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Prusa i3 MK2 Clone Extruder for 8/12/18 mm sensors and MK8 Hobbed Pulley

by geoffsim Apr 20, 2017
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Hi! Nice work, this is exactly what I need!
Is there a reason you removed the parts that you need to drill out after printing? Without this the print won't look very nice in the screw holes. Is there a way I can put these back in?

AFAIK I have not removed anything. What do you need to drill out after printing? I have printed many of these units, and have not had any issues with them...

If you open the original extruder body stl file from Prusa you will see that the holes in the Z direction that are counter sunk are not see through. This is because without the extra plastic that needs to be drilled out the printer has to try to bridge between a solid surface and nothing. This will lead to threads of plastic hanging hanging around the holes. It's not a big deal, but it will look better with the supports.

The design uses a technique to reduce the curved overhang of the hole, by starting the hole as a square.

Cool! Any ideas why Prusa made the stl the way that they did, so it has to be drilled?

Nope. This "square hole" was in their original design. Why they changed it, I don't know. This design print just fine.

Okay. Thanks for taking the time!

Hey, have you thought about doing something like this that allows to use a 40mm fan (I'm using a 40mm Noctua for my Prusa i3 MK3 clone)?

I think someone already has.
I am perfectly happy with the 30mm fans.

Hi Geoff, thanks for the great work! Just following up on this 40mm fan discussion, is there any chance that you can post the 8mm-sensor version SCAD files so I can try modify it and fit the bigger cooler in it?

Much appreciated in advance!

Look for the "MK2.3 extruder", it's what I currently use!

Thew scad provided has params in it that you can change to get the different versions.

Really? Can't find anything like that in the remixes.
Ok :) I have to say that I have a printer with 30 and one with 40mm fan and the difference is huge. Even with simple old drivers, that little fan can be the loudest thing on the whole printer.

I am looking to update my MK2S clone to a sort-of MK3, but am in no real hurry...

Thanks for your reply, I'll just wait for it

Hi - I'm new to 3D printing, and have a question about this design:
Is there a reason not to use PLA when printing this? Can't stand up to the temperatures?

You answered your own question there :-)
PLA starts to soften at around 35-50 degrees.

If you don't want to use ABS, try PETG, although watch you don't weld the parts to the bed (lean the bed with window cleaner to stop it sticking too much).

HI Geoff,
Any chance of MK3 Clone extruder for 18mm sensors and hobbed pulley?

I made the 12mm MK2 for myself, and am happy with it. I made the others just for completeness.
I may have a look at the MK3 some time, but not for a while.
You can always have a go yourself!

Thanks for this, what nuts/bolts are needed?

The same as what is required on the real printer.
Have a look at the Prusa online manuals for the details.

Any chance you would be willing to modify the 18mm sensor MK8 for use with a Bowden setup?

Quick answer, no.
I made these changes for the two clones I built. I have no plans to make any more changes.
You could have a go yourself...

"MK8 12MM" extruder cover on bottom side of photo. Could there be some error in dimensions?

Nope. By design. Allows insertion of hot end.

Is there a .3mm v6 version of this by any chance?

Hi Geoff,

Thanks for sharing this (and your other mods). It appears that even the 8mm version may position the probe too close (2-3mm) to the body of the part. The wire doesn't have enough clearance to pass between the probe and the body. Am I missing something?


I didn't move the 8mm version. It is the same as the stock part, apart from the MK8 modification.
I intend to update the parts to use a single screw to affix the sensor at some stage. I will increase the distance of the probe on the 8mm version at this time, too. Or you can do it yourself as the scad files are provided (look at soff and cabinc in extruder-main.scad).

How far are you on doing that?

Do what? I have decided not to do the single-screw fixing mod as I am happy with the 12mm version - that's what I use on my clones.
As far as the spacing for the probe wire for the 8mm version, you can do that yourself - I have pointed to you the changes in the scad file.

well, ive brought the part up to the latest singe screw version and modified the cable guide

Perhaps you can post on Thingiverse as a remix?

Prusa i3 MK2 Clone Extruder for 8/12/18 mm sensors and MK8 Hobbed Pulley 1 Screw better cable loop edition
by MaxRink

Hi Geoff,

Followed your suggestion and reworked the original Mk2 body. Filament holes are 1mm to the right & 8mm sensor mount is 2.5mm to the right. Thanks for the inspiration.


Prusa I3 Mk2 Dolly Extruder
by Cr3470r

nice work! I have a question! When I printed the body and the cover I noticed they have a different diameter is this ok?!!!

Yes, it's by design. If they were the same, it would be difficult to fit together with the e3d in place.

thanks you again for your time!

