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CBF125 Exhaust - It's Time

Blog Date - 09 May 2018

Some of you may recall I was griping about rust on the CBF 125's exhaust (CBF 125 Exhaust Rust). Well this last couple of weeks it's started to blow from the cylinder head to exhaust joint. 

Typically this would only require replacement of the cheap exhaust gasket. However as most of the metal ring that pulls the exhaust into the head via the studs is now rust rather than steel I cannot imagine that it will all come apart like a good little motorcycle, it will crumble and dissolve instead. 

The exhaust header is so rusty it it crumbling away
There's not much metal left these days.

I have ordered a used exhaust of Ebay. I hope it arrives intact and undamaged. I have ordered a new exhaust gasket too. That is the easy part. Removing the old exhaust is the bit that worries me.

If I put a 10mm spanner onto the nuts of the exhaust studs then apply torque to undo said nuts one of two things will happen. Either the torque will snap the studs clean off at the nut or the torque will snap the studs clean off at the base of the stud. Either way I'm going to end up removing the head and there'll be tears and tantrums.

The nut that holds the exhaust to the head is badly corroded
I dare not think how much of the stud is left beneath the dirt.

So...I have a cunning and devious plan. I shall do my best to remove the nuts by using my Dremel. The idea is to cut into the nuts and encourage them fall off rather than having to spanner them off. If I can achieve this I will be left with the exhaust removed and the studs still intact.

The flange and the studs and the nuts are in a poor state
Please PLEASE come out nicely.

I won't be out of the woods though. It all really depends on the state of the studs. I suspect they'll be rusted quite badly and unable to manage the torque required to fit the replacement exhaust. In which case I'll need to remove the head and get the old studs machined out. 

I want someone to wave a magic wand and make it all better. 

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Reader's Comments

Bill said :-
Heat the alloy at the stub area with blow torch to expand it and stud should unscrew with the seized nut and replace with new studs and nuts.
Good luck
9/5/2018 6:44:00 PM UTC
Snod said :-
I tried the Dremel trick just a few days ago, it worked enough but I did end up removing the thread from the end of the studs too so it wasn't perfect. I had much better luck hammering a socket on (to get it over the rust) and simply undoing them, sometimes you just don't know what is best to try.
9/5/2018 6:51:46 PM UTC
Ren - The Ed said :-
I really really don't know what to do for the best.

Bill - IF I heat the ally then the stud comes out then great. But...what if it doesn't? It's a risk, a big risk. I'll be as happy as a pig in mud if the whole lot comes out as I've plenty of M6 stainless threaded bar with which to create new studs.

You see - in the past with a previous 125 I had a firm grip of the nut and I turned it. Only to find the actual stud was stuck in the head so the weakest point in the chain was where the stud exited the ally of the head. I ended up with the stud broken flush to the head. Drill and helicoil from Mr Latchford soon followed after removing the head.

We're talking 73,500 miles and 9 years old here. Rain, grime, road salt and long cold damp winter's nights. I fear there ain't much left.

Think positive thoughts Ren! If you believe then it will be so! Erm. Yeah.
9/5/2018 7:23:43 PM UTC
Bill said :-
If the stud is seized into the head removing the nut with dremel is no advantage as you still need to get the stud out and have no nut to turn it. Be positive warm the alloy, it expands faster than steel, then give the nut a sharp tap towards the head to shock the stud then try to undo. If it moves turn it back in a little then out some more until it loosens. What have you got to lose.
9/5/2018 11:14:56 PM UTC
Ian Soady said :-
What Bill said. But first soak it in Plus-Gas or similar (not WD40 which isn't penetrating oil) and let it stand for several days before applying heat.

It will often move if you try to tighten it fractionally before loosening.

What have you got to lose indeed. The poor little thing deserves a rest......
10/5/2018 10:55:51 AM UTC
Ian Soady said :-
And of course it's too late for the 125 but I hope you've removed the studs on your other bike and applied copaslip or similar......
10/5/2018 10:57:05 AM UTC
CrazyFrog said :-
If it were me and I just wanted to get a few more miles out of the bike before it dies completely, I would follow your route Ren. I've used nut splitters and a dremel in similar circumstances in the past, and though seized into the head, the stud threads have always cleaned up well enough to re-use. Just go a bit easy when torquing back up. ;)
10/5/2018 12:17:52 PM UTC
Ian Soady said :-
Or just plaster it with silicone and wire it on with a bit of fence wire......

Mind you, Ren has probably already broken his budget buying a used exhaust from ebay.
10/5/2018 1:56:59 PM UTC
Ren - The Ed said :-
Aaaaah the sweet sounds of different opinions! Heat it up, whack it, cut it, stare at it...

I'm afraid I have made my executive decision and on my own head be it. I shall Dremel the nuts and get the exhaust off. I shall inspect the studs. If I deem there is insufficient material left I shall TRY to Plus Gas, heat, hammer and turn them out. If there is sufficient material left they're staying where the blooming well are.

Ian - I almost fainted at the £1.60 for the exhaust gasket. As for the actual used exhaust...£50! My budget for this bike is totally and utterly blown now.

I can got to far nicer places than this to be insulted you know.
10/5/2018 3:34:37 PM UTC
Bob said :-
Cutting them off as you plan is the best bet. I have used a die grinder (like a beefier version of a Dremmel, 26000 RPM) and grinding stone. Grind the nut away on one side until you start to see the sides of the threads, they will turn blue with heat and cause a zebra stripe effect. When you get to that stage lather it in your favourite releasing agent and try to unscrew it by drifting it round with a cold chisel. If no joy then grind some more of the nut away and try again.
I have used this method successfully several times and the stud has always been usable.
When you reassemble use lots and lots of Coppaslip.
13/5/2018 3:44:56 PM UTC
Ren - The Ed said :-
Bob - there will be a report on my progress soon...........
14/5/2018 6:29:27 AM UTC

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