Once you can see them bulging they're beyond bad, their ESR is probably 100x higher than it's supposed to be, so they've become almost worthless in the noise filtering function that they are needed for.
Sometimes motherboards can be surprisingly tolerant of the caps. You might get away with running the board like that - many people have done it. But in the long run these and other caps on the board are likely to keep getting worse. Sometimes a bad cap can short out, which will cause more serious component damage if it happens. Usually though you just get lots of noise on the affected voltage rails. This noise could also lead to component damage in the long run. So if you wait too long, it's possible it will fail in such a way that recapping will no longer be enough to fix it.

A board that still works is the most promising candidate for recapping because you know it should work when it's done. It's not as promising to do a recap on a board that's no longer running, because at that point you have to worry about whether the recap will still save it.

All similar caps to the ones that failed should be assumed equally prone to fail, so it's best to replace all of them. They all came off the same reel.
Keep in mind that bad caps don't always bulge, though the typical failure with motherboard caps does usually lead to that symptom. But even bulged caps had gone way out of spec *before* you could see them bulging.

Ideally I'd also replace those 6 bluish caps in the foreground, which are probably on the Vcore supply to the CPU. They look like OSTs to me, or whatever they are they aren't one of the few reputable brands anyway. But if you haven't done this before I can understand preferring to leave them alone.
The 4 brown caps with the "Mercedes" vent are made by Nippon Chemicon which is a good brand, but they might be KZG series which are an exception - KZG is failure prone. In that case, same remark as what I said for the OSTs.

Robotanv wrote on 2024-02-22, 23:00:

Hi All, […]
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Hi All,

In the process of building my retro 98/XP rig and all going well.
98 and XP installed.. Running without BSODs / crashes, devices working ok but...
I noticed some of the capacitors were bulging, fortunately without any obvious leakages.
The only ones bulging are 1000uF 6.3V caps so I've ordered new ones and planning to replace only these.
Just wanted to confirm that replacing these bulgers is actually necessary even if everything seems to be working?

Here's a pic of most of the bulging caps (circled) alongside apparently ok ones.
Appreciate any help!

I had 11 bulged caps (3 with leakage) on a mobo that was running flawlessly... But I would sweat from the inside each time I was turning the computer on. Since I was at it, I replaced any caps I could. Somehow, it was only a very specific type 10v 1000uf... So I would replace them all to be on the safe side.

Thanks shamino and Linoleum.
Now that I see the bad caps, I feel uncomfortable turning the system back on!
I'll replace the bulging caps to start with and if that goes without incident, I'll likely order replacements for the others.
There are some tiny (non-bulging) caps dotted around that I can't even see the values unless I remove them. Am feeling reluctant to tamper with those!

I have a dead Shuttle xpc sn41g2 that I examined, which didn't have any bulging caps but wonder whether bad caps are the problem here. I may do a practice run on that motherboard to start with.

Just to share that I've recently decided to finally get to it and start replacing capacitors for the first time. I had 4 socket7 and slot 1 motherboards with bulging, leaking caps and replaced them all. As recommended by Shamino, I assumed all similar capacitors were to be replaced even if looking good as they are prone to fail at some point. I never seen any tiny caps you mention looking bad so didn't touch those, always "big" ones.

Out of the 4, 3 posted just fine after replacing the caps and it has been so gratifying! I only have 1 mobo left that still does not post, a Shuttle HOT-597, I will probably post a new thread for some help about it at some point. It's not executing code at all. But the other 3 I've thoroughly tested and they work flawlessly so that's great, feels amazing to be able to give a new life to that old stuff! 😀

Boohyaka wrote on 2024-02-23, 09:40:

Just to share that I've recently decided to finally get to it and start replacing capacitors for the first time. I had 4 socket7 and slot 1 motherboards with bulging, leaking caps and replaced them all. As recommended by Shamino, I assumed all similar capacitors were to be replaced even if looking good as they are prone to fail at some point. I never seen any tiny caps you mention looking bad so didn't touch those, always "big" ones.

