My old Stanless steel nozzel used to have really good clearences with all my joints and prints.
I recently changed to the hardened steel nozzel, same 0.4mm as my previous.
But now my clearences are no good and everything is over extruding, even brim values are no longer accurate.
How to fix?
It doesnt seem logically that i would have to flow calibrate all my filiament for a nozzle replacement, considering bambu already did that flow calibration at the factory for the 0.4 nozzle.
Any ideas?
Did you try performing a full calibration after changing the nozzle?
I did yes, sorry should have mentioned that, i even changed the nozzle settings on the device
Do a filament calibration (pressure advanced and flow calibration) with the new nozzle. There are reports in the forum, that sometimes also a new calibration is needed after a firmware update. But I think, in any case if something changed on the hardware, what is relevant for printing (nozzle, extruder, stepper motors,…) a new calibration makes sense. 
Would the stainless steel nozzle and the harden tools steel nozzle heat up differently if so then you would have to recalibrate your filament.
Considering that you changed from an older stainless steel nozzle to a new hardened steel nozzle, it does make sense that, without a new filament calibration, you are now consistently getting overextrusion. If the former nozzle was slightly worn, it would have calibrated to a higher throughput than is ideal with the new nozzle.
New filament calibrations should fix this. Nozzle replacement is one of the prime reasons for Flow Dynamics Calibration | Bambu Lab Wiki
But i never calibrated my previous nozzle… these were bambulab already configured flow calibrations which means NEW nozzles should match there in house calibrations.
It doesn’t make sense to have to go and do a calibration 100X for all my filaments for a new nozzle that is insane.
I am afraid that printing is still sufficiently complex that we need to be careful about expectations. In particular the big (but definetily not final) step that Bambu made for the consumer market led to some misconceptions. There are still countless input variables and almost as many failure types in FDM so there is a bit of a mismatch between what “should” be and what physics actually implies. In particular when forcing a 1.75mm filament through a 0.4mm nozzle while melting with high flow rates and comparing different nozzles. Very minor differences can have big effects under these circumstances where high speeds and hence highly non-linear thermo-fluid-dynamics lead to effects amplification.
While PLA tends to be quite forgiving, other filaments require even more frequent calibration. You may want to have also have a look at the Flow Rate Calibration | Bambu Lab Wiki. Overextrusion is item number 1.
I definitely understand and hear what your saying.
but a bambu lab hot end should have the same specs especially since i have done 0 calibrations that are “custom” on my end. its all been stock and very accurate. For a a third party hot end I totally agree, but I digress. Ill run some more calibrations and see what results. Appreciate the feedback
Actually, another thought that may be helpful since this thread went very quickly to calibration. Did you tell your printer about the changed hotend?
Bambu Handy, Device, Settings, Nozzle?
The different coefficients of thermal expansion of the two nozzle materials will lead to slightly different effective (rather than nominal) nozzle diameters at different temperatures.
Not sure if this helps, but a quick and easy check.
The only thing the setting does is warn you if you are attempting to print an abrasive material with the OEM stainless nozzle, when you should have a hardened steel one. Unlike other printers, there are no device or filament settings that change when you tell the printer or slicer that you have a hardened steel nozzle fitted. The addition of the menu for the P1P/S in the machine that allows you to change what nozzle is fitted only came out a couple of firmware versions ago. I’ve only ever had a hardened nozzle (and extruder gears) fitted to my P1P, as I changed them out before I did my first print.
Many thanks for comfirming that the difference in CTE is not adressed by the setting and is not really significant. It would have been an easy fix for the OP if that had been the case.
The CTE will make very little difference anyway, even the actual nozzle diameter being on size isn’t a big issue (the roundness and surface finish of the tip is more important than the diameter being exactly on spec unless you’re right up against the flow limit).
You could pop a 0.35mm nozzle in and use the same settings as your 0.4mm in the slicer and unless you ran into flow rate limits you’d get pretty much the same print out.
Yes i mention that above, i changed the nozzle at the printer settings