The advice everyone gives is "read the number off the chamber". It is the right advice, and on a chamber that has spent years under a trailer it is often useless. Paint covers the stamping. Corrosion eats it. Some parts never carried a number in the first place — we supply sizes where the moulding has no code on it at all.
So: no number. Here is what still works.
1. Find the Type before you look for anything else
The Type is the one marking most likely to survive, because it is usually stamped into the chamber body rather than printed on a label. Type 16, Type 20, Type 24, Type 30 — the figure is the effective area in square inches, and it is the number the whole industry sizes by.
Clean the housing and look again before concluding it is gone. A wire brush and a wipe of solvent recover more markings than people expect.
If you find the Type, you are most of the way there. What remains is the stroke, and that is where the next two routes come in.
2. Narrow by vehicle and axle — with the caveat attached
Our product pages list vehicle applications, and each page names the catalogue the data came from. That is real, sourced information and it is not the vehicle manufacturer's own fitment record. The same model in a different production year can carry a different chamber.
So treat this route as narrowing, not deciding. It gets you to one or two candidates. It does not end the question.
3. Compare the part you are holding against photographs of the real one
This is the route most suppliers cannot offer, because most product pages show one rendering and nothing else.
Every product page on this site carries six photographs of the actual part: the front face with whatever code is moulded into it, a macro of the bead band, the cord texture, and the side profile from two angles. The side profile is the one that answers the stroke question — a long-stroke diaphragm has a visibly deeper dome, and once you have the two side by side the difference stops being subtle.
Put your old part next to the photographs on the product pages. If the bead profile, the dome and the face match, you have your answer without a single number.
The trap: do not decide stroke from a moulded letter
Some parts carry a letter moulded into the face. It is tempting to read it as the stroke — an L for long, say.
Do not. The letter comes out of the tool, and on parts we supply it does not reliably indicate stroke: there are normal-stroke mouldings that carry an L. We publish photographs of the markings exactly as they are rather than retouching them, which means you will see this for yourself on our pages.
Decide stroke from the Type plus the physical profile, or from a cross-referenced OEM number. Not from a letter.
When you still are not sure
Two things settle it, and both are faster than another hour with a wire brush.
Send photographs. The old part, the chamber body, and whatever marking you can make out. A person reads them, not a form — that is what contact is for.
Order samples. Up to ten pieces, free of charge, you cover the shipping. Fitting one is the only test that answers the question completely, and it costs less than a wrong shipment of a thousand. The free sample page has the details.
If the number is readable after all
Then start there instead, and use it as a check rather than a conclusion — we wrote about why in what an OEM number tells you. The cross-reference takes any format, with or without spaces.