If you're really paranoid, one way to ensure you aren't overheating is to buy a big 'ol Socket 370 heatsink, and stick it on Socket 7. It's essentially a perfect match, and the TDP's of the 1GHz+ Pentium III's are much higher than SS7 CPUs at stock voltage, but even with the 2.6v I doubt it will even be able to attempt to overtake even a middle-range Socket 370 cooler. I actually have a Socket 370 cooler designed for little Celerons installed on my Pentium 200 MMX, and it's twice the size of the Socket 7 cooler that originally came with the system.

In terms of your heatsink not getting hot, this can be caused by one of two things: A) your cooling is so ridiculous that the heatsink never even gets a chance to get hot, or B) the conductive bond between the CPU and heatsink is not good enough. One way to test this is to unplug your system fans and CPU fans temporarily, and turn the system on and leave it in BIOS for a couple minutes or run a Quake timedemo at a high resolution and then feel the heatsink. If it gets hot, you're golden, if it doesn't, you might want to see about getting some Arctic Silver MX-4 or a similar thermal paste to ensure you really are overkilling cooling the CPU. I even use MX-4 on my P200, and it doesn't stain the ceramic or anything- it's not necessary to use thermal paste atop ceramic based CPUs, but I use it just in case.

The 2.6v is probably okay, but with the voltage spec for this CPU being 2.4v +/- .1v, I'd try to reduce it to 2.5v if you can. It's not like you've immediately damaged your CPU, but running it like that for a long time might cause problems in the long run. If you can't reduce it to 2.5v at your current overclock, and you want to keep it, meh. It works, and as long as your system isn't acting funky, or doesn't start acting funky, you're probably good to go. It's only 0.1v over spec. If you were running it at like 2.7v or 2.8v or higher, I'd be more concerned, but just keep an eye on it and if it starts acting weird (i.e. giving you unreadable error messages, Windows protection errors if you use Windows, HDD reporting bad sectors, artifacting in games or in Windows/BIOS/DOS) you might want to reduce the overclock temporarily and see if it fixes it before you go to other troubleshooting options like checking the RAM, HDD, etc. because it might be the overclock/voltage causing it.

Hell, even if you break it somehow, which I doubt will happen in the near future, knock on wood, K6-2's and K6-III's are pretty cheap (ballpark US$15-US$20) on eBay, so you could just replace it with a higher clocked version and forget about overclocking a lower speed version altogether unless you just want to overclock, which is a completely valid hobby in its own right.

I'm not sure about the PS/2 stuff being iffy, you obviously know more about motherboard level component repair than I do if you recapped an entire board- the only troubleshooting options I have is checking to see if it still does it at the processor's stock speed/voltage, checking the RAM, and checking the PSU to see if it's working okay (though I doubt it's the cause if it's a modern one, which is always best to use- I don't trust any PSUs made before, say 2003 or something in that ballpark region, and I especially don't trust anything made before 2000 to not blow up and take my expensive vintage system with it).

My AMD K63+450 CPU had 2.3v written on it so that is what I used and it worked fine.
Passed all benchmark tests fine.
I am using the same motherboard as you.
I over clocked it to 500mhz with same voltage. Works fine.
I would try to find a large heat sink with quiet fan.
Also I am using “cooler master drive 4” temp sensor
to monitor the temps. But a case with good ventilation and fans is good too.
There are allot of computer panels that monitor temps on eBay.
You can check the operating temp of your CPU on “CPUworld” for reference.
Use good CPU thermal paste too.

I don't get it... why don't you buy a 500AHX?
They are available for less than 10 euros.

@athlon-power The cpu cooler is pulled off from a S370 board and a Celeron 450 MHz and I'm using the Mx-4 already. I also strengthen the heatsink connectors to the socket connectors.
I will try to reduce the voltage to 2.5V and do some stress tests.
I have two K6-2 at 450 MHz one from 1998 and one from 1999, I don't remember the models, but K6-3s are faster.
I didn't replace the capacitors by my self, I have a very skilled friend who had studied electronics and mechanotronics for his bachelor and when I need him he doing the job for me. The ps/2 header is probably defective. Maybe from rust, because the metal parts on the I/O panel are rusted a little bit.

