Cb250 Engine Rebuild

Green199

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Ok, some of you may have seen the overhaul of my 1973 Cb250 (Link in my signature). Well, I didn't have time over the summer to completely finish the bike, and after taking it for a little spin, I lost compression on one cylinder.

I didn't have time to rebuild the top end, or the option to bring the bike back to university with me. Luckily I did have a spare motor which I brought back with me. I was planning to get it rebuilt and upgraded before Christmas so I could take it back, quickly swap motors and be done. Unfortunately, I have only just got round to starting work on it so that looks less likely as the days pass.

Anyway, save the essay, here's the start of the engine tear down and rebuild.

Obviously started by pulling the rocker cover off. The cam/timing chain was not present, neither were the rocker arms. So I will probably end up pulling the rockers from the current blown motor (after checking their condition), and getting a new cam chain.

I haven't managed to pull the valve springs yet, but will be getting hold of a spring compressor soon. The valve area (my lack of knowledge now becomes apparent) seems in pretty good shape. No dings or dents, just some carbon build up. The intake (the smaller ones?) valves seem to be pretty rusted from what I can see without pulling them, and the exhaust valves seem to be ok, just covered in carbon again.

Im not too sure about the gasket/surface area. It looks free from obvious deep scratches and dents, but Ill inspect it with a DTI later. It does seem to be covered in lots of small scratches.

The valve springs seem pretty clean and in good shape too, but again, I will be able to tell better once they are removed.
 

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Intake valves are the larger ones and the health of the valves (usually) has little to do with their appearance on the bottom and almost everything to do with how they look around the valve seat area.

Check this thread for more details: http://www.dotheton.com/forum/index.php?topic=39814.msg462905#msg462905
 
Ahh ok, thanks very much Sonrier. As I said, once I pull them I'll be able to check their condition better, was just trying to analyse where I am now. I will be consulting that thread a lot over the next few months, thanks for the link.



The pistons aren't in the best condition. There doesn't seem to be excessive carbon build up, just a tad, and no massive scoring on the sides of the piston.

The top edge does seem to have an area which has been slightly chewed up, so they will probably be replaced.

I had a quick look into the main case through the gap by the piston to check the conrod etc. The pistons and crank seem to move freely, and their doesn't seem to be any debris or metal chunks in their, which suprised me.
 

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The cylinders themselves didn't look too bad, but they will be honed anyway.

One of them had a slight bit of surface rust, but I don't think that will be a problem...am I correct in assuming that?
 

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The main thing I'm worried about is the Camshaft. The lobes seem to have a bit of pitting on them, and it seems chewed up pretty badly, so this will probably need replacing!
 

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Get out a set of calipers and check the bore widths. Possible that they may need to go .25mm over and an hone job won't be quite enough.

The surface of the piston isn't usually a good measure for wear, you'll want to check the ring lands and skirts.

The ring lands on your pistons don't look great, but I've seen worse. Looks like you may be getting a bit of blow by and the oil coking is never a good sign. Here's a pic of the pistons I pulled from my 360 (basically the same engine as the 250):

37201_492634470158_103256_n.jpg


Do you have a copy of the service manual yet?

And, yup, your cam is shot.
 
The cam is fine. It can be used to prop open the garage door or as the basis of a nice art piece. It's burned, gouged, worn and generally not much use as a cam unless the bearing surface is repaired and the cams reground. Call Joy. The cam people, not the girl at the chippie.......

Once your valve spring compressor comes in, remove each valve, springs and collets and put each into a marked bag. Measure the springs to see how much they sagged. Valves can be cleaned up and seats cut on a SERDI machine.

The pistons and bores are a bit the worse for wear. If it were mine, I'd look for a set of larger pistons. O/S stock work or maybe something larger. What size is the liner as it sticks out of the bottom of the barrels? That will give you an idea as to how large a piston might fit in that liner.

If there is no noticeable play in the bottom end, it will be fine. Cranks last a long time unless they get rusty.
 
Sonreir said:
Get out a set of calipers and check the bore widths. Possible that they may need to go .25mm over and an hone job won't be quite enough.

The surface of the piston isn't usually a good measure for wear, you'll want to check the ring lands and skirts.

The ring lands on your pistons don't look great, but I've seen worse. Looks like you may be getting a bit of blow by and the oil coking is never a good sign. Here's a pic of the pistons I pulled from my 360 (basically the same engine as the 250):

37201_492634470158_103256_n.jpg


Do you have a copy of the service manual yet?

