Possible Engine Seizure - REBUILD THREAD: *HELP!!*

Right, so I had a previous thread about my engine going bang the other day, and the good folks here convinced me it was a better idea to get stuck in myself than pay someone too much money to do it instead. So far, I'm glad they did as it's been pretty easy!

But I'll start at the beginning....

So, I went to work getting the engine out of the frame. I already had the gas tank off and some other bits disconnected so it was just a matter of unplugging all the wires, dropping the pegs and brake lever off then unbolting and lifting out:



So now it was on my bench...




Now, here's where it gets interesting and very apparent why it stopped working:



...now, I know my knowledge on bike engines is small, but experience with old cars tells me there should be a chain there somewhere!! :


Valve head off





...everything looks okay there

No piston damage that I can see of. Just a load of carbon which I expected:



The bit of metal sticking up between the cylinders, bottom right, doesn't look right, and feels sharp on top. It that damaged?



Cylinder head was on fast, and I unfortunately broke a tiny piece of a cooling fin off easing it, but all the cylinder walls look sound





Still, I got it off and the pistons are fine:



I think this thing is the cam chain tensioner, it looks a little worn... would you agree?




...And here is the offending article. Little bastard...




SO

Things I definitely need include a new cam chain, I'm guessing a fresh tensioner too as that looks a bit knarled-up...and a gasket set of course. What is the likelyhood of the bottom end being a big hot mess? Any advice? There's obviously a piece of chain in there somewhere too which doesn't fill me with much joy.

Help on what you'd recommend doing next would be greatly appreciated

Re: Possible Engine Seizure - REBUILD THREAD

Did you drain the oil out of the lower? If it was full of big metal shavings that would be an indication that you might have issues down there, but Im betting it's not too bad. I would rebuild it since you're this far. Good luck, should be fun.

Re: Possible Engine Seizure - REBUILD THREAD

Looks like a blast.
Tensioner is toast, its supposed to be round with teeth on it and should rotate with the chain, that ones obviously been sitting still hence the huge flat spot.

Re: Possible Engine Seizure - REBUILD THREAD

Alaskanrocket said:

Did you drain the oil out of the lower? If it was full of big metal shavings that would be an indication that you might have issues down there, but Im betting it's not too bad. I would rebuild it since you're this far. Good luck, should be fun.

Click to expand...

Not yet. Pretty much all the oil came out when I pulled the LH cover yesterday so I was planning to pull the plug tomorrow or at the weekend.

I'm guessing there's no easy way to get a new chain on? Bottom end open?

Re: Possible Engine Seizure - REBUILD THREAD

JohnnySomersett said:

Not yet. Pretty much all the oil came out when I pulled the LH cover yesterday so I was planning to pull the plug tomorrow or at the weekend.

I'm guessing there's no easy way to get a new chain on? Bottom end open?

Click to expand...

Yup gotta break her open.
Which is a good thing anyway so you can see what kind of shape its in.

Re: Possible Engine Seizure - REBUILD THREAD

SONICJK said:

Looks like a blast.
Tensioner is toast, its supposed to be round with teeth on it and should rotate with the chain, that ones obviously been sitting still hence the huge flat spot.

Click to expand...

I don't think it's supposed to have teeth on it, the replacement looks like this:




...but yeh, mangled is a good word for it!

To do list so far in my head is:

Clean pistons and valves of carbon buildup
Fit new piston rings - it was burning oil before and I was getting a little fumes through the breather so might as well renew them.
Replace cam chain tensioner
Replace can chain (dont know how to do this yet!)
Clean out engine and oil filter
New gaskets all 'round
Clean up engine while its out, might 'shot and paint the cylinder tube
Pull sheered nuts out and replace all dubious fixings with new ones


any glaring gaps, please shout. I am open about my ignorance

Re: Possible Engine Seizure - REBUILD THREAD

My bad amigo,
I was thinking of this:
http://www.ebay.com/itm/251107806767

Re: Possible Engine Seizure - REBUILD THREAD

The sharp piece of metal opposite the cam chain tensioner is a guide, and though I've not been into a CB350, yours looks pretty well fucked. You'll want to replace that as well.

