CB250 Engine rebuild

This section is for discussion on old/classic motorbikes. Also please use this section to put your project and restoration threads.
Message
Author
fastbob
Posts: 3296
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 1772 times
Been Liked: 1314 times

Re: CB250 Engine rebuild

#16 Post by fastbob » Mon Nov 26, 2018 9:39 pm

CD 250 U , that's the one I was trying to remember ! I had one of those very briefly. I could never understand why it existed at all because I think the N series or Superdream engine also existed at the same time . I used to refer to the bike as a " Big Benley " I also had a CM 200 a CM125 and a 125 Benley that I took my test on . I've always liked the elegant styling of your CB 250 , although it resembles a Night Hawk , it also reminds me of the old Triumph Hurricane . Have you got a side view picture of it when you were touring on it ?

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#17 Post by Snod Blatter » Mon Nov 26, 2018 10:41 pm

More CB pics, you say.. Hmm I might have the odd one (!)
Image
Image
And how about one without all the luggage on? This was before I recovered (and refoamed!) the seat:
Image
It is exactly a big Benley and despite being much less flash than a Superdream they do prove to be much longer lived. Personally I was never much for the styling of it but I do appreciate the upright riding position, big seat and the large front wheel 8-) I'd like it to have steel collars in the mounts because it's tougher that way and there are still places I think it needs to see, which is part of why I'm keen to go down this route..
Image

fastbob
Posts: 3296
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 1772 times
Been Liked: 1314 times

Re: CB250 Engine rebuild

#18 Post by fastbob » Mon Nov 26, 2018 10:42 pm

Snod Blatter wrote:
Mon Nov 26, 2018 8:28 pm
fastbob wrote:
Mon Nov 26, 2018 12:06 am
It wouldn't happen to be the same as a CMX 250 Rebel by any chance ? https://m.ebay.co.uk/itm/Honda-CMX250-C ... r=1&_rdt=1
I do believe it is, the MC24 Nighthawk and CD250U are also the same - the only differences between the motors, as far as I know, are the ratios of the gearbox - the MC26 Two Fifty (mine) runs different sprocket sizes to the Rebel and Nighthawk for absolutely no reason at all but Honda changed the gears so the end result was almost exactly the same. They clearly love a faff and for that I respect them. I'm not sure but even the CB125T and CB125TDC may use the same KB4 crank cases - KB4 is apparently the CM250C from 1982 so the vintage is correct.

However, it would be nice to save the original cases with steel inserts because 1) The engine number will still be original and 2) A steel surface has got to be better in this application than aluminium. But I'm not buying anything until this is all in bits and I know what I'm dealing with 8-)

Oh I should also mention that the clutch cover is also knackered, the tacho drive is ovalled and broken. The first person to point me to a clutch cover that will fit and also house a kickstart gets a big like from me!
This any good ? https://m.ebay.co.uk/itm/Honda-CB250-Ni ... r=1&_rdt=1 ahh , no kick start , sorry.
You do not have the required permissions to view the files attached to this post.

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#19 Post by Snod Blatter » Mon Nov 26, 2018 10:48 pm

A bit, certainly the condition is in keeping with the rest of the bike, but the kickstart hole is still blanked off - how can I impress girls with that?? I think something off a Chinese bike will likely be the answer. The boss is still present in the crankcases by the way, I just need the right cover and gubbins to slot in.
Image

fastbob
Posts: 3296
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 1772 times
Been Liked: 1314 times

Re: CB250 Engine rebuild

#20 Post by fastbob » Mon Nov 26, 2018 10:50 pm

Regarding the pics , now that's adventure touring ! Sod yer BMW RS 1200s with their bloody great tin boxes. You can tour on anything if you have the right attitude. And some bungees !