And, yup, your cam is shot.

Yes sir, I do have a manual, but only a digital one. Could do with printing it so its more at hand.

I will get a bore gauge and check as well, but I was planning on getting some oversize pistons. The only thing is i'm not certain of the model...whether it is a k model or a g5. D'you know if pistons for them would be interchangeable, i'm guessing they have the same bore?
 
teazer said:
The cam is fine. It can be used to prop open the garage door or as the basis of a nice art piece. It's burned, gouged, work and generally not much use as a cam unless the bearing surface is repaired and the cams reground. Call Joy. The cam people, not the girl at the chippie.......

Once your valve spring compressor comes in, remove each valve, springs and collets and put each into a marked bag. Measure the springs to see how much they sagged. Valves can be cleaned up and seats cut on a SERDI machine.

The pistons and bores are a bit the worse for wear. If it were mine, I'd look for a set of larger pistons. O/S stock work or maybe something larger. What is the liner as it sticks out of the bottom of the barrels? That will give you an idea as to how large a piston might fit in that liner.

If there is no noticeable play in the bottom end, it will be fine. Cranks last a long time unless they get rusty.

Thanks very much! Looks like i'm gonna have to swap the camshaft over from the motor currently in the bike. (Hoping its in decent condition, should be as it was running well....until I blew the top end ahah)

I will try and measure the liner over the weekend to see what pistons are compatible. Looking at cb250 G5 +1mm pistons, but I 'think' this engine is a k model, either k3 or k4.

Sweet! Yeah the bottom end seems pretty solid so may refrain from splitting the cases.
 
Ok, havent managed to spend hardly any time on the engine due to uni work etc.

I have however managed to locate a valve spring compressor, so I will be pulling the valves as soon as I can.

Trying to pull my finger out and get this thing rebuilt, so I've got a complete gasket kit on its way, and after much ebay searching, these have just arrive at my door! Shiny 'new' old 1mm os pistons!
 

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Managed to pull the valves. They don't seem too bad in my opinion, and should clean up quite well. But, I don't know a lot and will probably be proven wrong.

There is a bit of corrosion on one of the exhaust valves. Should I be worried about this?

Plan is, obviously, to get the head cleaned and then lap the valves.
 

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The valve seat area also seems ok, so once its cleaned and lapped, should be good.

And yes, there is a picture included to show you that my top end currently lives in my chest of drawers in my room! haha
 

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Just got my gasket kit through today so I can start making some headway...plan to get it done before christmas, whether or not that happens....who knows.

slowly making progress. Got the clutch plates off but not the actual basket. Hope to get that off soon so I can split the cases and have a look. Looked over the friction plates. No obvious signs of wear but i'm no expert. Maybe Ill just replace the springs and go from there.
 

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Cleaned the valves up as best as I could for the moment. Will probs hit them with the wire wheel to get the excess carbon off them. Used some oven cleaner to get most of it off and cleaned the head with it aswell. This stuff worked amazingly well!
 

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Used it on the valve seat area aswell. Cleaned all the carbon build up off in minutes.

Got some valve lapping compound today as well so as soon as I clean the actual valves, I can get them lapped and ready.
 

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You have any close up shots of the valves? They look a little pitted in the shots that I can see.
 
Sonreir said:
You have any close up shots of the valves? They look a little pitted in the shots that I can see.

I tried taking some but i'm using my phone and it doesn't seem to want to focus. I'll try and dig up a camera and take some soon.

The little black marks on it look like pits, but when I look closely they don't seem to actually 'indent'or go in if you catch my drift. The surface seems to be smooth and level but obviously not.

Will lapping get rid of them?
 
If it does, then you know it's not pitting. ;)

It's not uncommon for a bit of oxidation to occur and it can discolor the metal without ruining it.
 
Finally back home with my bike!

Brought the motor down to carry on the rebuild.

Ran into a bit of a problem with the oil filter. I know you need a custom honda socket, or to make one, but mine is looking a bit ruined. Seems the previous owner tried to take it off without the tool, and its pretty chewed up, so I may have to try and get it off and then replace it.

So until I sort that, I cant split the cases.
 

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Also managed to lap the valves and get the head re-assembled.

A few of the valves seem to still have a bit of discolouration on the surfaces after being lapped, not sure if that is ok or not?

Going to do the kerosene test to check everything is ok tomorrow.

I also cleaned up the gasket surfaces ready for it to be put back together.
 

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