Jump to content
  • Sign up now

    Registration is quick and easy 

CB250 Engine rebuild


Recommended Posts

A little background, perhaps. In 2010 I purchased a 1994 CB250 for £430, hated it for a bit but grew to respect it because I couldn't break it. Fast forward to 2017 and I'm using it to go to work and tours, culminating in a frankly ludicrous trip to the bottom of Spain:

R6dOo9E.jpg

By the time I got back the poor thing had 93000 miles under its spindly wheels but I was more in love with it than ever. Sadly it wasn't in the best of health, having a broken frame and some ovalled mounting holes in the motor:

jCutYOL.jpg

The frame wasn't too hard to sort, I just needed to find a bloke dodgy enough to do it, but the holes in the motor are proving rather more troublesome. It was suggested on here to fill them in with low melting point aluminium weld stuff but the cases are split vertically and I'm not sure the gasket would take that much heat, plus how on Earth would you heat that much metal that well anyway? I also recently found that, despite all documentation from Honda saying there isn't a dowel there, there is a dowel pin in one of these holes:

0tGkhHa.jpg

And it's 12mm diameter while the mounting bolt is 8mm. This also means it'll be a nightmare to drill out from the outside to make it round again as I'll likely be drilling through the dowel. The only reasonable course of action seems to me to be to split the cases and drill the holes out from the inside, then probably sleeve them with steel down to 8mm at the outside edges - this will be better than stock as it seems there is a similar amount of metal there from the factory, but of course it's just aluminium. Also it should cost nothing to at least dismantle the motor and see what's what, and if it seems to be worth doing I can also fit a new cam chain/blades and some new gearbox bits to hopefully make it do things like change gear nicely and then stay in that gear. So, first things first, will all the bolts undo? No great loss if they don't, I'll just get another motor and save myself all the hassle.. This one definitely won't want to know anyway.

yqsBTEG.jpg

Oh no.

K74G7Dr.jpg

Another one came undone. Ugh.

PoV6RTF.jpg

All the bolts holding the cases together also came undone. Guess I'm all out of excuses. Let's get this cam and head off then..

wlyXcNe.jpg

This bolt is actually broken! There are no signs of anyone having been in there before so I can only imagine it has been like this since it left the factory, which may explain why oil gets past the head gasket so readily.

mBrEAV6.jpg

No studs broke! Success.

U7IKe7j.jpg

If anyone knows their history, Honda spent decades trying to get cams to run directly in the head without bearings with disastrous consequences. They overcame this with the CMX250C by fitting steel sleeves around the ends of the cam. I can't really believe this is any cheaper to do than fit some bearings but perhaps it was a matter of principle by the early 80s!

fpNfY9P.jpg

Spring loaded rockers, and perfect surfaces of the rocker faces and cam lobes. Unbelievable.


The next problem was the cam chain tensioner - how does it work?? Eventually, with judicious use of mole grips (a tool that should never be anywhere near any engine internals!) I figured it out.. Press the square button thing (cam side) down and then heave the other bit upwards, until you can shove a thing in there to stop it going back down.

Z2Pkogj.jpg

2wqTZWb.jpg

This still doesn't give me enough slack in the chain to get it off the cam sprocket though, I think I'm going to have to unbolt the sprocket to get a little more room to work with. But I'll consult google first, I can't believe it needs to come to that. After so much faffing it was dark so that's that for this week..

Link to comment
Share on other sites

Errrr.....have you actually got a manual for this bike? Just that I've never stripped an engine (especially with vertically split cases) down by removing the crankcase bolts before removing the head/cylinders/pistons.


Or is the narrative somewhat truncated?

Edited by Anonymous
Link to comment
Share on other sites

Errrr.....have you actually got a manual for this bike? Just that I've never stripped an engine (especially with vertically split cases) down by removing the crankcase bolts before removing the head/cylinders/pistons.


Or is the narrative somewhat truncated?

I haven't removed them, I just made sure each one came undone and then nipped them back up, one at a time - I've been bitten by crank case bolts before, absolute nightmare if they're seized solid. I wanted to know if there were any obvious "blockers" to this before I put some time into it, having been stopped by a seized bolt before.


Having checked all that, and feeling like I knew what I was doing, I was disappointed that it took me so long to figure out how to let the tensioner off! And I do have a Honda workshop manual but it could be more informative.. At least I have the torque figures to hand though.

