 10 thou fatter.

I think what happened is that the threads seal themselves but not quite. The leaking oil takes ages to reach the seal, in this case 3 months. Then the oil found the seal built up pressure and because the seal was to thin it didn't have enough nip. And once the oil got past the pressure pushes the seal down and oil dribbling out of the valve.

(Message approved by david_gore)
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 458
Registered: 5-2012
Posted on Sunday, 17 August, 2014 - 12:41 pm:   Edit PostDelete PostView Post/Check IP

Chris

Absolutely - this is my contention. When I measured the failed sphere/ACV O-ring on my car it had a cross sectional diameter of 0.128", 11 thou less than the AS568A spec. In my view, this is the 11 thou that prevents RR363 appearing as a puddle on your garage floor.

But here's a thing. The design of these cars pre-dates the AS568A spec. They were designed in the 1960's and the spec was introduced in the early 1970's. Furthermore, AS568A is an American standard and the cars are British. I am in no doubt that Brian is correct when he specifies these O-rings. He has measured the originals and has sent the AS568A equivalents to many owners, myself included, and has had no reports of O-ring failure. It would be interesting to speculate on why the AS568A O-rings work. Maybe the Americans designed the spec to conform to earlier standards or maybe RR just used the soon to be introduced new American spec.

Geoff
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 459
Registered: 5-2012
Posted on Sunday, 17 August, 2014 - 12:51 pm:   Edit PostDelete PostView Post/Check IP

Bob,

I absolutely agree with you. What you describe is precisely what happened with my car. The leak didn't show for a few months and then got steadily worse. This is why whenever anyone mentions O-rings I'm here yelling "measure them". I'm trying to save them the pain of having to re-do the job again a few months later.

Geoff
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 978
Registered: 6-2009
Posted on Sunday, 17 August, 2014 - 02:43 pm:   Edit PostDelete PostView Post/Check IP

Geoff,

I'm too tired to post "the history of AS568" but it is the an offshoot of an SAE spec, and even though the SAE is based in the USA, they're pretty influential as far as "de facto" worldwide standards (during a certain era, anyway). See the "Dimensional Tolerances" section on this page which gives a brief history of the ARP568 spec, the progenitor of AS568 (in all its lettered [AS568A-D] forms) which came out in the late 1950s and was adopted quite widely.

I will honestly admit that the calipers I've used to measure things aren't nearly as precise as the stated tolerances in the specs. My measurements far more closely match the nominal values, and that's how I ended up mapping the O-rings to their dash numbers.

I started taking photos of the measuring process later on, but was not doing that when I started this process of documenting the seal kits.

Unless you have some very high precision calipers you (for any you) are going to pretty much have to believe the seal manufacturer as far as their statements regarding ring size.

Brian