Out of the 4, 3 posted just fine after replacing the caps and it has been so gratifying! I only have 1 mobo left that still does not post, a Shuttle HOT-597, I will probably post a new thread for some help about it at some point. It's not executing code at all. But the other 3 I've thoroughly tested and they work flawlessly so that's great, feels amazing to be able to give a new life to that old stuff! 😀

That's very encouraging and reassuring to hear!
I think I'll likely replace all the big caps too once I get into the swing of things.
I really hope I can bring my shuttle xpc back to life with a re-cap. That was one of my favourite rigs back in the day!

Judging by the pictures, the motherboard in question is from AsRock (939A8X-M?).
The small bulging capacitors are likely OST brand, and either RLP or RLS series (likely the former), rated for 6.3V and 1000 uF, 8 mm diameter. If so, these are known to go bad very frequently (I've encountered many of these bad on Biostar motherboard too.) I would suggest replacing all of them at once, just like shamino said.
The bigger dark blue ones by the CPU are very likely OST RLX series (probably 6.3V 2200 uF or 3300 uF). These are so-so for reliability - sometimes they last for years and never give problems, other times they go bad pretty quickly. ASUS an AsRock motherboards usually have pretty well-designed CPU VRMs, so in my experience, OST RLX caps tend to do OK there. If you want to replace them, you can (and probably should for extra long-term reliability.)

Just BEWARE that ASUS and AsRock motherboards mark the negative side on the circuit board with the NON-SHADED side of the semi-circle. The SHADED (typically white) semi-circle side is where the POSITIVE lead of the electrolytic cap should go. This is backwards to how all other motherboards and electronics are done. So be careful not to install your new capacitors backwards, because they will blow up / bulge / leak almost instantly.

Also, start by replacing the small OST RLP/RLS caps first, and then attempt the bigger blue ones. The smaller ones should come out fairly easy with a decent soldering iron. The big blue OST RLX ones you might struggle with a bit, because they are connected to a thick copper plane on the CPU VRM. Using a soldering iron with a big, (dull) chisel-shaped copper tip would be advisable here... unless you have a T12 tip soldering station, of course (the latter is much better.) Avoid using a soldering iron with a small pencil tip - these are usually no good for motherboard work.

Robotanv wrote on 2024-02-22, 23:00:

The only ones bulging are 1000uF 6.3V caps so I've ordered new ones and planning to replace only these.

Just curious, what capacitor brand and series did you get? Hopefully it's not some no-name stuff from eBay, Amazon, or Ali. That's not to say there aren't OK capacitors sold on those platforms, but there are quite a few countefeits and no-name garbage brands sold there. My general advice to novice recappers is to avoid these. If you have a known good supplier in your country (Mouser, Digikey, Farnell, RS components, and etc.), I would suggest to go with those instead and look for these Japanese manufacturers: Rubycon, Panasonic, United Chemicon, and Nichicon.

In the case of the 6.3V 1000 uF OST RLP/RLS, I often use Rubycon ZLQ (6.3V 1200 uF) or ZLH series (6.3V 820 uF). I think Panasonic FR/FS series also offers caps with similar ratings and sizes, but it's been a while since I checked. You may be able to also get NOS (new old stock) Panasonic FL/FJ on eBay. Forgot which series corresponded to these from Nichicon and UCC, but I think HW and KZH/KZM respectively.

shamino wrote on 2024-02-22, 23:54:

The 4 brown caps with the "Mercedes" vent are made by Nippon Chemicon which is a good brand, but they might be KZG series which are an exception - KZG is failure prone.

Yup, KZG are indeed failure-prone, though their 16V -rated caps usually seem to fare a little better.
That said this motherboard appears to have KZE series, which have excellent reliability. So those can stay.

Robotanv wrote on 2024-02-23, 09:22:

I have a dead Shuttle xpc sn41g2 that I examined, which didn't have any bulging caps but wonder whether bad caps are the problem here. I may do a practice run on that motherboard to start with.

Have a look inside the PSU of that system. Shuttle PSUs are pretty decent but often came with OST -branded caps, which sometimes didn't last too long.
Alternatively, try the system with a standard ATX PSU. If it doesn't work, then the issue really might be on the motherboard. Caps were a common problem for Shuttle PCs. Even if none look bulged, some (or many) can still be bad. It really depends on what brand of caps they used on the motherboard. I've seen quite a few crap brands fail "silently" without any signs of bulging, but their capacitance and leakage current was through the roof.