Well, all 386DX I know need CLK2 input.
That means that your OSCillator needs double clock frequency: 80 MHz for 40 MHz operation frequency.

Isn't there a jumper for this? I remember my 386DX board had a jumper on it where I could select 33/40 Mhz. It looks like one of compact late 386 boards, so it seems unlikely it would only support 386 20Mhz.

AlexZ wrote on 2022-12-04, 09:54:

Isn't there a jumper for this? I remember my 386DX board had a jumper on it where I could select 33/40 Mhz. It looks like one of compact late 386 boards, so it seems unlikely it would only support 386 20Mhz.

Checking the board thoroughly and also the retroweb tells me there are very few jumpers to alter.
So i hope i can source an 80mhz xtal easily and put it in. Thanks

On close inspection of picture of your board and the one on theretroweb for identical board it really seems they have 80Mhz crystal while you got 40Mhz. If you get to replace the crystal it may be worth getting both 80Mhz and 90Mhz one for a small overclock. For higher OC cache faster than 20ns and 60ns memory will likely be needed. Also fit it with 256Kb cache.

AlexZ wrote on 2022-12-04, 18:03:

On close inspection of picture of your board and the one on theretroweb for identical board it really seems they have 80Mhz crystal while you got 40Mhz. If you get to replace the crystal it may be worth getting both 80Mhz and 90Mhz one for a small overclock. For higher OC cache faster than 20ns and 60ns memory will likely be needed. Also fit it with 256Kb cache.

I can confirm the 90MHz crystal idea. I'm running my 386DX at 45MHz on M321 motherboard with 90MHz crystal. Love it!
I also bought 100MHz crystal, but it seems to be just dead. Maybe there are some different types of 4-legged crystals? (I don't have oscilloscope to confirm)