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Need replacement O ring - how to choose?

Need replacement O ring - how to choose?

Need replacement O ring - how to choose?

I've got an o ring that seals the oil pickup filter in an automatic transmission (ZF 4HP22). I contacted ZF, but so far no reply and I'm in a hurry to get the car back onto the road.
The O ring that came from the car is obviously not elastic anymore (24 yrs old most likely, as old as the car).

I can buy those from a nearby automotive shop, that sells them per piece and they have an enourmous range of dimensions and materials.

The o ring goes over an locator extrusion on the filter side, about 2-3 mm high (external dimension 39.30 mm diameter), and when in position, into a recess with internal diameter 42.75 mm
Current thickness of the o ring is 2.2 mm.

How do I choose the correct dimension?

As for the material, I guess with NBR shore 70, I can't do much wrong (is resistant against hydraulic oils like Dexron III, which I am using), and is very common. Please correct me if this is not a good choice.

RE: Need replacement O ring - how to choose?

That is indeed a possibility, but I prefer the option where I come into a shop, prepared with the necessary information, so that I do not depend on the experience or willingness of the guy that's paid minimum wage to help the customers to sell me the correct ring of which they probably got hundreds in stock, based on what information, a worn-out, hardened o ring of unknown dimensions?

I was hoping for someone to tell me that for instance a 3-5% stretch of the inside diamater would be OK (?), en how to determine the needed cross section.

RE: Need replacement O ring - how to choose?

I'd go through a selection process based on the application, existing groove and the parker guide o ring guide or their calculator



and then go to harbor freight (if Buna is OK) and get the $8-10 O ring kit there. Otherwise, for other materials order from mcmaster carr and you'll have tomorrow or the next day most likely.

Edit, oh, I see, good supply at the shop.

RE: Need replacement O ring - how to choose?

The shop will have a sizing chart or card for you to compare your o-ring to.
In reality there are only a few specific cross section sizes. Though you need to know if this is metric, imperial, Japanese, or some other standard.
If metric a 2.4x37.6 sounds likely, though it could be a 2x37.6.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

RE: Need replacement O ring - how to choose?

And really, if it's an automotive application, it's almost surely cataloged and cross referenced. I hear they have cut down to essentials to 5S for covid sanitation but to get an idea, go to the main NAPA store for your city and look for the old guys sitting behind or on top of a mountain of catalogs. They're like the IC and transistor catalogs my dad used to have lining the wall.

RE: Need replacement O ring - how to choose?

@ Moon, it looks like the calculator gives groove dimensions based on o-ring size, but I can't see if it also works the other way around. I'll play a bit with this...
Also, there is no old guy there, just a pimple face that hands you your order. I really haven't got any luck, the "good" parts store I always go to is closed until further notice.

@ Ed, I called, they have diameter 37.0; 37.21; 34.42; 37.5; 37.6; 37.77; 37.80; ... in cross section diameters every tenth mm...

RE: Need replacement O ring - how to choose?

There are only a handful of standard sizes but there are millions of sizes in actual use. Of the dozens of o-rings I've used in products I've designed, very few of them are standard sizes. I would not take for granted that the subject o-ring is a standard size. The same goes for compounds. I would go to a dealership or an online place like Rock Auto and buy the original part.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Need replacement O ring - how to choose?

Somewhere I have the original paper in electronic form, the engineer who designed the Chevrolet Corvair air cooled car engine gave a lecture about the engine design. There was a question about oil leaks from the pushrod tubes. His reply was that the o ring material and sizing were critical. These were factors not in the possession of makers of non oem parts.
'Our o rings do not leak' he asserted.

RE: Need replacement O ring - how to choose?

Thx for the replies.
I will wait for the original manufacturer to get back to me and report back.

RE: Need replacement O ring - how to choose?

It seems to me that the calculator can still be usable, but you'd need to iterate a bit. You already know the groove characteristics, ID/OD/depth, and using https://www.efunda.com/designstandards/oring/desig... you can enter a nominal o-ring and see whether you achieve 40$ compression.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Need replacement O ring - how to choose?

Thx for that link.

This gets me to a 37.5-38.0 X 2.2-2.8 O ring. Seeing as those cost <1$; I'll slap on one of these (38 x 2.5) until the OEM gets back to me.

RE: Need replacement O ring - how to choose?

Don't hold your breath for ZF to get back to you. Go through the normal replacement parts channels, you can get almost any part in a day or two.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Need replacement O ring - how to choose?

DO NOT BUY FROM YOUR LOCAL AUTOMOTIVE SHOP! Well, not out of the o-ring bin. The o-rings they sell are NBR rubber and will have a very short life at the temperature seen in an automatic transmission. You want to use HNBR at a minimum and FKM would be the premium option.

When selecting automotive o-rings the flavor options are typically:

NBR - low temperature air, water, and oil applications. These should be avoided at all costs as they will have a very short life at normal engine temperatures (especially in water). They also deteriorate in sunlight.

HNBR - good all purpose material. This works for high temp oil and is generally acceptable for water. It's also used for refrigerant.

Silicone - Good for high temp water and air but is highly permeable and will seep oil. It has poor mechanical properties as well.

EPDM - the best choice for most water and brake fluid applications. It's highly sensitive to damage from contact with oil.

Viton/FKM - the most expensive choice. The best choice for oil and fuel at all temperatures but does not perform as well as EPDM and HNBR for water.

Neoprene - good mechanical properties with average performance.

RE: Need replacement O ring - how to choose?

Thx a lot for that. This shop has viton in their selection, I'll get one of those.
I guess design is similar (few % elongation, 40% compression) ?

RE: Need replacement O ring - how to choose?

Viton o-rings are usually slightly harder, Shore 75A vs 70A for NBR. There is no practical difference. Dimensions are identical.

RE: Need replacement O ring - how to choose?


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