Is the CBX worth saving?

[Timmay]

Here is the full story of the recent failure of my CBX250RS (1983 50,000K):

An oil leak caused the engine to run dry (150ml of thick black oil in the sump):gob:

A split washer in the head broke and found its way into the sump, luckily only damaging one gear in the starter system.:argh:

The head gasket has been leaking allowing soot to blow by as visible in the photo.:ar15:

Oil has been leaking somewhere but I believe it to be at the pump and with the addition of a paper washer it is now fixed.:done:

There is evidence of oil leaking down the side of the cylinder from the head gasket, which possibly suggests rings or the oil seal.:argh:

The cam-shaft bearings feel sloppy and probably should be replaced.

My questions are: Is this bike worth reassembling? I have a new starter gear and head gasket already paid for. Is running the bike dry going to have stuffed anything else? Should I just reassemble it and ride it anyway? Should I fork out more money for rings and seals at this stage seeing as it is all apart?

I have brought the XL/XR manual which covers the RFVC engine but only the single camshaft version. Does anyone know the correct torque to tighten the head bolts? I want to avoid the blow by problem again.

Could the oily residue down the side of the cylinder (starting at the head gasket) be from something other than worn rings or seals?

Do I need to clean the piston and cylinder head or have the cylinder re-surfaced due to the blow by? I have my own mill I can do this on but don't want to risk removing too much material. There is a bit of build up on the piston top as visible in photo.

Thanks to all who give me advice.

Timmay

[xwhatsit]

Feh, you could get that up and running quite happily for little expense. I took the third option in your poll with my CB250RS when the camchain jumped off and caused major trauma. Still running the same piston tens of thousands of kilometres later with two dents in it from the valves.

Really, you should check the oil every time you ride it. Even if it doesn't smoke or leak a drop, there's stuff all in the sump in the first place -- and with the simple lubrication systems they have, it doesn't take much of a drop in oil level for overheating and nasty noises to appear.

Little air-cooled singles like this are very resilient. You'll be amazed how cheaply you can get by.

With the cylinder head gasket blowing like that, what's the mating face of the cylinder head like? It's not damaged at all? If so, you might need to get it planed flat again. Dunno how much that is. You'll need a new gasket anyway, just get a complete top-end gasket kit, with new rubber oil seals. Camshaft bearings can probably be left quite alone.

Rings -- was it smoking heaps? If not, forget about it. Just looks like a blowing head gasket, notice it's blowing right where there's an oil feed.

Put it back together, run it for another 50,000kms+ as a commuter.

[koba]

With the cylinder head gasket blowing like that, what's the mating face of the cylinder head like? It's not damaged at all? If so, you might need to get it planed flat again. Dunno how much that is. You'll need a new gasket anyway, just get a complete top-end gasket kit, with new rubber oil seals. Camshaft bearings can probably be left quite alone.


If it aint too bad it can be lapped flat on a sheet of glass, use some valve grinding compund and rub it in a careful figure 8 pattern to flatten it.

To measure if it is warped put a good straightedge across it and try to poke a thin feeler guage under it, Dunno what the max distortion for one of these is but I wouldn't be too worried if it is under 2 thou.
Max distortion figure may be in your manual.

Is head tourque not in the manual?
If you are really stuck without a figure a guess can be made based on the size of the thread, 8mm is normally around 26Nm.
Make sure you do the studs/bolts up in sequence, doing a wee bit at a time.
Also it is a good idea to get the engine hot, maybe run down to the shops for a pie and then re-check the head.

Its worth having a crack at fixing, if it dies horribly maybe you could find another engine, perhaps even from a dirtbike?

[Timmay]

Hi all, thanks for the replies. I checked the bore this mornig and it looks fine, no scoring or marks. I think option 4 may have to be changed to camshaft bearings rather than piston rings. The head looks fine too. I think I am lucky that nothing else went wrong. Ok, i'll try 26nm and retorque if needed.

There are no oil feeds near the blow by either. I have no idea where all the oil is coming from. The only holes near are not connected to oil passages.

[Katman]

Those heads are well known for developing cracks in them.

[xwhatsit]

Those heads are well known for developing cracks in them.
The early RFVC ones (single camshaft), yes. By the time they did the DOHC, it was pretty much sorted.

In either case, not nearly so bad as the CB250RS/XL250S/XR250 pre-RFVC. Every single head I've seen on those is cracked.

RFVC or pre-RFVC, they will all run very happily with a cracked head -- provided the crack is the usual minor one between the exhaust valve and sparking plug boss. The crack fills up with carbon and effectively seals itself.

Check for a small crack in the place mentioned above. It'll be hard to see. If it's small -- again, don't worry -- still possible to get >180psi compression.

[Timmay]

I can't see that a crack here would be casing oil loss to the outside of the engine. Thanks to everyone for your help. I think I am just going to put it back together and hope nothing else goes wrong while keeping a closer eye on the oil level.

[Timmay]

After a long hard day with spanner in hand the CBX is back together and running well. Other than the chain tensioner mechanisms, the reassembly was very straight forward.

With a new fill of oil she is running reasonably well, other than a flat spot around 6k-7k rpm under load(up hill) which may be due to 2 week old fuel or the new spark plug I put in, other possible suggestions to this flat spot are welcome.

A look at the head gasket shows no sign oil leaks or blow by and other than a drip or two off the bottom of the crankcase indicating further tightening of the cover cap screws is required everything looks sweet.

The new parts are:
Head gasket
Wave washers for the cam arms and starter idler gear
A new starter gear
A new starter idler bearing.

I didn't bother with the camshaft bearing, rings or piston seal as they all looked good(rings and bore) or ok (Camshaft bearings)

So it will be goodbye to pillioning on the boss' BMW K1200rs and back to sleeping in until 15 min before lectures start.