@Intel486dx33 mine is 2.4/3.3 V model. The fan is very quiet. It's a new one from startech. I think the case has good ventilation. I will take a laser heat thermometer to measure the cpu temperature. Unfortunately our board doesn't shows it. The P5s-99VM which I've installed the Pentium MMx has it. Also the QDI Titanium IB+, the P5A and the MSI MS-5169 Ver 4.0 also have this option. But I have them on packages for spare.

@retardware I cannot find a K6-III 500/AHX on ebay. I found a K6-III+ 400/ATZ (1.6v reaches 600 MHz at 2.1V) model. But it is very expensive. 135.00 € asks the seller.

JSO wrote on 2020-01-01, 21:43:

@retardware I cannot find a K6-III 500/AHX on ebay. I found a K6-III+ 400/ATZ (1.6v reaches 600 MHz at 2.1V) model. But it is very expensive. 135.00 € asks the seller.

Ooops, didn't notice that you were about K6-III+... thought K6-II

And...ouch!
The prices have gone crazy.
I should have kept my K6-II+/550ACZ instead of selling it on ebay for 19.80 Eur a year ago... 😢

JSO wrote on 2020-01-01, 21:43:

@retardware I cannot find a K6-III 500/AHX on ebay. I found a K6-III+ 400/ATZ (1.6v reaches 600 MHz at 2.1V) model. But it is very expensive. 135.00 € asks the seller.

What the Hell? Who is going to spend that much money on a friggin K6-III (no offense to the K6-III, but seriously)? You could build an entire Pentium II or Pentium III system with that kinda money, mine is worth roughly ~US$160 or so right now, and that's just because I have spent ages working on it and have bought parts for it over a long period of time, and that price includes the case.

Don't worry, I have found a PCI Pine TNT2 M64 on sale for US$20,000 on eBay.

Maaan, that seems like a really good deal, the ultimate performance of a TNT2 M64 for the low, low price of 20,000 dollars. The boxed TNT2 M64's normally go for a ridiculous 100 dollars, that's a steal!

JSO wrote on 2020-01-01, 21:43:

@athlon-power The cpu cooler is pulled off from a S370 board and a Celeron 450 MHz and I'm using the Mx-4 already. I also streng […]
Show full quote

@athlon-power The cpu cooler is pulled off from a S370 board and a Celeron 450 MHz and I'm using the Mx-4 already. I also strengthen the heatsink connectors to the socket connectors.
I will try to reduce the voltage to 2.5V and do some stress tests.
I have two K6-2 at 450 MHz one from 1998 and one from 1999, I don't remember the models, but K6-3s are faster.
I didn't replace the capacitors by my self, I have a very skilled friend who had studied electronics and mechanotronics for his bachelor and when I need him he doing the job for me. The ps/2 header is probably defective. Maybe from rust, because the metal parts on the I/O panel are rusted a little bit.

The cooling seems to be optimized as well as it can be already. If your heatsink doesn't get hot without airflow for a little while, especially when overclocked with a higher voltage, I'd be worried.

Regarding those PS/2 ports, If the pins inside the connector managed to get corroded (or the pins on the motherboard, whichever one the mobo happens to use), I'd say it would make it go kaput. USB with Windows 98 (assuming you're using Win98) is a bit ugly from my experience. Had a USB mouse and the thing glitched out several times, and this was on a 440BX board. If you've just got USB legacy mode enabled for the keyboard and are using DOS and a serial mouse, it shouldn't matter. Do you have spare PS/2 port bracket things if it uses the external connectors? If you do, you could try and test it and see if it's the original ports or if it's the headers.

I hope reducing the voltage goes well. If it works at 2.5v, I'd try going down to 2.4v. It may or may not crash at that voltage. If it doesn't, then you are right on target for the voltage, but even 2.5V is considered solid and acceptable for a K6-III like yours (at least, according to CPU World, and I trust them as they usually have their stuff right, unlike INTEL, which says a 200MHz MMX has 512KB of L2 cache- good lord people, at least fix your stupid datasheets, when third parties get your stuff right more than you do, there's a problem, fIX IT, thank you).

Yes prices for K6-3s are very crazy, even for the K6-2s.

But...

The SS7 It's the best platform for Windows 9x and pure DOS. I have two PIIIs at 550 MHz, Coppermine and Katmai for Slot 1 and the very good Aopen AX6BC plus motherboard, but due to locked multipliers of the cpus, SS7 is the best approach for Windows 9X era for me. A K6-3 at 450 MHz and over is fast enough as PIIIs I believe for this use.

