CB250 Engine rebuild

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Snod Blatter
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CB250 Engine rebuild

#1 Post by Snod Blatter » Sun Nov 25, 2018 5:34 pm

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:
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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:
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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:
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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.
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Oh no.
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Another one came undone. Ugh.
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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..
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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.
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No studs broke! Success.
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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!
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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.
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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..
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Re: CB250 Engine rebuild

#2 Post by Bungleaio » Sun Nov 25, 2018 6:49 pm

Nice little project and like you say if it doesn't work you can source a new one. It's always a relief to get things undone without shearing anything though.
You can't beat a nice bit of faff

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Re: CB250 Engine rebuild

#3 Post by Speedy23 » Sun Nov 25, 2018 7:17 pm

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?
Last edited by Speedy23 on Sun Nov 25, 2018 7:20 pm, edited 1 time in total.
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Re: CB250 Engine rebuild

#4 Post by megawatt » Sun Nov 25, 2018 7:20 pm

Have I missed something? You do have a workshop manual?
Bike work, I come to you and you can learn to do it yourself click the link below for more info
http://themotorbikeforum.co.uk/viewtopi ... 5&p=949652

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Re: CB250 Engine rebuild

#5 Post by Snod Blatter » Sun Nov 25, 2018 7:49 pm

Speedy23 wrote:
Sun Nov 25, 2018 7:17 pm
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.
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Re: CB250 Engine rebuild

#6 Post by Gerontious » Sun Nov 25, 2018 8:47 pm

He is an expert at pretending he has no expertise whatsoever.
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Re: CB250 Engine rebuild

#7 Post by fastbob » Sun Nov 25, 2018 9:23 pm

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

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Re: CB250 Engine rebuild

#8 Post by Speedy23 » Sun Nov 25, 2018 9:42 pm

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.....
Last edited by Speedy23 on Sun Nov 25, 2018 10:08 pm, edited 1 time in total.
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Re: CB250 Engine rebuild

#9 Post by Snod Blatter » Sun Nov 25, 2018 9:48 pm

Gerontious wrote:
Sun Nov 25, 2018 8:47 pm
He is an expert at pretending he has no expertise whatsoever.
You're too kind :scratch:
fastbob wrote:
Sun Nov 25, 2018 9:23 pm
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..
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Re: CB250 Engine rebuild

#10 Post by Mickly » Sun Nov 25, 2018 9:51 pm

Speedy23 wrote:
Sun Nov 25, 2018 9:42 pm
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 !!
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Re: CB250 Engine rebuild

#11 Post by Speedy23 » Sun Nov 25, 2018 10:08 pm

Mickly wrote:
Sun Nov 25, 2018 9:51 pm
Speedy23 wrote:
Sun Nov 25, 2018 9:42 pm
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?
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Re: CB250 Engine rebuild

#12 Post by Speedy23 » Sun Nov 25, 2018 10:15 pm

"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.
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Re: CB250 Engine rebuild

#13 Post by megawatt » Sun Nov 25, 2018 10:25 pm

Old Air cooled Suzuki cams have been running in non bearing heads for decades.
Bike work, I come to you and you can learn to do it yourself click the link below for more info
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Re: CB250 Engine rebuild

#14 Post by fastbob » 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

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Re: CB250 Engine rebuild

#15 Post by Snod Blatter » 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!
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