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: 4000
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 2189 times
Been Liked: 1663 times

Re: CB250 Engine rebuild

#31 Post by fastbob » Sun Dec 09, 2018 7:37 pm

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
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#32 Post by Snod Blatter » Sun Dec 09, 2018 8:06 pm

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

User avatar
Speedy23
Posts: 1619
Joined: Wed Oct 25, 2017 9:33 pm
Gender: Male
Bike(s): Yamaha FJR 1300A
Location: nr Shipston on Stour, Warwickshire
Has Liked: 190 times
Been Liked: 572 times

Re: CB250 Engine rebuild

#33 Post by Speedy23 » Sun Dec 09, 2018 8:15 pm

Snod Blatter wrote:
Sun Dec 09, 2018 8:06 pm
Here's the photo again, edited just for bob!
Image
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?
Oh my God, what's happened? I don't know man...I just got here myself!

Snod Blatter
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#34 Post by Snod Blatter » Sun Dec 09, 2018 9:41 pm

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!
Image

User avatar
Speedy23
Posts: 1619
Joined: Wed Oct 25, 2017 9:33 pm
Gender: Male
Bike(s): Yamaha FJR 1300A
Location: nr Shipston on Stour, Warwickshire
Has Liked: 190 times
Been Liked: 572 times

Re: CB250 Engine rebuild

#35 Post by Speedy23 » Sun Dec 09, 2018 10:10 pm

Snod Blatter wrote:
Sun Dec 09, 2018 9:41 pm
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.
Oh my God, what's happened? I don't know man...I just got here myself!

Snod Blatter
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#36 Post by Snod Blatter » Sat Feb 09, 2019 6:28 pm

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.
Image
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!
Image
Image
Image
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 penny 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:
Image
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:
Image
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:
Image
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..
Image
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..
Image
Keep sliding..
Image
and chuck it in the bucket for later!
Image
Getting there now 8-)
Image

Snod Blatter
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#37 Post by Snod Blatter » Sat Feb 16, 2019 7:50 pm

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:
Image
And simply screw one of these into it..
Image
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:
Image
The smallest Torx bit hammered into the small hole just nice..
Image
And it turned out no problem. Pah!
Image
Image
A quick check of the thread and all seems well, success!
Image
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...
Image

fastbob
Posts: 4000
Joined: Sat Sep 03, 2016 1:42 pm
Gender: Male
Bike(s): gsxr1100 wp and er5
Location: Coventry
Has Liked: 2189 times
Been Liked: 1663 times

Re: CB250 Engine rebuild

#38 Post by fastbob » Sun Feb 17, 2019 10:34 am

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
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#39 Post by Snod Blatter » Sun Feb 17, 2019 2:27 pm

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 8-)
Image

Snod Blatter
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#40 Post by Snod Blatter » Sun Mar 10, 2019 6:29 pm

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.
Image
Right, clean ("clean") the joint between the cases to appease the fastest of bobs:
Image
Remove the 3 bolts that are holding the cases together on the inside, sneaky buggers!
Image
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..
Image
Malleting was still required but eventually progress was made:
Image
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.
Image
Image
It still wouldn't come to bits though! Oh wait, I see.. The piston won't pass between the cylinder/head studs, ha.
Image
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.
Image
Image
Okay, make sure the floppy connecting rod is not in the way..
Image
Aaaand ta-da!
Image
Image
This is largely how bike gearboxes work. Amazing they last any length of time at all, these things take such a mashing.
Image
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..
Image
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!
Image
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 8-)
Image

skyrider
Posts: 406
Joined: Fri Jan 18, 2019 7:12 pm
Gender: Male
Bike(s): yamaha 225 serow
Location: preston lancs
Has Liked: 33 times
Been Liked: 77 times

Re: CB250 Engine rebuild

#41 Post by skyrider » Sun Mar 10, 2019 8:16 pm

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

Snod Blatter
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#42 Post by Snod Blatter » Sun Mar 10, 2019 10:39 pm

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 :seeya:

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!
Image

User avatar
MrPickwick
Posts: 953
Joined: Wed Jul 13, 2016 8:23 am
Bike(s): vfr750 CBR954
Has Liked: 376 times
Been Liked: 186 times

Re: CB250 Engine rebuild

#43 Post by MrPickwick » Mon Mar 11, 2019 12:38 am

Will this help
You do not have the required permissions to view the files attached to this post.

Snod Blatter
Posts: 405
Joined: Tue Jul 11, 2017 9:07 pm
Gender: Male
Bike(s): CBX250RS-E CB250N Two Fifty TRX850 K100RS Z250SL
Location: Leicester
Has Liked: 399 times
Been Liked: 170 times

Re: CB250 Engine rebuild

#44 Post by Snod Blatter » Mon Mar 11, 2019 8:35 pm

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!
Image

User avatar
smallfrowne
Posts: 57
Joined: Fri Dec 11, 2015 4:40 pm
Gender: Male
Bike(s): NC30, CG125, DR650, CD250U, VFR750FL
Location: Rov'rum
Has Liked: 73 times
Been Liked: 31 times

Re: CB250 Engine rebuild

#45 Post by smallfrowne » Wed Mar 13, 2019 2:16 pm

lol