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#21 Post by Snod Blatter » Mon Nov 26, 2018 11:02 pm

You can green lane too, can't imagine why the frame ever cracked up..
Image

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#22 Post by Snod Blatter » Sun Dec 02, 2018 3:48 pm

Another Sunday, another spanner day.. At least when it wasn't raining. So, before I took the cam sprocket off I marked it up with some very gloopy Tipp Ex, just in case it fits a special way:
Image
Stuck a 14mm socket on the flywheel bolt to stop the motor turning while I undid the cam sprocket bolts:
Image
And job's a good 'un.
Image
The cam was still a bit tricky to pull out with those long studs in the way but it was eventually wriggled out. The bits are multiplying..
Image
The cam chain tensioner is held in place by three bolts - one inside the cases, one at the cylinder and one at the head. It turns out that the one in the head is hidden behind the inlet manifold:
Image
So off that came and then the bolt came undone:
Image
The quality of this bike is really shining through right now. Every bolt has come undone with a click, I can't thank Honda enough. As you can see, this lets the tensioner come away and then there is nothing else holding the head or barrels in place:
Image
A few light taps with a rubber mallet and..
Image
What are these, valves for ants?
Image
Crusty valves for ants..
Image
So, turn the crank with the ratchet until the pistons are at the bottom of their stroke (so the connecting rods don't fall down and smack the crank cases, which could lead to a failure later on!) and get the mallet again, dock dock dock..
Image
Here's the end of that bolt that was broken at the start - that won't be easy to get out!
Image
I also didn't know about this bolt holding the cases together. As ever though, it came undone with a click.
Image
Cylinder 1, on the left - not bad..
Image
Cylinder 2, oh shiiit:
Image

It is rough to the touch and well within the stroke of the piston, the motor doesn't burn oil any faster than it used to and it's never nipped up so I don't know how this has happened but.. Damn. So there I stopped. Is there any point going further? Has anyone had a rebore recently? How much did it cost? I could fit a Chinese cylinder and pistons for about £150 last time I looked but that is well into "just get another motor" territory. Oversize pistons and rings are £££ from Honda, but I might have a search for some new old stock just in case..

EDIT: Forget the rebore, there are barrel and piston sets on ebay for £50, like this. Hmmm, maybe I will carry on after all..
Image

fastbob
Posts: 3296
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 1772 times
Been Liked: 1314 times

Re: CB250 Engine rebuild

#23 Post by fastbob » Wed Dec 05, 2018 9:26 am

Not trying to teach you to suck eggs but I would have thoroughly cleaned the outside of the engine before I opened it up .

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#24 Post by Snod Blatter » Wed Dec 05, 2018 9:42 am

If I'm splitting the cases I'm going to be all over it for ages scraping the old gaskets off, when this is mostly exploratory I don't see the point of being that surgical.

Some searching reveals a rebore can be £5 a cylinder at some specialist/bulk car places, but nearer to £30 per cylinder seems normal. Oversize Honda bits are incredibly expensive at around £70 for a piston and rings, oversize 253FMM (Chinese version) are cheap but Chinese. There is a +0.5 oversize CD250U head and pistons on ebay for £70 which would bump it up from 233cc to (apparently) 242cc, cor. This all seems a bit silly compared to just chucking another motor at it though, one appeared last week for £90 - it was known to be running but had lived in a kart for a while and was covered in green. I was still tempted but I hadn't seen the cylinder at that stage.. Now it's sold.
Image

fastbob
Posts: 3296
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 1772 times
Been Liked: 1314 times

Re: CB250 Engine rebuild

#25 Post by fastbob » Wed Dec 05, 2018 11:13 pm

Why not buy some new rings and measure the end gap first . You never know, you might be able to get away with a light hone instead of a full rebore and oversize pistons . You can get a honing tool from Machine Mart and do it yourself. I recently did this on my replacement ER5 engine . Definitely worth watching a few YouTube videos first though.
You do not have the required permissions to view the files attached to this post.

User avatar
Joeman
Posts: 11295
Joined: Mon May 28, 2012 2:25 pm
Gender: Male
Bike(s): BMW S1000RR, KTM300.. etc
Location: Sevenoaks, UK
Has Liked: 1679 times
Been Liked: 1477 times

Re: CB250 Engine rebuild

#26 Post by Joeman » Wed Dec 05, 2018 11:44 pm

JB weld/Chemical metal would sort those holes out.
Fill them up and drill them out. Easy.
Image

User avatar
Speedy23
Posts: 1340
Joined: Wed Oct 25, 2017 9:33 pm
Gender: Male
Bike(s): Yamaha FJR 1300A
Location: nr Shipston on Stour, Warwickshire
Has Liked: 148 times
Been Liked: 460 times

Re: CB250 Engine rebuild

#27 Post by Speedy23 » Sat Dec 08, 2018 8:07 pm

fastbob wrote:
Wed Dec 05, 2018 11:13 pm
Why not buy some new rings and measure the end gap first . You never know, you might be able to get away with a light hone instead of a full rebore and oversize pistons . You can get a honing tool from Machine Mart and do it yourself. I recently did this on my replacement ER5 engine . Definitely worth watching a few YouTube videos first though.
......yes, there's a good video from Ichiban Moto on cylinder boring.....