TNT2 M64 is very good for DOS, especially the PCI version, I using it on my Pentium MMx build and it has Windows 3.x drivers!

No issues at all on Win 98SE using usb mouse and keyboard. New models, not vintage hardware.

At 2.4 V doesn't posts, so I'll try at 2.5 V.

I have a ps/2 header, newest than the one that board has, but I need my friend to do the job for me!

17.000 € for a PCI card??? I'm buying a house with this amount...

I bought an K6-3+ 400/ATZ about 45.00 € including shipping.

It reaches 600 MHz 24/7 stable with only 2.0V as the seller told me.

JSO wrote on 2020-01-02, 08:48:

Yes prices for K6-3s are very crazy, even for the K6-2s. […]
Show full quote

Yes prices for K6-3s are very crazy, even for the K6-2s.

But...

The SS7 It's the best platform for Windows 9x and pure DOS. I have two PIIIs at 550 MHz, Coppermine and Katmai for Slot 1 and the very good Aopen AX6BC plus motherboard, but due to locked multipliers of the cpus, SS7 is the best approach for Windows 9X era for me. A K6-3 at 450 MHz and over is fast enough as PIIIs I believe for this use.

TNT2 M64 is very good for DOS, especially the PCI version, I using it on my Pentium MMx build and it has Windows 3.x drivers!

No issues at all on Win 98SE using usb mouse and keyboard. New models, not vintage hardware.

At 2.4 V doesn't posts, so I'll try at 2.5 V.

I have a ps/2 header, newest than the one that board has, but I need my friend to do the job for me!

17.000 € for a PCI card??? I'm buying a house with this amount...

500MHz is too much for a K6-III AHX 400.
Be happy when you get 450MHz stable.

It's stable on 500 MHz area with 2.6 V, but as I mentioned on previous post, I've bought and ATZ 1.6 V model.

See my posts:
AMD K6-lll+@500mhz., Voodoo 3000 (desktop)

Coincidentally I just replaced the DIN connector on a super 7 board. On mine, all of the pins toward the connector side (which IIRC were connected to the ground shield) were broken from flexing. The loss of that connection made the keyboard not work, so it would only work if I jammed a piece of paper against the port so the pins made contact.
I think it was caused by my historical use of one of those big and clunky PS2->AT adapters that stick directly in the port. Nowadays I use the kind that's on a flexible piece of wire.
I've killed keyboard controllers before by hotplugging, so I was worried about this situation.

I'd be kind of surprised for this to happen to a min-DIN PS/2 connector though, since they probably don't get flexed as much when used.
If nothing is broken then it might just need solder reflowed on all the pins (adding a bit more in the process). Reflowing is easy, you can do it without experience. But if it really is a corrosion problem then it probably has to be replaced which is harder.

I had a K6-3 450MHz 2.2V on the above board when new. I don't know if it was ever stable at that setting, but it certainly wasn't by ~6mos into ownership. I ended up running it at 400MHz because back then I didn't know about increasing voltage, nor would I have been comfortable doing it.
Mine needs 2.3V minimum to be usable at 450MHz. I think my CPU has always been prone to occasional quirky behavior like failed POSTing or whatever, even at 400MHz. Back in the Win98 days I was more accepting of things like that.

I don't like the original K6-3 very much. After upgrading to a K6-3+ I'm a lot happier with the machine. It runs cold at 1.8V, it can clock to 550MHz easily and it's made the machine more stable than the original K6-3 ever was. I believe it's also better for the longevity of the motherboard since it doesn't put as much stress on the onboard power circuitry. You did a good thing by upgrading to the K6-3+.

For anyone in a similar situation who doesn't want to pay the price of a K6-3+, consider the K6-2+ also. They aren't much slower but they're usually cheaper, and when you clock them up they will still be faster than the original K6-3.

It happens that issue with the din connectors and the adapters.

But on mine situation, is something different. I'm definitely assured that is a corrosion problem inside the headers.

I forgot to use a thin metal rod or steel pin to rub the rust on the header holes.

The K6-3s+ are better for sure. Especially the low voltage parts that can handle temperatures of about 80 degrees of Celcius.

K6-3s+ are hard to find nowadays. K6-2s+ are easier.

The ATZ arrived and I'm running it stable with 2.0V at 550 MHz.

I think is a very good overclock.