Jonsered 920 air leak- need help

[A_sharp]

Hi everyone. I am relatively new to rebuilding chainsaws and after spending countless hours on this site I finally started to rebuild this 920 I aquired about 15 years ago. The saw had an air leak at the shaft seals so I replaced them, cleaned up the cylinder and installed a new piston. I did another air test and still had a leak whick I could not find, so plugged all the holes and dipped in a bucket of water. What I found was a bit of a surprise. The leak was right below the bar studs around a small pin in the case. This pin is folded over kind of like a cotter pin but really small and is loose in the case. I have attached a pic with a black arrow pointing to this pin. Anyone have a similar problem? Any help is much appreciated!!

 

[Johnwell]

I’ll check my 930 over tomorrow evening—I don’t think it has a pin there. Maybe the casting had a hole there and the factory plugged it with a pin? You could probably jb weld it to seal it up.

 

[SteveSr]

Hi everyone. I am relatively new to rebuilding chainsaws and after spending countless hours on this site I finally started to rebuild this 920 I aquired about 15 years ago. The saw had an air leak at the shaft seals so I replaced them, cleaned up the cylinder and installed a new piston. I did another air test and still had a leak whick I could not find, so plugged all the holes and dipped in a bucket of water. What I found was a bit of a surprise. The leak was right below the bar studs around a small pin in the case. This pin is folded over kind of like a cotter pin but really small and is loose in the case. I have attached a pic with a black arrow pointing to this pin. Anyone have a similar problem? Any help is much appreciated!!

Is this possibly a vent for the oil tank and is there a leak between t he crankcase and oil tank?

 

[cbfarmall]

Here's the backside of the split crankcase. You should see if air is leaking out of the oil cap as well. Confirm what Steve suggested. Fortunately, aftermarket gaskets are available.

 

[Bob Hedgecutter]

The cotter pin is the vent for the oil tank as suggested above- if the saw is showing an air leak from the engine internals exiting this point then yes (as also stated above) you have an internal leak from engine interior into the oil tank interior.
Likely to be a failed gasket- but not impossible for it to be a hole in the crankcase if something hard got lodged between crank lobe and case at some stage- something hard like a bit of bearing- ball, needle or race.

 

[heimannm]

I had a 920 in early last year with exactly the same symptoms. The owner decided not to proceed with repairs and took it home partially assembled.



Mark

 

[A_sharp]

I had a 920 in early last year with exactly the same symptoms. The owner decided not to proceed with repairs and took it home partially assembled.

View attachment 1235306

Mark

Thanks everyone for the replies, looks like I have more work to do! The air leak is not that severe so am hoping it is the gasket and not a hole in the case. If it is a hole in the case I would assume it is not repairable??

Thanks again,
Harry

 

[Johnwell]

…If it is a hole in the case I would assume it is not repairable??

Thanks again,
Harry

I’ve fixed aluminum-cased two strokes (lt500r) that had a hole in the case from the piston skirt breaking up; just had to find a good welder.

I’ve heard you can weld magnesium but I have never attempted it; and somehow you’d have to keep it from catching on fire. I’m sure it could be done. Probably easier, cheaper, and safer to buy used cases on e-bay.

 

[heimannm]

JB Weld.



I can't find the video of the saw running (McCulloch BP-1) but in this instance a replacement crankcase/housing was not an option.

Mark

 

[A_sharp]

JB Weld.

View attachment 1235434

I can't find the video of the saw running (McCulloch BP-1) but in this instance a replacement crankcase/housing was not an option.

Mark

Wow, quite the repair. How big was the hole?

 

[heimannm]

Something was loose inside the engine while it was running and was caught between the crankshaft and crankcase causing the stress fractures you see here.



The red circle indicates where opening through the crankcase occurred.



The culprit found upon tear down.




Collateral damage was minimal, thankfully.



Mark

 

[A_sharp]

Thanks for the info Mark, I got something to look for.

Harry

 

[Real1shepherd]

People who work on smaller assemblies that require machining and welding can weld mag with no problem. Easy to test between aluminum and magnesium as well.

Also, there are better commercial epoxies out there than JB Weld. You won't find them by Googling 'best epoxies'. Expensive....like a small tub might run $100.

As was said, J'red put those little cotter pins in to vent the oil tanks. On my 80 there is one just below the muffler on top of the oil tank. Once the pin vibrates enough from long use, the hole is enlarged. Fix is just to go to the next size cotter pin. Don't overthink this. As far as a pressure or vac leak into the oil tank....yes, there is a compromise as the chambers are not connected. You can probably find good color pics of similar case sets broken down.....it will become clearer.

Take your J'red questions over to us at @the actual Jonsered thread. I only found this thread by accident.

Kevin

 

[A_sharp]

People who work on smaller assemblies that require machining and welding can weld mag with no problem. Easy to test between aluminum and magnesium as well.

Also, there are better commercial epoxies out there than JB Weld. You won't find them by Googling 'best epoxies'. Expensive....like a small tub might run $100.

As was said, J'red put those little cotter pins in to vent the oil tanks. On my 80 there is one just below the muffler on top of the oil tank. Once the pin vibrates enough from long use, the hole is enlarged. Fix is just to go to the next size cotter pin. Don't overthink this. As far as a pressure or vac leak into the oil tank....yes, there is a compromise as the chambers are not connected. You can probably find good color pics of similar case sets broken down.....it will become clearer.

Take your J'red questions over to us at @the actual Jonsered thread. I only found this thread by accident.

Kevin

Thanks for the info Kevin. I'm sure there will be more questions and will post on the J'red section

 

[johnfawn2000]

Hi everyone. I am relatively new to rebuilding chainsaws and after spending countless hours on this site I finally started to rebuild this 920 I aquired about 15 years ago. The saw had an air leak at the shaft seals so I replaced them, cleaned up the cylinder and installed a new piston. I did another air test and still had a leak whick I could not find, so plugged all the holes and dipped in a bucket of water. What I found was a bit of a surprise. The leak was right below the bar studs around a small pin in the case. This pin is folded over kind of like a cotter pin but really small and is loose in the case. I have attached a pic with a black arrow pointing to this pin. Anyone have a similar problem? Any help is much appreciated!!

Dear A_sharp , is it possible to find a driil bit this size drill this then thread it, with same screw threads with a really good caulking and seal it , caulking and a screw.
I am asking A_sharp , not sure.
Buck

 

[A_sharp]

It could be done but I would think the oil tank needs to be vented to work properly. Perhaps others can verify this??

 

[Bob Hedgecutter]

Dear A_sharp , is it possible to find a driil bit this size drill this then thread it, with same screw threads with a really good caulking and seal it , caulking and a screw.
I am asking A_sharp , not sure.
Buck

Why would anyone want to seal the vent of an oil tank?
If air cannot get in, oil dont come out.

 

[johnfawn2000]

It could be done but I would think the oil tank needs to be vented to work properly. Perhaps others can verify this??

A_ sharp , I thought was a defect.

 

[A_sharp]

Initially I was not sure what the purpose of the cotter pin was but others replied that it was indeed a tank vent.