Re: Possible Engine Seizure - REBUILD THREAD

JohnnySomersett said:

I'm guessing there's no easy way to get a new chain on? Bottom end open?

Click to expand...

Really glad to see you're having a go at it, nice work so far. Yes, would be a good opportunity to clean out the bottom end as well, since there may be bits of metal and tensioner down there now.

Re: Possible Engine Seizure - REBUILD THREAD

Rich Ard said:

The sharp piece of metal opposite the cam chain tensioner is a guide, and though I've not been into a CB350, yours looks pretty well fucked. You'll want to replace that as well.

Click to expand...

Haha, I thought as much. Not an expensive part thankfully (like, £12). In case it makes a difference later on in the thread/rebuild, this is a CB250... not that there's much difference.

Re: Possible Engine Seizure - REBUILD THREAD

Well, I clearly haven't opened a CB250 either

Re: Possible Engine Seizure - REBUILD THREAD

HerrDeacon said:

Really glad to see you're having a go at it, nice work so far. Yes, would be a good opportunity to clean out the bottom end as well, since there may be bits of metal and tensioner down there now.

Click to expand...

Cheers man, and thanks for the push. I actually feel really good about this now, already a massive sense of satisfaction.

Should I take the pistons off before I turn it over? What order should I attack this in?

Re: Possible Engine Seizure - REBUILD THREAD

SONICJK said:

Yup gotta break her open.
Which is a good thing anyway so you can see what kind of shape its in.

Click to expand...

Yep, sounds like you were due for some seals anyhow.

Re: Possible Engine Seizure - REBUILD THREAD

Not sure about the CB250 but the 350 needs the bottom cracked open to replace the chain. As well now that you have the upper Cam Chain sprocket off, get new bolts for it as well. The old ones, once re-torqued will not hold up. A few years ago they were still available OE.

Re: Possible Engine Seizure - REBUILD THREAD

CB250 = CB350 with a smaller bore....Otherwise identical.

The Manual for the CB350 covers the 250...only the cylinder and piston dimensions are different. they even weigh the same.

Re: Possible Engine Seizure - REBUILD THREAD

TheCoffeeGuy said:

Not sure about the CB250 but the 350 needs the bottom cracked open to replace the chain. As well now that you have the upper Cam Chain sprocket off, get new bolts for it as well. The old ones, once re-torqued will not hold up. A few years ago they were still available OE.

Click to expand...

That's exactly the sort of information I need, I'd have never thought to change those! Already found them online and now on the list

Re: Possible Engine Seizure - REBUILD THREAD

CB 350 and 250 are pretty much identical, except for jugs and pistons.

Poor maintenance was what killed this bike.

If the chain broke while running, you must, MUST check your valves, they may have made contact with pistons. One or more valves may be bent. Remove the valves and roll the stems on a sheet of glass to test for bends. The naked eye is not enough. May as well do a valve lapping since the valves are going to be out.

Crack the cases and make sure that ALL bits of metal are out of whatever grunge has built up below the plate at the bottom of the engine.

No reason not to get the teflon KA cam chain tensioner. Will not fail in the way the stock ones do.

Honestly, may as well get 350 jugs and pistons. an extra 75cc never hurt anyone.

Re: Possible Engine Seizure - REBUILD THREAD

Also, kinda skimmed over your other thread... what broke off of where now? Show me a pic and describe the breakage.

I'll put this bluntly: It sounds to me like a used cb350 engine will be a lot less work and less expensive than fixing your 350 engine.

Re: Possible Engine Seizure - REBUILD THREAD

I'm at work so can't provide a pic but basically the bolt that holds the rotor on the end of the crankshaft sheerd. I can get a replacement and my bolt extractors turned up today - no major drama I guess.

Replacement engines aren't so common over here and you could easy get one that needs a load of work anyway!

Re: Possible Engine Seizure - REBUILD THREAD

I don't know where "over here" is, but there were over a quarter million cb350's sold worldwide.

But, go for it.

Have fun cracking the cases and dealing with the gear shift linkages .