This is much more than i have experienced with the other Sun3's. Especially the server machines are already near their limits and become instable if overclocked more than say 10%.
The 3/60 does have a 40 MHz quarz oscillator from which the CPU clock of 20 MHz is derived. You can replace this one by a 45 MHz (very rare), 48 MHz (rare) or 50 MHz (common) type.
Trying even more with 55 MHz and 60 MHz i wasn't sucessful. After some minutes (30 MHz) or hours (27.5 MHz) the machine crashed with errors. With another machine i experienced a funny behaviour: It ran slower at 25 MHz than at 24 MHz. A couple of other 3/60 run fine at 25 MHz and are 25% faster.
One should expect that the 100ns SIMMs wouldn't do at more than 20 MHz, but most of my SIMMs work fine even at 25 MHz. If you can find those with 70 ns or 80 ns, the better.
Important! Clean the air inlet at the bottom of the machine! If the fan is weak, replace it by a newer (stronger) one. Mount cooling devices on CPU and FPU. If you have a cg4 color graphics card in the 3/60, cool the RAMDAC too. To get a better airflow, put the machine on higher feet (i use pieces of wood).
A higher clock means more heat! Make sure the 3/60 can get away with that! Else it dies from the heat.
Ok, enough teachers speach. The 3/60 is a tough machine and i have seen some that ran for months with totally clogged air inlets. But of course the 3/60 will thank it you with a long life, if you get her more cooling.
There are other means of reducing the heat: If you have to replace the SIMMs, get 3 chip types. These consume less power and more air can flow through them. Replace the CPU (and FPU) with ones of a higher rating. By the way: the 68030 easily is available with rates of 25 and 33 MHz. Remove the cg4 color graphics card, if you can place it in another (not overclocked) 3/60. Avoid to run a hard disk internally in the 3/60. This would overload the power supply too.
In this partial view of the mainboard you can see the two quarz oscillators which provide the clocks. The right one gives the bus and CPU clock.