Can I cut down a headset cup?

Old headset bottom cup on the left, and new one on the right. The new one is about 6mm too deep to fit the frame. Any reason I can’t cut that much off with a Dremel and then use it?

They are completely different, one is internal and one is external

Yup, you’ve got 2 different headset types – left is semi-integrated and right is press-in. Looks like you’re trying to fit a tapered steerer fork – not sure whether you’ll find a headset compatible with your frame to do this…

I’m planning on doing the same…? 

Interested to hear what everyone else thinks of the plan.. 

I suspect you are planning to fit a tapered fork in a straight headtube? 👍

Are you fitting a tapered fork to a straight headtube?

What bike is it?

I wouldnt use a dremel but yes you can. The indents on the cup almost suggest your supposed to dependant on frame.

You can swap ex 1.5 and internal 1- 1/8th headsets round in many cases if you can live with the extra stack. I’ve done just that on my 9zero7 to accommodate a shand 1.5″ fork.

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.hopetech.com/_repository/1/documents/HOPEHEADSETS2011Web.pdf&ved=2ahUKEwjgqom5joODAxVxT0EAHWZaDAAQFnoECBYQAQ&usg=AOvVaw2JhR24v5ag6gw4SKzBmgIN

Second column sounds like what op is looking to do.

I take it your putting a taper fork onto a frame with a straight headtube by swapping the semi integrated cup for an external one?

We did the same on my daughter’s bike & had the same problem. We took a little off the cup on a lathe so it could be fully inserted.

It’s only soft aluminium, so you could probably clamp it down and take a few mill off with a file rather than trying to use inappropriate power tools. There’s more chance of failing in future if it’s not a nice consistent smooth finish, maybe a risk your happy to take though.

I did it to fit an external cup to my Voodoo Hoodoo. It’s a 44mm headtube and came with a straight steerer fork, the original headset was like your one on the left. Moved the Fox fork from my old Trek to the Voodoo and just needed to git an external cup to make it work but the Voodoo frame only has 8mm of depth for inserting the lower cup and the headset I had (Brand X) had a 15mm depth. A quick chop with a hacksaw and making a bevelled edge with a file and it went in perfectly first time. Hasn’t given me any issues whatsoever in three years of pretty hard use.

Just keep as much of the cup as you can, give it a bevelled edge to aid insertion and make sure it’s fully seated against the bottom of the headtube and you’ll be fine.

Thanks all.

Yes, swapping to a tapered fork on a Voodoo Bizango. Plan B was a hacksaw, maybe I’ll try that instead. Good to know others have had success with this!

Spares here if you don’t get it right first time? – Nukeproof Neutron Bottom Headset Cup | Wiggle

At 49p it might be worth picking up a few for the bearings.

It wouldn’t worry me to cut it down, I’d do as reluctantjumper says and cut as little as possible and chamfer the top edge. The whole headset assembly is under compression so it shouldn’t be able to fall out anyway 

I definitely wouldn’t be using a Dremel though.

Take your time to get it as even and level as possible with a nice smooth chamfered edge as well, it will go in wonky if it’s not, I did it years ago with a junior hacksaw, it wasn’t cut that accurately and wouldn’t go in straight so I spent more time getting it as perfect as possible with a file then stuck some wet n dry onto a piece of flat granite to sand it all level and even.

Prob OK. That Nukeproof cup is very deep — 14.8mm according to the specs.

The equivalent Hope lower cup is only 9.5mm deep, so after your surgery yours should be basically the same.

I cut it down with a hacksaw, it turned out like this. It went into the frame pretty easily, and the new fork is now fitted. Thanks for the advice all!