Is there some reason you can't bring the o-ring to the shop and ask them?

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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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.

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

http://solutions.parker.com/ORingSelector

https://www.parker.com/literature/Praedifa/Catalog...

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.

Geez, I was gonna ask merc or beemer, but that transmission is far more widely used than that:
https://en.wikipedia.org/wiki/ZF_4HP22_transmissio...

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

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.

@ 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...

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.

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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.

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.

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

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
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

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.

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.

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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.

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.

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) ?

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

Thanks!