Link to comment
Share on other sites

Mounting holes can be drilled and sleeved - camchain will probably come off when sprocket is unbolted from camshaft allowing you to lift off chain and wiggle the camshaft out? Never done an OHC bike with a hyvo camchain, though.


Some of the American sites reckon you can split 'em but I'm a bit dubious......we shall see.....

Edited by Anonymous
Link to comment
Share on other sites

He is an expert at pretending he has no expertise whatsoever.

You're too kind :scratch:

 

Would it not be a whole lot less hassle to transfer all your components into another set of crank cases ?

This is a distinct possibility if I can find some that aren't incredibly expensive - same thing with replacing the whole motor. This is really a "make do and mend" scenario but if something better comes along before I'm in way too deep with this then I'll happily change tack. I've just discovered this springy tensioner thing is £65 so with some gearbox bits and a cam chain and a load of gaskets thrown in I must be looking at £200 before I even start. Oh also the clutch is worn out (including the steel plates, I don't know why they wear but they do) so that's another £50 or so. Hmmm.


Some googling reveals I do indeed have to unbolt the sprocket from the cam shaft, oh joy. I should've just got on with it I suppose, I could've had the head off today - oh well!


Speedy - People have been splitting hyvo chains for a long time but I like the idea as much as you..

Link to comment
Share on other sites

Mounting holes can be drilled and sleeved - camchain will probably come off when sprocket is unbolted from camshaft allowing you to lift off chain and wiggle the camshaft out? Never done an OHC bike with a hyvo camchain, though.


Some of the American sites reckon you can split 'em but I'm a bit dubious......we shall see.....

 

Bet there’s an Itchybum video showing how it can be done with a tin opener and a small avacado !!

Link to comment
Share on other sites

Mounting holes can be drilled and sleeved - camchain will probably come off when sprocket is unbolted from camshaft allowing you to lift off chain and wiggle the camshaft out? Never done an OHC bike with a hyvo camchain, though.


Some of the American sites reckon you can split 'em but I'm a bit dubious......we shall see.....

 

Bet there’s an Itchybum video showing how it can be done with a tin opener and a small avacado !!

 

As long as it's a Snap-On avo, I don't see a problem?

Link to comment
Share on other sites

"If anyone knows their history, Honda spent decades trying to get cams to run directly in the head without bearings with disastrous consequences"....


They did succeed in getting their camshafts to run directly in their cylinder heads, alright......they just didn't run for very long, that's all. Coupled with that bloody so-called centrifugal "oil filter" (impossible to get at and therefore even more impossible to clean...oh, how we used to laugh...)....a recipe for disaster indeed.

Link to comment
Share on other sites

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!

Link to comment
Share on other sites

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 ?

Link to comment
Share on other sites

More CB pics, you say.. Hmm I might have the odd one (!)

S4GEBIt.jpg

sElwFtN.jpg

And how about one without all the luggage on? This was before I recovered (and refoamed!) the seat:

ZWMJEH7.jpg

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..

Link to comment
Share on other sites

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-Nighthawk-1996-Clutch-Cover/183526646187?_mwBanner=1&_rdt=1 ahh , no kick start , sorry.

wp_ss_20181126_0007.thumb.png.4cb130130e74abb6ff72f5cf180a4dac.png

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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 !

Link to comment
Share on other sites

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:

7qvXtd6.jpg

Stuck a 14mm socket on the flywheel bolt to stop the motor turning while I undid the cam sprocket bolts:

CvzrdwC.jpg

And job's a good 'un.

uBzGhRq.jpg

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..

nyH0XLk.jpg

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:

mpkbuyg.jpg

So off that came and then the bolt came undone:

30BG3IR.jpg

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:

BKedmdv.jpg

A few light taps with a rubber mallet and..

Aju9JcS.jpg

What are these, valves for ants?

M2ewm2D.jpg

Crusty valves for ants..

h0uqAHg.jpg

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..

pdkmKy4.jpg

Here's the end of that bolt that was broken at the start - that won't be easy to get out!

pkzM5d0.jpg

I also didn't know about this bolt holding the cases together. As ever though, it came undone with a click.

ZyT2Rkj.jpg

Cylinder 1, on the left - not bad..

GIGaIle.jpg

Cylinder 2, oh shiiit:

hV0FeWS.jpg


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..

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

wp_ss_20181205_0001.thumb.png.4daf35816585c6cd90b1cd1f9226e5f8.png

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

Terms of Use Privacy Policy Guidelines We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.