Very nice. This "thing" saved my butt. I ordered an E3D hobbed pulley from Filastruder and the filament path was too tight with the original MK2 extruder housing. This one worked perfectly. Can someone tell me what size screws and nuts are needed for securing the pinda probe? I had the old style before.

M3 square nuts and 12mm bolts.

Comments deleted.

Great updates, thanks! With the MK8 hob gear, won't the e-steps need to be updated to accommodate this different size?

Updated from what? However, the short answer is yes. It's important to tune your extruder on every new build, anyway.

There are also 10mm inductive probes (mentioned with 10mm detection distance on ebay, not sure about that detection distance but i would try).
Could you also make extruder for 10mm probe? =)

You sure it's a 10mm diameter probe? I have never seen one.

It is unlikely that an 8mm sensor has a 10mm detection range, especially when sensing aluminium. My tests showed about 2mm for an 8mm sensor, 3-4mm for a 12mm sensor, and 6-8mm for an 18mm sensor. Capacitive sensors have a better range, but are more sensitive to temperature. If you have bought this one, let us know what sort of range you get from it.

Doh.. It is not inductive sensor it is actually hall effect, does only detect magnetic field.
Test results with magnets and ruler: 8x1mm magnet ~5mm, 8x4mm magnet ~9mm.
Can be used if magnets are glued under heatbed. Maybe original Prusa MK2 use hall effect sensor and magnets inside heatbed for XY aligning?

No, no magnets. Prusa PINDA is a custom inductive sensor. The sensor senses simple copper circles on the top of the bed. A standard 12mm inductive sensor with a 4mm range detects a 3mm aluminium heat bed at around 2mm. The 18mm device is about 4mm.

Hi! Thank you very much for these! What do you use as the idler shaft?

I just used a 5mm bolt, cut to size.

Thank you for making these, exactly what I've been looking for! Just a question regarding the 18mm: Did you incorporate the changes that you mentioned in the summary regarding the sensor holder? Thanks!

Yes I did.
The main picture has not been updated yet.
However, the part has been updated, and the actual picture of the part show the change.

Thank you for making these, they would come in handy - unfortunately, the extruder body for the 12mm sensor is missing most of the screw holes. Could you upload a repaired version? Thanks!

Just testing you ;-)

Hi Geoff,

Can you tell me what are the bearing you used to work with your mk8?
It's look like yours is bigger than the original?
Am i wrong?

I assume you mean the idler bearing. This is a standard 625 as per the Prusa instruction manual.

Hi Geoff,

I have build my clone using Tom's guide as well. I am going to print your part and change things out.
But I have another issue that you might have a solution to - I also used TR8x8, but the brass nuts don't fit into the original Prusa parts - what did you do to fix this?

Print a shim or collar to make the brass nuts a snug fit; cut out the piece of brass between each of the holes and the outer edge of the nut; place the nut in the shim, and the shim into the prusa part. The bolts will then fit. You can use washers, but I found I didn't need them.

Can you post the ship or collar you used to make the brass nuts a snug fit? can you take pictures of the modifications you made with the T8 brass nut?

ah - yes, that makes sense.. Thank you for swift reply :)

Prusa i3 MK2 Clone Brass TR8 Nut Adapter

You can also print these if your in a pinch :)


Trapezoidal Nut for Prusa i3 MK2 MK3
by Mach0gr

Thanks :)
My china E3DV6 clone is printing like SH**. So I am currently waiting for an original set. It will arrive Monday. I hope that I can obtain decent PETG prints with this build and an original hot end.

As you can see in the last photo, I used those at first. I replaced them with the brass ones a few days later, once I had worked out how to fit them.

I am working on an 18mm sensor version, and will post it here when done.

can you post please the psu cover??and what are the dimensions??

I am working on this at the moment. I will post it when done.

Can you post the SCAD files?


NIce work! What type of bearing did you use for the x axis idler? Thank you in advance.

I used a 16 tooth idler timing belt pulley with dual bearing.

The Original one uses 20 (without tooth) idler timing belt pulley and on the stepper motor the 16 tooth one

I am also using 16 tooth idler timing belt pulley with dual bearing but it is smaller than the original one. Did you modify the x carriage to align the belt. One more question, what's the part number of the heat bed that you used? Can't find one with connector on the side..

I only noticed that when my genuine prusa arrived.
I may swap out the idler with bigger ones - when I find them...

I need one for the 18mm probe. :)

Working on it.

My SCAD changes don't work well for 18mm probes. I optimised the changes for 12mm only.
If I get enough requests, I will see what I can do.

For now, get a 12mm probe. It works quite well, albeit only 4mm sense range (about 2mm with an aluminium plate).

You don't perhaps have one with for the M8 Probe? Or maybe the Openscad files?
Great job on the clone. I'm waiting for a few parts to arrive then I'm going to build one too.

That was quick thanks!