CB250 Engine rebuild

[Joeman]

JB weld/Chemical metal would sort those holes out.

Fill them up and drill them out. Easy.

[Guest]

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.....

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Just the ticket!

[Snod Blatter]

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..

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..

Clickety click go the bolts, I've been in here a few times.

Remove the drive bit section for the oil pump as it will be in the way:

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!

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.

[Guest]

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!!

[Snod Blatter]

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

[fastbob]

We'll I've had another look at your picture of the cylinder where you say " oh shiiit " and although I completely take your word for it that its knackered I can't quite make anything out from the photo. So a light hone obviously won't save the day . Regarding the engine bolt holes , they could possibly be drilled over size and then sleeved down to match the bolts . Failing that, you could simply insert some strips of metal alongside the bolts and tighten everything up with a few friction washers for good measure. As long as the engine can't move it'll be ok plus nothing will be visible.

[Snod Blatter]

Here's the photo again, edited just for bob!

I can only guess this happened either when it was stood outside after it was found to be right mega broke, or perhaps something fell in from a plug changing session and broke the surface which has then be worn away more and more. In time I'll also check if the bores are oval, that will be a good way to find if they can even be rebored or if a replacement cylinder is the only option.

Drilling out the holes and putting in sleeves is the plan, but from the "inside" so that I can be fairly sure they're in the right place.

[Guest]

Here's the photo again, edited just for bob!

I can only guess this happened either when it was stood outside after it was found to be right mega broke, or perhaps something fell in from a plug changing session and broke the surface which has then be worn away more and more. In time I'll also check if the bores are oval, that will be a good way to find if they can even be rebored or if a replacement cylinder is the only option.

Drilling out the holes and putting in sleeves is the plan, but from the "inside" so that I can be fairly sure they're in the right place.

Resleeve it? Possible?

[Snod Blatter]

If by resleeve you mean replace the liner then I'm sure it is possible but why bother when there are so many decent blocks around?

If you mean resleeve as in sleeving the engine mount holes with steel after drilling them out, anything is possible. There are no problems, only opportunities to improve!

[Guest]

If by resleeve you mean replace the liner then I'm sure it is possible but why bother when there are so many decent blocks around?

If you mean resleeve as in sleeving the engine mount holes with steel after drilling them out, anything is possible. There are no problems, only opportunities to improve!

 

I was on about pressing new liners into the block, but, as you say, if there are loads of s/h blocks out there, then probably not worth it. But keep us posted - interested to see how it's going.

[Snod Blatter]

Right, where was I? Who's that Speedy bloke? Why is it February already?

So, I gave up finding my clutch nut tool and bought another one. Thankfully the Far East has gotten in on the game and this was mine for under £3, quite different to my old one which I think was more like £15! I just had to wait a while for it to arrive.

The next problem made itself immediately apparent, the nut is on the countershaft which is the first shaft of the gearbox (so called because it turns the opposite way to the output shaft and the crank). With the engine out of the frame I couldn't use the rear brake to stop the gearbox from moving, but thankfully an electric impact driver did the trick!

The clutch basket is decidedly worn with bits eaten out of the fingers and the splines of the inside but I'll think about that later. So, stick the [strikeout]penny[/strikeout] special tool in between the clutch basket and the primary drive gears yet again and use the glorious impact driver to undo the nut with hardly any fuss:

Sadly this bent the special tool, this strip down is costing me a fortune! Slide the clutch basket off the countershaft, pull the primary drive gears off the end of the crank and then take the oil pump off:

I always treat Japanese screws like bolts, use a proper bit that has a nice snug fit and use a ratchet so you can press the bit into the screwhead while you turn it. This has given great success so far, I always cringe when people recommend buying JIS screwdrivers because how are you going to tighten them up properly? How are you even going to undo them if they're as tight as these were?? Just use bits. Anyway.. With the pump out of the way we can see the detent mechanism for the gearbox, this is what makes it "slot" into different settings:

The "star" is on the end of the selector drum, without that snicky mechanism it'd all just kind of flop about. The thing that really struck me about this is the circle on the end of the springy arm which is being pressed into the detent star - surely it shouldn't be like that? Remove the star, remove the arm and..

Yeah that's worn alright! That should be a round pin holding the circle in place, worn down to an ellipse.. A new one of those may well be all the gearbox needs! Sometimes I forget that 93K is a lot of miles.. Next up is the selector shaft, which is the thing with the springy jaws on it that moves the selector drum. Clean it up, start sliding..

Keep sliding..

and chuck it in the bucket for later!

