CB350 1971 Engine rebuild (white smoke out of breather pipe -> Obliterated head gasket)

[ManuR]

Hey,

Litlle introduction about my new Bike.
Its a CB250 1968 frame, with a CB350 1971 engine.
According to the previous owner, he bought the bike from a person who revised the engine.
He couldn't give me any more details, so it might as well just be painted and not have any of the critical components inspected/replaced.
Never the less, the bake has had some care and some paint work done.

When visiting the bike i took a compression measurement, which was around L: 12bar, R: 10bar.
Not perfect, nor horrible.

The unknown state of the engine became painfully obvious when after the sale, (and the trip home) the bike started smoking more and more out of the breather tube.
So i suspected a leak from the a valve, Piston rings or head gasket. Yet the compression stayed the same as originaly.
An oil flow test can be seen here, as well as the smoking: Video

After openingen up the top end, the problem was obvious.
A completly obliterated head gasket.

So next step frome here:
1. Try and analyse why this head gasket failed so catastropicly. My assumption is that this gasket was reused after opening up the engine, in combination with not tightening the head bolts enough. Maybe some warped surfaces could also have contributed to the bad clamping in the middle.

2. inspect if the mating surfaces on the cilinder block, and cilinder head can be machined, or if they are at there end.

3. if the cilinder head can be reused, inspect:
3.1 Valve play
3.2 Valve spring condition

4. If the cilinder block can be reused, inspect:
4.1 Cilinder conditions
4.2 Piston and rings conditions

5. Inspect Cam and rockers

6. Hesitate to compleatly open up the bottem end, and clean all the oil channels. etc...

 

[ex119x]

I have never seen a cb350 head gasket blown that badly.
I have reused plenty of 350 head gaskets on racebike when I have been in a hurry. I have never had a head gasket failure. I suspect from looking at this failure that either the center studs were not torqued properly or the head and cylinder are not mating well there due to a warped cylinder (never seen it) or a warped head (only seen once). If it were me, I would clean off the mating surfaces and check them with a straight edge or run them on some sandpaper on a flat surface and see if you see any warpage. I suspect they will both be fine and the blown gasket came from improper stud tension due to installation error. I would not disassemble the head nor would I pull the cylinder off, at this point. I would put in a new head gasket and run it. If you plan on keeping and using this bike, I would source another engine and rebuild it from top to bottom so you know it is done right. Then put it in the frame and freshen up this motor to have as a spare. With 55 year old bikes, I always like to have spares.

 

[teazer]

Good feedback from ex119x. Get a sheet of wet and dry 30 or 400 grade and put it on a very flat surface. Use a little WD40 to "lubrictae it and run the head back and forth a few times and check to see the clean shiny parts versus the dull parts. That will show you if it's bowed or has raised pats where it might have been damaged during removal.

 

[teazer]

As to why it failed, another possibility is too long head dowels. To check, place the dowels into their holes and lower the head into place without a gasket and measure the gap between head and barrels. It should be at least say .010 smaller than the thickness of your head gasket.

 

[MathiasVogel]

Hey,

Litlle introduction about my new Bike.
Its a CB250 1968 frame, with a CB350 1971 engine.
According to the previous owner, he bought the bike from a person who revised the engine.
He couldn't give me any more details, so it might as well just be painted and not have any of the critical components inspected/replaced.
Never the less, the bake has had some care and some paint work done.

When visiting the bike i took a compression measurement, which was around L: 12bar, R: 10bar.
Not perfect, nor horrible.

The unknown state of the engine became painfully obvious when after the sale, (and the trip home) the bike started smoking more and more out of the breather tube.
So i suspected a leak from the a valve, Piston rings or head gasket. Yet the compression stayed the same as originaly.
An oil flow test can be seen here, as well as the smoking: Video

After openingen up the top end, the problem was obvious.
A completly obliterated head gasket.

So next step frome here:
1. Try and analyse why this head gasket failed so catastropicly. My assumption is that this gasket was reused after opening up the engine, in combination with not tightening the head bolts enough. Maybe some warped surfaces could also have contributed to the bad clamping in the middle.

2. inspect if the mating surfaces on the cilinder block, and cilinder head can be machined, or if they are at there end.

3. if the cilinder head can be reused, inspect:
3.1 Valve play
3.2 Valve spring condition

4. If the cilinder block can be reused, inspect:
4.1 Cilinder conditions
4.2 Piston and rings conditions

5. Inspect Cam and rockers

6. Hesitate to compleatly open up the bottem end, and clean all the oil channels. etc...

That gasket failure definitely points to more than “normal wear” — reused gasket and/or uneven torque would be high on my suspect list too. On these twins it’s worth checking the head and cylinder deck with a straight edge before doing anything else, especially around the cam chain tunnel and center area where clamping can be less forgiving.

Before you decide on splitting the bottom end, I’d first inspect the oil for debris, clean the centrifugal oil filter/sludge trap if fitted, check the oil pump screen, and look closely at the cam/rockers since they suffer quickly if oiling was poor. If the bores, pistons and top end look good, you may not need to go all the way in — but with an unknown “rebuilt” engine, I understand the hesitation.