Should I DIY it?? – rebore a 125cc bike engine barrel

[Russ BParticipant @russb]

I have a CBR 125 engine with a scored barrel, if I can fit it on my faceplate (myford super 7) do you think I should have a go a boring it out and honing it to size?

At this stage, I've yet to consider if the barrel is sleeved, and/or coated which is a big consideration.

I'm relatively sure my lathe now turns very close parallel after a full day setting up and adjusting the feet.

I would guess I'd mount some blocks on the faceplate to allow the barrel to sit on its base and face these blocks so they're flat – I could perhaps even line them up roughly and drill and tap them so I could hold the barrel by its stud holes at the correct torque value while machining the bore?

 

Any advice or tips on technique or tooling would be very welcome, it would be nice to do this myself even if it's not the most time effective solution.

[Russ BParticipant @russb]

[john carruthersParticipant @johncarruthers46255]

I did a Bantam using an expanding hardwood mandrel between centres and fine grit, took a while though.

[Russ BParticipant @russb]

I'm not sure how to setup the barrel on the cross slide and ensure its square and on centre but I guess this might be a better option – is it difficult?

[Clive HartlandParticipant @clivehartland94829]

For interest purposes it is not often that bores are, 'Bored' out now. With a honing machine the bore is honed to size and it is quite quick.

Rather than faff about it would be cheaper and easier to have it honed to size.

Clive

[Gordon WParticipant @gordonw]

Yes, it can be done, but is it worth it? First thing is how deep is the score mark, and can you get O/S pistons ? I once "rebored" a 500 norton that was so worn all I had to do was scrape the top with a half-round scraper & fit the next size up piston. If there is a piston with a small dia. increase the a hone will do it.

[Russell EberhardtParticipant @russelleberhardt48058]

If the scoring isn't too deep I would try one of these first. Even if you do have to bore it you will probably want to finish the bore with a hone anyway.

Russell.

[BowberParticipant @bowber]

I've just done it with a DR350 barrel.

You may need to drill and tap holes in the face plate to get the mounting right and you also need to space the barrel off the face plate to allow the boring bar to pass the top edge.

If you have a shop with the facilities then you'll find they'll do a better job and it should be quite cheap.

I'm not sure the CBR125 barrel has a liner though, it may be plated but it should be easy enough to find out from a honda dealer if there are oversize pistons available.

You'll get a parallel bore if you mount it on the cross slide but I'd think that would be quite a faf.

Steve

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