Getting there now

[Snod Blatter]

Before splitting the cases I thought I'd better have a go at removing what is left of the bolt that was supposed to be clamping down the head and cylinders but had instead been broken for a long time, possibly even from initial construction. So, drill a small hole:

And simply screw one of these into it..

Yeah nah. I tried and tried, drilling the hole bigger and deeper (risky when I'm holding the drill!) but these blasted things just weren't interested in turning it out. So, plan B:

The smallest Torx bit hammered into the small hole just nice..

And it turned out no problem. Pah!

A quick check of the thread and all seems well, success!

Shall I split the cases tomorrow or go for a ride instead? Life is becoming a bit all work and no play but I'm so close to getting to the proper gubbins, hmm...

[fastbob]

I hope you're not offended but I've just got to say it . I'm amazed that you are going ahead and opening up this engine with all the dirt still in place on the outside.

[Snod Blatter]

I think I will clean around the join between the cases, since this has all gone so well so far and it doesn't appear to be a lost cause just yet.

Today I have done nothing at all, lovely

[Snod Blatter]

Slow burner this one, eh? As if to suggest I should get on with it the wind blew the cover off, ah yes the red frame! I should get on with that.

Right, clean ("clean") the joint between the cases to appease the fastest of bobs:

Remove the 3 bolts that are holding the cases together on the inside, sneaky buggers!

Needless to say, after 25 years the two halves did not exactly fall apart. Fed up with faffing about I ran some washing line through the dipstick and clutch pivot shaft holes and hoisted it up, that'll do it..

Malleting was still required but eventually progress was made:

They still wouldn't separate though. Turns out the right bearing for the output shaft is quite a good fit into the right case, a bit of levering with wood and some more malleting sorted that out.

It still wouldn't come to bits though! Oh wait, I see.. The piston won't pass between the cylinder/head studs, ha.

Pistons are easy enough to remove, just pick the clip out from one side and slide the gudgeon pin out. Of course the clip sprang away with the ferocity of a thousand things that never want to be found but never mind, there should be more clips with the next set of pistons anyway and if not I hope to God they are cheap to buy.

Okay, make sure the floppy connecting rod is not in the way..

Aaaand ta-da!

This is largely how bike gearboxes work. Amazing they last any length of time at all, these things take such a mashing.

Now, one of the main things I want to do is replace the cam chain. I was hoping the teeth on the crank would be easily accessible, or at least that the chain could be replaced with the crank still fitted to the left case half. Apparently this is wishful thinking..

This is actually a bit of a disaster. My only hope is that I can undo the nuts/bolts holding the carrier and it'll give me enough room to slip the chain through but right now it looks like I'm going to have to remove the crank, which means getting the flywheel off, which means putting it back together so I can lock the crank up again while I muller the rotor. Even if the bolt comes out the end I've seen stories of people having to cut the rotor up to get it off the taper, I really hope there's an easier way!

Anyway, those mounting holes - that was the original point of this. Guess what, they're not all that oval at all but they're also apparently not sleeved down to fit the small bolts that go though them!

Surely this can't be right? They are ever so slightly stepped down in size but only so the dowel pin locates properly. I'm not quite sure how to tackle this now, maybe drill the holes in the holding bracket to take bigger bolts??

On the plus side the motor is now in the living room so I can check things over in the warm

[skyrider]

strewth what about oil on the carpet will you not get chastised

[Snod Blatter]

I have put the left case in a box, with the crank pointing upwards so hopefully it will hold the oil. The rest is wrapped up in cloths/plastic bags in the garage.. If it does leak I will indeed be in trouble though

Right, time to hit the 2fiftycc.com forum and see if anyone knows any short cuts for this chain.. I might even consider one with a rivet link because these rotors/flywheels/whatever you want to call them simply do not come off. The whole flywheel area runs dry, not in oil like most bikes, and therefore it all rusts into place. Eep!

[ThePhatomfart]

Will this help

[Snod Blatter]

Not really, I've already seen it.. Thanks though. That picture is misleading, the end of the crank that is nearest to you in that picture is the end with the flywheel on so they've already pulled it out the crankcase. It's probably what gave me the idea I could get away with half a job!

[smallfrowne]

lol

[Snod Blatter]

lol

no u

Moving on.. I was faced with this, the "carrier" for the middle bearing.

I undid all the bolts (one was threadlocked, that had to be rattled out!) in the vain hope I could turn the carrier and open up spaces to get the cam chain out. This didn't work. Right, long way round we go.. Alternator cover off:

Don't lose the gear that goes on the end of the starter motor! I still can't believe the starter clutch is chain driven, even the '84 CBX250 is gear driven but whatever.

Don't faff about.

