455 rancher low compression after cylinder/piston/ring swap

[theWyatt]

New here so be kind. .

The story: spark plug end broke off and obliterated the piston head and cylinder head. Actually poked a small hole through piston head. Guessing it finally made its way out to the muffler. My pal was a cub cadet dealer and said in 20 years never saw a spark plug fail like that. I am jus outside the warranty window too.

So did a piston/ring/cylinder swap. Compression testing at jus under 30 now. I lnow it for sure has to be in the 100’s and more likely the 130-150’s depending on decomp valve being opened or closed.

What am I missing here. Seemed very staight forward on the swap. Took about 10 mins once everything was a part.

I did change the gaskets on the bottom end of cylinder thinking that was the problem but still jus under 30. I also swapped the decomp button valve and the same result. Did i maybe assemble it wrong?? What am I missing? All videos on youtube look 1:1 what I did as well. Kinda at a loss.

What can I look into to check to point me in the right direction please? Pics for fun

 

[Bob Hedgecutter]

Okay, did you test the compression of the old cylinder before you tore it down?
If that had given you a reading of 90 psi ish and the new one gave 30 then I would be concerned.
More likely is you are using an incorrect style of gauge meant for general automotive compression test use- not specifically designed for small engines- or you are performing the test incorrectly.

New cylinder, piston and rings -even Chinese cheap ones should give starting strength compression- not much you can do to botch assembly up that bad that you are missing 100 odd psi.
Does the saw fall down the starter rope quickly and of its own accord?

Better yet- does the saw start and run?

 

[theWyatt]

I sure didnt. It would start with the bad parts and run for like 2-3 seconds and die. Any load would kill it too. I used this tool from Harbor Frieght. LINK

I need to do the hang test still. Ill report back when I can get to it.

 

[Bob Hedgecutter]

The ex- hire saw thing and likely being well used and abused before you got it opens a whole other can of worms and an air leak might be your problem.

 

[theWyatt]

I got the saw from a buddy as Cub Cadet dealer, new. Ran great, with zero issues until the unfortunate spark plug deal. I am sure we can get there just was hoping others here might of had an issue w/ compression after a swap of this nature and if so, what things did you/they try to right it.

Ill still try the hang test when I get home.

 

[Bob Hedgecutter]

Easiest way to figure out the compression thing is to find a known to be good saw and test it with your tester- if it comes back with a similar low reading- you do not have a compression problem.

 

[theWyatt]

Great idea! I have a brand new 460 so Ill try that when I get home.

 

[theWyatt]

Update: tested the 460. Very similar readings. . Thx for the idea to test another working saw for a control.

Put the 455 back together. Started perfectly! Need to put the bar on and cut a little in the coming week.

Any tips for proper breakin in of a saw after this typenof replacement/swap?

 

[Bob Hedgecutter]

Update: tested the 460. Very similar readings. . Thx for the idea to test another working saw for a control.

Put the 455 back together. Started perfectly! Need to put the bar on and cut a little in the coming week.

Any tips for proper breakin in of a saw after this typenof replacement/swap?

Myself, I tune them a tad rich on the H from ideal tune, run them like you stole it for 5 hours or so run time, tune to ideal and cut wood with it.
Others might doo dad around with heat cycles and periods at idle and so such, but my method has never failed me yet in 40 odd years of use.

 

[theWyatt]

This is great Bob. Ty. I will do jus this. I have about 3-4 80-100 footers to take down. Plenty of cutting there.

 

[Bob Hedgecutter]

Like I said, its what I do- new saw or new rebuild- but I can also tune saws by ear and am confident doing so.
I have no idea if your rebuilt saw had other issues regarding air leaks or bearing wear, if you used OEM parts or Chinese cheap outs- I can only tell you what I do, I cannot tell you to only do that and it will be the perfect outcome for someone else in a different part of the World with a rebuilt ex hire saw.