Don’t Miss This Step In Your Rebuilds..

[Canyon Angler]

Seals can last for a very long time but if the saw is 10-15 years old I replace them. Seals are cheap

That's a pretty big job, no? Don't you have to split the case? My 361 is getting to be about 14 yrs old...but I figured I'd prolong a rebuild until it started leaning out due to an air leak from one of the seals...

 

[Vintage Engine Repairs]

Tom did you make up the plug adapter? Snap us a picture.

Yep, cut the metal crimp on the spark plug, it falls apart. Epoxy a brass fitting and add an o-ring to the threads.

 

[Jasonrkba]

I've never changed them on a 361 but most times you can do them without splitting the case. I work on primarily vintage stuff. Maybe the new saws are different. I've made a number of different size drivers some with a recess to seat them a little deeper depending on the model. Pulling the old ones are usually straight forward.

 

[Vintage Engine Repairs]

That's a pretty big job, no? Don't you have to split the case? My 361 is getting to be about 14 yrs old...but I figured I'd prolong a rebuild until it started leaning out due to an air leak from one of the seals...

It’s very simple, no need to split anything! Skip to 8.30 for seal install process, different model saw but identical process.

 

[PV Hiker]

Thanks Tom for taking the time to create this short video that show how easy and simple to do this important testing!

I was taught to always do a vacuum test first then pressure.

If you do a pressure test first, then you risk pushing the lips out reshaping the lips and recreating a seal. If you then vacuum test you might not show a leak missing why the motor was not running correctly.

While the motor is running it always wants to pull in on the lips under vacuum. By doing vacuum test first you are representing current conditions and not changing shape of the lips.

 

[Canyon Angler]

It’s very simple, no need to split anything! Skip to 8.30 for seal install process, different model saw but identical process.

Thank you!

 

[Vintage Engine Repairs]

Thanks Tom for taking the time to create this short video that show how easy and simple to do this important testing!

I was taught to always do a vacuum test first then pressure.

If you do a pressure test first, then you risk pushing the lips out reshaping the lips and recreating a seal. If you then vacuum test you might not show a leak missing why the motor was not running correctly.

While the motor is running it always wants to pull in on the lips under vacuum. By doing vacuum test first you are representing current conditions and not changing shape of the lips.

Pleasure mate! That’s an interesting theory, though when the piston goes up and down, the seals are constantly under alternating pressure and vacuum pulses each stroke so I’m not sure doing vacuum first or pressure first would make any difference in terms of seal shape. I just do pressure first in case there is any fuel / vapours in the top end that I don’t want to suck in to the mityvac.

 

[Jasonrkba]

Tinman did a good video explaining the positive and negative pressure the seals take using a cut out of a 028 super. He does a good job of explaining where the pressure is coming from and at what time in the cycle.

 

[stihltech]

Thank you to the person that posted the Vacuum and Pressure test. THE secret to getting the unit fixed right. It is advisable to do a vacuum test first as pressure may temporarily seal a leaking crank seal.
INVALUABLE tool!

 

[Vintage Engine Repairs]

Thank you to the person that posted the Vacuum and Pressure test. THE secret to getting the unit fixed right. It is advisable to do a vacuum test first as pressure may temporarily seal a leaking crank seal.
INVALUABLE tool!

Pleasure! That’s the first time I have heard about this (well second now as someone mentioned the same thing above!). Though I’d not have thought it’s the case that pressure will close a seal, I’m certainly open to taking the advice of those with more experience. I’ll do vacuum first from now on. Thanks!

 

[Automender]

Pull out the fuel filter and attach same Mityvac and you test fuel lines and needle valve in a minute.

 

[Okie]

Very good info in this thread: I've got it saved in my files.
Couple things I do:
I use a small Briggs fuel filter in the mityvac hose. This will prevent/trap/filter small particles being sucked into the mityvac check valves. (I've had to take my mityvac apart few times and carefully clean the check valves with Q-tips to re-store operation before I started using the fuel filter)

When making the spark plug adapter I usually take a tire valve and remove the rubber from the brass section and epoxy or braze the threaded portion schrader valve into the spark plug instead of using a hose barb. I can then install a low pressure schrader valve and use this adapter on my compression gauge and remove the valve for vac/pressure tests. (use the same homemade spark plug adapter for both compression and pressure/vac tests.

When a chainsaw or 2 cycle is not running properly and the problem is not readily apparent I take time out and sometimes have to make up block off plates etc and do the pressure/vac test. You then know that the main problem is most likely external to the block if the pressure/vac test is ok.

 

[Dennisthemenace]

Real time pressure and vacuum test, it’s really this easy and will save you so many potential running problems

Very good info, I must make a plug adapter

 

[Brufab]

Great info!

 

[64poncho]

Was checking over an 028 Super that I just got. While doing Pressure / vacuum test, found out how important vacuum test is!
Did Press test first and didn’t leak at all. Vacuum was another story, wouldn’t hold any. Found clutch side seal leaking. Couldn’t
believe a seal that was worn that flat would hold pressure.

 

[Okie]

64poncho:

It might have leaked pressure if the crank shaft was wiggled or rotated by hand while watching the pressure gauge but yes about doing vac also.

I've seen some saws that power head by itself would run fair but after the bar and chain was installed and the chain tensioned the idle would change, loosen the chain and the idle change again. this is a hint of the clutch side crank seal leaking and the bearing can still be ok. The seal is just dry enough that it leaks when the crank has little side torque.
AND the saw might also pass the tilt test with a seal of this type leaking more when the chain is tensioned..