• PiperForum.com is a vibrant community of Piper owners and pilots. Our over 1,500+ active members use Piper Forum to swap technical knowledge, plan meetups and sell planes/parts. We host technical knowledge of general aviation topics and specific topics on J3-Cubs, Cherokees, Comanches, Pacers and more. In addition to an instant community of pilots for you, PiperForum.com is a library of technical topics, airplane builds, images, technical manuals, technical documents and more.

    Access to PiperForum.com is subscription based. Subscriptions are only $49.99/year or $6.99/month to gain access to this great community and unmatched library of Piper knowledge.

    Click Here to Become a Subscribing Member and Access PiperForum.com in Full!

O-320-E3D low compression following new cyl

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

mobilepolice

Well-Known Member
Joined
Jun 16, 2013
Messages
1,048
Reaction score
101
Worried I'm having a recurring nightmare here. Sorry for the super long post...

About 30 hours ago I had a mechanic (whom I no longer trust or use) change a cylinder out repaired by John Jewell. The story is the annual was done and all the other cylinders came out fine except for this guy, which clocked in at 58/80 cold.

The mechanic insisted I continue running it for at least 10 hours to make sure it wasn't aligned compression rings causing the low numbers. I also noticed my oil consumption was trending up, from a normal 15hr/qt to more like 8-10hr/qt but it had just started doing this in the previous 20-30hrs.

I had been collecting oil samples for sending off to Blackstone but had been lazy about getting them in the mail. The poor comp check at annual made me drop them in the mail immediately.

Four or five days later those came back in and it wasn't good. The previous three oil changes had been showing a trending analysis suggesting valve guide wear and potentially more problems. I called the mechanic up, told him about this, and told him I wanted to retest immediately since it had been about 5 hours already.

Flew the plane this time, did the compression check with the engine piping hot. Results? 58/80.

Mechanic said he'd fly it another 50 hours. So began the short road of losing all faith in a guy I trust my life with.

I INSISTED we pull the cylinder and told him I'd help. He got around to it the next day and called me with shock "I'm really glad we pulled that cylinder!" broken top compression ring, and from the best that I could tell, possibly a burnt/burning exhaust valve.

https://lh3.googleusercontent.com/-...FUw/KNKN1E-X3ks/s2048/IMG_20140709_123656.jpg

(big photo, I have plenty more)

John Jewell Aircraft said the cylinder was shot (cracked somewhere), but the valves (to my surprise), piston, and wrist pin were good

The biggest difficulty in the entire ordeal is that all of my cylinders are previously reworked, P010 oversize. A newer cylinder would not be oversize, and would weight a little less than the other three, and the bore would be a little smaller.

This was the logic that I was given when the mechanic told me it would be fine to just leave it up to John Jewell to take care of it to find a P010 replacement.

In the days and weeks since that decision I've learned that balance on the assembly is negligible (the deposits on the piston crown likely matter more) and the power output difference is negligible. Had I known this at the time, I would have opted for a complete new power assembly for a measly extra $200. Alas.

I asked the mechanic during the cylinder installation if the piston came installed in the bore with the right size rings on it, already trimmed down. He said yes (but I was curious as to why there was an empty box for the piston) and that John Jewell takes care of all that. I was skeptical, but still I left it to the guy who had been doing the work for 40 years or more. I know the mechanic didn't bother to check the ring gap.


I don't have the facilities where I'm at to do a differential compression check, but just on cursory pulls of the prop I've noticed one cylinder gives very little resistance to the compression stroke, I popped the valve cover on #4 after a six hour flight yesterday and confirmed it was cylinder 4 that was easy to pull through, even on a hot engine that had just been flown at high power for six hours (2.6 hour, then 3.5 hour legs)

So I do have an automotive compression gauge, and while it is a different style test than the typical differential test, it still produces interesting results. Given a healthy engine, every cylinder should produce similar pressure +/- 15%

My procedure was:
- Warm the engine to 300CHT @ 1000RPM
- shut off the fuel at the selector valve
- shut off the electric fuel pump when fuel pressure gauge drops to 0psi
- when the engine dies (this takes awhile, surprisingly) attempt a restart at low throttle settings, full rich
- wait for the engine to die, or fail to start. rapidly advance the throttle to full open (triggers the carb accelerator pump to toss fuel) then try to start with the throttle full open. (this gave me about four or five blades on restart)

pull all the plugs on top, then ground out the magnetos with alligator clips. probably made all the above redundant but eh.

one cylinder at a time, full throttle, full lean mix, with the compression tester screwed into each cylinder, motor the engine on the starter through five compression cycles on the cylinder.

my numbers were as follows:

Cylinder 1: 100PSI
Cylinder 2: 125PSI
Cylinder 3: 120PSI
Cylinder 4: 75PSI


My questions are thus:

Should the mechanic have checked the ring gap prior to the cylinder installation? Is it normal for the shop to size the rings to the cylinder or do they depend on the mechanic to do it?

what are the chances that I have a burned exhaust valve? The CHT on this cylinder is running hotter than the others by about 20F (and hotter than #3, the "hottest cylinder") But I didn't have an engine monitor prior to the cylinder change, so I don't have good numbers to go on for a "healthy engine".

How the **** do I escape this situation without it costing me another $1500 either due to a ****ty shop, ****ty mechanic, or both? Sigh.

Thoughts appreciated.
YNYmjv7mTO8FjfZltN_CJfPMPm8y5EDiEytoUiZq7sI
 

Latest posts

Back
Top