Just the ticket!
Oh my God, what's happened? I don't know man...I just got here myself!

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#28 Post by Snod Blatter » Sun Dec 09, 2018 4:53 pm

I don't know what's going on with bob and Joe but at least I can tell Speedy's post is definitely a joke.. Anyway, the rain cover of the CB250 has been torn up in the wind and the gorgeous red frame is flashing me from the top of the garden. I was disheartened with the damage to the bore (which is never going to magic away with a hone, wtf) but decided to see if at least the crank is dead. If there is play at the big end (crank end) or small end (piston end) then it's all definitely scrap, and that will be nice to know as it means I can give up on this altogether. So, bring the pistons up from their holes..
Image
And wiggle the rods up and down and side to side, then try and rock the pistons (well actually the gudgeon pins) side to side to see if the small ends are no longer a good fit. Sadly it is all rock solid and perfick, goddammit. Right, onwards..
Image
Clickety click go the bolts, I've been in here a few times.
Image
Image
Remove the drive bit section for the oil pump as it will be in the way:
Image
Image
Jam a pre-1992 (so it's bronze, not steel) penny between the primary drive gears and the clutch gears to stop it all turning and remove the clutch, err, disengagement (?) mechanism. Try not to let the fancy radial bearing in the middle fall on to the floor and pick up loads of dirt!
Image
I stopped there as it was getting dark and I couldn't find the special tools to hold the clutch basket still or the special tool for the ridiculous special nut in the middle. In theory it is easy from here on (once I've found those blasted tools) but I have a funny feeling it's going to be a faff of the highest order as I'll probably end up putting the motor back in the frame so I can hold it still while I remove the clutch nut and the primary drive nut - they be tight as.
Image

User avatar
Speedy23
Posts: 1340
Joined: Wed Oct 25, 2017 9:33 pm
Gender: Male
Bike(s): Yamaha FJR 1300A
Location: nr Shipston on Stour, Warwickshire
Has Liked: 148 times
Been Liked: 460 times

Re: CB250 Engine rebuild

#29 Post by Speedy23 » Sun Dec 09, 2018 5:43 pm

Ichiban Moto aside......I think this is a goer. I reckon you could rebore that barrel......just gotta get a piston to fit. All getting a bit interesting now.......is the motor somewhere you can work on it in the warm? I've had a number of bed/front room rebuilds........much nicer when you can feel your fingers! Anyhow Snod........press on!!
Oh my God, what's happened? I don't know man...I just got here myself!

Snod Blatter
Posts: 322
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 284 times
Been Liked: 140 times

Re: CB250 Engine rebuild

#30 Post by Snod Blatter » Sun Dec 09, 2018 6:29 pm

The barrel could of course be bored, and oversize parts from Honda are freely available though surprisingly expensive. But then, in the grand scheme of things with this bike (such as the ££££ in petrol I've thrown in it over the years) it's not expensive at all. I spent a lot more sorting the CBX250 out back in 2011-ish, a bike with which I have very little history compared to this one.

I am working outside currently but may be able to bring it indoors over Christmas, thankfully my factory shuts down over the festive season so I'll have a couple of weeks to dismantle and inspect at my leisure. The only blocker left is the condition of the crank case holes internally, and no I'm not filling them with JB Weld - it's a stressed member of the frame! They'd be oval again and the frame cracked again in no time.

Meanwhile I will continue to wish I had a proper workshop.. This place got listed on Friday, I shouldn't be so interested but look at that garage space!! Plus I think you get the whole building, so it's detached, so the Sabbath could be as loud as it should be :mrgreen:
Image