Chasing air leak in 3120xp

[MS260 Fan]

Hey guys,

I just picked up a 3120xp with minor scoring on the exhaust side of the piston. It won't hold vacuum. I'm trying to find the air leak before pulling it apart. Under pressure there are no visible leaks at the seals, base gasket or decomp valve.

I found clear silicone sealing the perimeter of the oil pump to the case. Great. Someone's been in it.

The seal is installed in the oil pump and an o-ring seals the oil pump to the case. This is new to me. I checked over the o-ring. It looked good so I installed it on the oil pump and bolted it back to the case. I could still hear a leak. No bubbling at oil seal. Took it apart and installed a new, thicker o-ring. No difference.

The tolerance is super tight where the oil pump fits into the case. Oil pump is stamped 5018973. I'm not sure if it's seating. No visible damage to mating surfaces. I've played with the o-rings and bolt torques so many different times I've stripped out one of the m4 threaded holes in the case. Still fuming about. If anyone in the area has an M4 helicoil or timesert kit, I'll gladly pay for one insert and the use of the kit !!!

Any suggestions? It's tough because the oil pump covers up everything so I can't actually see where it's leaking.

 

[MrGiggles]

Check cylinder base, especially if using a FIPG. Unless you're positive it's in the oil pump area.

 

[MS260 Fan]

No bubbling at the base. Relying a lot on my ear. Only sound coming from the oil pump area

 

[lone wolf]

Use a big gob of grease to stop the suspected leak that will prove it.

 

[MS260 Fan]

Grease is a good idea. Unfortunately what I'm trying to seal is the entire clutch side crank bearing. I used a 3" square piece of a zip lock bag with a hole in the center to allow the crank to slip through. I put the bag over the crank and sandwiched it between the case and oil pump. Under pressure, the seal in the oil pump starting leaking. It hadn't before so that's telling me the air was previously escaping between the case and oil pump. The bag worked. I need to get this to seal long term.

To bad I've been so hard on the seal. I was protecting it at first but 20 reassemblies later and I stopped caring.

5018973 on the oil pump casting isn't listed in any IPL's that I can find. Anyone know if this is correct for use with a 1999 3120XP case? There must be interference between the pump and case that doesn't allow the o-ring to compress. The OD/ID is so close you think you'd have to heat the case to get them to fit.

Why do I do this stuff so late at night. The aggravation is going to keep me from getting to sleep :0)

 

[Nitroman]

One thing to remember which is critical when using/installing O-rings: they have a set when they are molded. You can see the very faint ring around the outside or the inside of the ring. When you install the ring, it must not be twisted, or it will not seal. I use a small piece of wire to slip under the ring, then pull the wire around the shaft a few times, that will allow the ring to untwist if it has a twist in it.

I would use a little water with some soap in it applied with a small paint brush to find the exact area of leakage.

 

[MS260 Fan]

Back at trying to get the oil pump to seal in the crank. Based on a conversation with another member and the thread below, it seems the silicone could be from the factory. I was impressed with how neat it had been applied. It was sealed nicely except at the oil adjustment screw.

http://www.arboristsite.com/community/threads/silicone-whats-the-purpose.109178/#post-1719717

Pics of my crank case aren't pretty. Crud was getting in. It had to be getting in around the 740 42 22 o-ring.

I'm a little confused on the hole in the crank. You can see it in the picture. It aligns with the clutch bearing. I assumed you put grease in the threaded end of the crank and it came out the hole. Their not connected. I don't know why the hole is there.

 

[Big_Wood]

your leak is a controlled leak. it is leaking through the crank journal. you can see the hole for it in the last pic. on alot of saws the hole is there to manually grease the clutch bearing from the crank end but on some saws including the 3120 the hole is drilled right through the the counterweights. there was a thread on here where a guy filled it with JB weld and it fixed it. not sure what husky do the control the hole (maybe a check valve of some sorts) but whatever it is it does fail. i just clean it up nice and do a spot weld in the hole and it fixes it up good. i have actually only ever done this on 2 3120's as i have never had a problem with other models. when vac/pres testing it will seem like you have a leak and the sound is noticable. plug that hole and test again, she'll hold vac/pres after that.

 

[LegDeLimber]

One thing to remember which is critical when using/installing O-rings: they have a set when they are molded. You can see the very faint ring around the outside or the inside of the ring. When you install the ring, it must not be twisted, or it will not seal. I use a small piece of wire to slip under the ring, then pull the wire around the shaft a few times, that will allow the ring to untwist if it has a twist in it.

When I was in the steering gear shop, We mainly liked a dental probe for that check & roll.

 

[MS260 Fan]

Found it. The oil pump is easy enough to get on there cockeyed but the hole in the crank was leaking. At first only under pressure but then under vacuum. I filled the hole with jb weld. It worked pretty slick. I would get as much in as I could and then draw a bit of a vacuum. It would suck in and I would repeat. Got quite a bit of JB in the hole. Hopefully enough to hold. Time to get the build back on track!

 

[olyman]

sure am glad I read this thread!!!!!!!!!!!!!!!!! thanks boys....

 

[olyman]

one other thing ms260 fan,, why is the inside of the case so full of chips and dirt???

 

[MS260 Fan]

Best I can tell it was ingested through the hole in the crank or by the o-ring. Both are sealed now

 

[olyman]

Best I can tell it was ingested through the hole in the crank or by the o-ring. Both are sealed now

and totally washed and cleaned out?????

 

[MS260 Fan]

I split the case and scrubbed everything.

 

[cwebefree]

Ah, so that's it.
Back in my neo-neophite days, I once built an XPW w/ a spanky new 375K crank, same issue.
I plugged it.
Damn, that thing had some wicked compression.
Now I'll plug that rascal on the 3120 I just built

 

[Gord404]

I could be wrong but I think K saws used that crank setup to lube the clutch bearing with a seal on the clutch to stop dust and dirt from getting in, had to deal with it a couple of times- one was a 371 husky.