Can a head gasket leak into the crankcase?

[this old xs]

Sounds stupid, but I'm wondering if it could leak to one of the oil galleries on the head studs maybe? Reason I ask is I've put new pistons and rings in, the bores were good, end gap is good, but I have terrible crankcase pressure which blows a lot of oil out the breathers. I've done most every breather mod on here and then some to no avail. Bike runs great, plenty of power, doesn't burn any oil that I can tell, starts easy.

I borrowed a mates leak down tester and just then used it. The air is coming out the oil filler hole (or breather if the dipstick is in), which says rings but I don't understand how. I squirted some wd40 in the bores and repeated the test with no change.

When riding, I've had some rides where it barely passed any oil, then sometimes it pukes heaps. I think this might be important - often it leaks oil from the cylinder head to cylinder joint (mostly left side) and base gasket. At the same time it will still blow oil out the breather.

I retorqued the main head stud crown nuts, they were loose but it didn't help.

I'm a bit stumped. I feel like this is a head gasket issue (head was lapped flat but I didn't check the cylinders) but leak down test says rings?

 

[joebgd]

There is supposed to be case presure. Where are the Brother tubes running. If you habe one into 2 or old.style 2. When on side stand the left one will fill w steeled oil. I eventually just ran the right one to my Air filter box amd capped off the left so the right cylinder pulls in the 12,000 miles of oil that the left one got 90% of. After another qp15,000+ miles maybe ill do something else w it.

 

[Highpower]

Crank case pressure is directly related to:

A. The size of the bore and the length of the stroke.

B. How much compression is leaking past your rings.

Your leak down test told you exactly what the problem is, your rings are the culprit. I would just about bet money that your rings were installed wrong and/or the bores weren't properly prepared before piston installation. With properly installed pistons/rings that are sealing properly, you will not see excessive pressure being pushed out of the breather.

I don't know why you insist that the evidence is lying to you, it is obvious what the problem is. Unfortunately, you are faced with having to pull the motor to service the top end.

 

[this old xs]

There is supposed to be case presure. Where are the Brother tubes running. If you habe one into 2 or old.style 2. When on side stand the left one will fill w steeled oil. I eventually just ran the right one to my Air filter box amd capped off the left so the right cylinder pulls in the 12,000 miles of oil that the left one got 90% of. After another qp15,000+ miles maybe ill do something else w it.

I've got the forked breather at the moment. Instead of going to the air boxes where it was clogging the filters with oil, I capped the right hand side and with the restrictor plug in the left it runs to a brake check valve and then on to a container. I had thought maybe I should swap. Couldn't hurt to try before I go tearing everything down again I guess.

 

[Jim]

It's a 360° twin. That means both pistons rise and fall together. When both pistons go down, it tries to compress the air inside the crankcase. That gets forced out the breather. In other words, these engines are famous (infamous?) for high crankcase pressure.

Depends on what you consider excessive I guess...

 

[this old xs]

Crank case pressure is directly related to:

A. The size of the bore and the length of the stroke.

B. How much compression is leaking past your rings.

Thanks Highpower, I know that, that's why the question was is it possible for combustion pressure to make its way into the crankcase via the head gasket. I know liquid cooled motors can for eg pressurise the cooling system via a head gasket leak but I don't know if a similar situation is applicable to XS650's.

Your leak down test told you exactly what the problem is, your rings are the culprit. I would just about bet money that your rings were installed wrong and/or the bores weren't properly prepared before piston installation. With properly installed pistons/rings that are sealing properly, you will not see excessive pressure being pushed out of the breather.

I don't know why you insist that the evidence is lying to you, it is obvious what the problem is. Unfortunately, you are faced with having to pull the motor to service the top end.

I didn't think I was insisting, just questioning if there could be an alternative. The reason being I measured the bores, I measured the ring end gap and honed the cylinders before reassembly with new parts. Heck I even staggered the end gaps. I'm not sure what else I could have done. After only a few hundred miles it seems odd to me that the rings would not be sealing, but I guess stranger things have happened.

I would have thought that squirting some oil (albeit only wd40) I the bores should have aided ring seal for the leak down but it made no difference. Either way the motor is coming out again, just trying to prepare a plan of attack. Also I can't imagine what I would have done different regarding honing and new pistons and rings so it seems foolish to repeat that and expect a different outcome.

 

[this old xs]

It's a 360° twin. That means both pistons rise and fall together. When both pistons go down, it tries to compress the air inside the crankcase. That gets forced out the breather. In other words, these engines are famous (infamous?) for high crankcase pressure.