With the bolt out the middle it's time to use a 12.9 bolt to pull the flywheel off. This took quite a bit of doing with tightening, hammering, tightening, hammering, cringing, tightening, hammering, tightening, cringing.. But eventually it gave in!

Once loose the flywheel falls off the taper, no problemo.

Annoyingly the rollers for the starter clutch a) are covered in rust and b) fell out. This will be a game to put back together, ho ho!

I'll stick them in there for now, good job it's magnetic eh?

In preparation for crankshaft removal remove the other piston so it'll go past the studs for the cylinders/head..

To try and prove that I'm not always a complete numpty here's proof that I managed to not lose a gudgeon pin clip!

So, undo the bolts for the carrier again:

And use Thor's leather hammer to persuade the crankshaft out.

A few taps and out she comes, lovely.

By gum I've got a lot of gasket surfaces to scrape. So, that's the cam chain replacement sorted - now what about those oval holes?? Going to take some thinking about.

[Snod Blatter]

The oval holes have been thought about and I have a plan, but for now I've still got all this incredibly boring gasket scraping to get on with. The only interesting thing I can report is that I've replaced the cam chain and this was made strangely difficult because the carrier for the middle bearing had actually slid off the bearing and wouldn't let me get the chain out or take the tensioner off its pin! It took me a while to figure this out, the crank wouldn't even go round..! I can only guess this must have happened when I malleted the crank out of the left case.

It was nothing a few blows with a wooden mallet couldn't sort, once the carrier was moved back (upwards in that shot) there was enough room to get everything off. In the interest of science, here are the new and old chains side by side:

The new one is at the top, if that isn't clear. So the old one is slightly worn and a little bit stretched, I guess this would put the cam timing out a few degrees but considering a new Morse chain was £36 I'd rather see a more dramatic difference! Considering this chain has done 93K I'd suggest most of these engines will never need a new chain.

More interesting updates coming soon, promise.

[Guest Richzx6r]

The oval holes have been thought about and I have a plan, but for now I've still got all this incredibly boring gasket scraping to get on with. The only interesting thing I can report is that I've replaced the cam chain and this was made strangely difficult because the carrier for the middle bearing had actually slid off the bearing and wouldn't let me get the chain out or take the tensioner off its pin! It took me a while to figure this out, the crank wouldn't even go round..! I can only guess this must have happened when I malleted the crank out of the left case.

It was nothing a few blows with a wooden mallet couldn't sort, once the carrier was moved back (upwards in that shot) there was enough room to get everything off. In the interest of science, here are the new and old chains side by side:

The new one is at the top, if that isn't clear. So the old one is slightly worn and a little bit stretched, I guess this would put the cam timing out a few degrees but considering a new Morse chain was £36 I'd rather see a more dramatic difference! Considering this chain has done 93K I'd suggest most of these engines will never need a new chain.

More interesting updates coming soon, promise.

 

I'm surprised that the old chain doesnt look that knackered, would have expected it to be falling apart

[fastbob]

The oval holes have been thought about and I have a plan, but for now I've still got all this incredibly boring gasket scraping to get on with. The only interesting thing I can report is that I've replaced the cam chain and this was made strangely difficult because the carrier for the middle bearing had actually slid off the bearing and wouldn't let me get the chain out or take the tensioner off its pin! It took me a while to figure this out, the crank wouldn't even go round..! I can only guess this must have happened when I malleted the crank out of the left case.

It was nothing a few blows with a wooden mallet couldn't sort, once the carrier was moved back (upwards in that shot) there was enough room to get everything off. In the interest of science, here are the new and old chains side by side:

The new one is at the top, if that isn't clear. So the old one is slightly worn and a little bit stretched, I guess this would put the cam timing out a few degrees but considering a new Morse chain was £36 I'd rather see a more dramatic difference! Considering this chain has done 93K I'd suggest most of these engines will never need a new chain.

More interesting updates coming soon, promise.

 

I'm surprised that the old chain doesnt look that knackered, would have expected it to be falling apart

 

Is it possible to fit a manual cam chain tensioner ? I fitted one to my ER and it worked wonders. I doubt that anyone makes one but one could be adapted .

[Snod Blatter]

Is it possible to fit a manual cam chain tensioner ? I fitted one to my ER and it worked wonders. I doubt that anyone makes one but one could be adapted .

Given that it isn't a standard plunger style tensioner I think not, but really why would you bother? The original never gave any trouble, the chain never made any noise and yet it hasn't been stretched to oblivion so I'd say it works absolutely perfectly. Now if a manufacturer can find a way to tension a final drive chain in the same manner and bother to fit hydraulic buckets for automatic valve clearance adjustment then we'll be set!