Ever have a new cylinder not hold vac or pressure?

[Mark71gtx]

I recently purchased a 660 cylinder kit and I was installing it tonight. I got everything together and went to do my post install vac and pressure test. Vacuum, fail - pressure, fail. Break out the soapy water and it is leaking at the brand new NGK plug and the decomp. Tightened the plug more, no better. Tried another plug and get the same result. Tried a Bosch plug. Same result. Any ideas, or is it just piss poor machining?

 

[jmssaws]

I recently purchased a 660 cylinder kit and I was installing it tonight. I got everything together and went to do my post install vac and pressure test. Vacuum, fail - pressure, fail. Break out the soapy water and it is leaking at the brand new NGK plug and the decomp. Tightened the plug more, no better. Tried another plug and get the same result. Tried a Bosch plug. Same result. Any ideas, or is it just piss poor machining?

How tight are you getting them?

 

[Mark71gtx]

Tight enough that I am beginning to fear I will pull the threads out...

 

[Mark71gtx]

I have done this many times, in fact, it is the second time I put a new top end on this very saw. I have never had this trouble before.

 

[Canadian farm boy]

Probably chitty machine work. Do you have a tap you can run through the plug hole?

 

[Mark71gtx]

The threads are all the way through, but I do have a tap for the plug hole, but not for the decomp. It is leaking around the crush washer on the plug. Not certain what tapping the cylinder would do for me?

 

[Canadian farm boy]

The threads are all the way through, but I do have a tap for the plug hole, but not for the decomp. It is leaking around the crush washer on the plug. Not certain what tapping the cylinder would do for me?

I was just thinking about possible burrs or questionable threads.

 

[bmwpowere36m3]

Probably crappy facing of spark plug hole... a little leak won't matter.

 

[5000+]

If the threads are good, then the surface for the seal washer or porosity/gouge in the casting is the only thing left. You may have to dress the top of the cylinder. Could try a soft coper washer used for indexing plug electrodes. How bad of a leak? If it's slow with foamy micro bubbles, I wouldn't loose sleep over it. Over time and use carbon may stop it.

 

[Chainsaw Jim]

It sounds like you're not getting it tight enough. When a spark plug hits the end of the line it feel like your saw will twist in half before it'll strip out.
You can thread the decomp half way on and apply loctite before cranking it down the rest of the way.

 

[jmssaws]

I've changed plugs several times trying to find one that will seal when looking for a airleak in a bucket of water.
Stihl says to plug the decomp for p/v test.

 

[Mark71gtx]

Spark plug is the worst leak. Can't do much about that... With the current sale price of the Cross MMWS kit, this one is coming off... There will soon be a tree service field test of the Cross MMWS Performance kit in the Raleigh, NC area...

 

[fatboycowen]

I had an aftermarked 346xp cylinder, which had a casting defect. It had a small hole in the top of the exhaust port, that went straight up and through. It was far enough back from the exhaust gasket face to make it tough to notice. I had it all assembled. Took a bit to figure out why i couldn't seal the exhaust.

 

[redtractor]

Had the same spark plug leak on an AM cylinder. No visible flaw. A very light sanding and polish around the hole helped.

 

[Al Smith]

Several things depending .It could have a badly threaded hole which would be an over sized hole that's threaded,wrong reamer, not full profile threads .If it's a taper fit seal might be a bad taper .Try a gasketed plug before you throw in the towel .