Depends on what you consider excessive I guess...

Thanks Jim. One short ride it pushed 200ml (sorry, don't know imperial) out which seems like a lot. I talked to the previous owner and he said he used to catch it in a container and pour it back in! So I guess this bike has always been a bit of an oil thrower.

 

[Jim]

There's two things Yamaha did to "help" control oil loss.
They put a baffle in the oil return between the head and jugs, and also one inside the breather housing. Are both yours installed?

 

[Jim]

Here's the one inside the engine, 'tween the head and jugs...

 

[Jim]

... and here's the baffle inside the breather housing. Also, you have the oil tube outlets at the top of the breather when mounted, correct?

 

[Jim]

I wouldn't be ready to condemn your top end job just yet... nor pull the engine. One way to cut oil loss out the breather is to add some stainless scouring pad inside the breather housing. What happens is the oil vapor clings to the stainless mesh as it passes through, where it turns back into a liquid and falls back into the sump.... and not out the breather. Believe it or not, this was common on car engines (inside the oil cap) all the way up into the 70's.

I'd definitely give that a go before redoing the top end.

 

[this old xs]

Thanks Jim. I definitely have the one in the box on the back of the head, been in there more tines than I care to admit. I've also thought about the scourer, was worried it might clog but it's worth a try.

I will have to check the other breather between heads and jugs (not sure how). Speaking to the previous owner he holed a piston and had it rebored to 1st oversize though he said the bores were ok. I've had the motor apart a couple of times and I can't recall if I saw that breather. Will review my photos.

 

[Jim]

was worried it might clog but it's worth a try.

It won't clog. As I said... very common on engines back in the day.

 

[Highpower]

The thing is, if squirting lubricant into the cylinder didn’t change the amount of leak down, you have a serious problem in there. Stuffing kitchen products into the breather isn’t going to actually fix the problem. It’s kind of like putting a band aid on a broken arm.

You just shouldn’t have that much leak down on fresh rings. What was the percentage of leakage into the crankcase? It sounds almost like you have a bad head gasket.

 

[this old xs]

No compression test, but I think my friend has a tester so I might borrow it. Somewhat ironically I only replaced the pistons and rings because although the previous set's compression ring end gap was fine, the oil control rings end gap was huge. I thought this was causing the carbon I saw on the plug but it was more likely coming in via the airbox. It's a 78 Special so the breathers are fed to the air boxes.

 

[this old xs]

Found it. The breather is just visible in the top left hand corner, so I'm relieved that it's in there (this is before lapping the valves BTW).

I'll tinker with the breather some more before tearing everything apart. I have fiddled extensively with the breather arrangements so one more time don't make no nevermind as they say.

 

[AUTOMAN]

YOU did hone job and news rings?

YOU sprayed wd40 to see if it helped leak-down?

YOU did leak-down and what were the numbers?

YOU are not helping us much.....

 

[Its been a long time]

Just thought I'd mention. Where is the oil level on the dipstick? I'm running my 650D with the oil level only about 1/3rd of the way up the marked area on the dipstick. Depending on where the OP has his oil level, if it's high, that might be an easy thing to try before pulling the engine out. I have had more than one wet sump engine where I've had to run the oil level low to prevent losing it through the breather. As a result, I always run the oil on the lower side of acceptable.

 

[Tiggertoo]

Just thought I'd mention. Where is the oil level on the dipstick? I'm running my 650D with the oil level only about 1/3rd of the way up the marked area on the dipstick. Depending on where the OP has his oil level, if it's high, that might be an easy thing to try before pulling the engine out. I have had more than one wet sump engine where I've had to run the oil level low to prevent losing it through the breather. As a result, I always run the oil on the lower side of acceptable.

Yeah, its counter-intuitive as folks will run the engine with a higher oil level thinking that will prevent the level getting too low, whereas in reality it is probably causing more oil to be dumped through the breather.

 

[Its been a long time]

Yeah, its counter-intuitive as folks will run the engine with a higher oil level thinking that will prevent the level getting too low, whereas in reality it is probably causing more oil to be dumped through the breather.

Indeed. The classic for me was my Guzzi. One day a bloke riding behind me showed me the small spots of oil on his visor and the front of his bike that my Guzzi was throwing out above about 60mph. Fitted an extended sump and also dropped the oil level a touch too. Completely stopped oil blowing from the breather.