Exhaust disconnected

[gammaxy]

I heard a ticking sound the other day that I assumed to be caused by too much valve clearance. I decided to cancel the flight and returned later to investigate. After removing the cowl, I noticed both bolts on the #1 cylinder exhaust flange were completely missing. The gasket was still in place and the exhaust was still mostly positioned correctly, held in position due to the #2 exhaust flange. There was no visible exhaust stains near the disconnected flange, so I'm pretty sure it hasn't been that way very long. Unfortunately, I don't have telemetry from the previous flight to see if it's obvious when the disconnect happened.

https://youtu.be/dzh0RvVxsAY

I checked the head bolts and valve clearances and only had to make minor adjustments. When I re-attached the exhaust flange the ticking sound went away and it sounded normal. Mentally, I think it sounded better than normal.

I thought I had been careful with the exhaust bolts. I had one back out a few turns during phase 1, so I've paid attention to them ever since. I recently worked on the engine and replaced the exhaust gaskets when re-assembling the engine, so checked the bolts a couple of times as the gaskets seem to shrink with use for a while.

I don't believe Sonex mentions a torque for these bolts and I was concerned about stripping out the aluminum heads, so I just snugged them by feel. When I put a torque wrench on them yesterday, they were all around ~5 ft/lbs. I believe this is likely to be too low, so I torqued them all to 10 ft/lbs and used safety wire on the ones that fell out. I suspect even 10 ft/lbs may be too low, but haven't done my research to figure out a better torque.

[mike.smith]

That seems to be an elusive value. From the few web locations I found, the value appears to be 25 ft/lb. I've never torqued mine (tough to get a socket on all 8 of those bolts), but have certainly snugged them down good and tight. I'm sure well beyond 10 ft/lbs. Never had them strip on me.

[gammaxy]

Yeah... I really should have thought it through to figure out a reasonable number. The spark plugs get torqued to 12 ft lbs, but I believe they engage with less material and have finer threads, so a higher torque for the exhaust bolts makes sense.

[mike.smith]

Glad you found it before the flange moved!

[Jim1342]

"How to Build a Reliable VW Aero Engine" by Rex Taylor on page 82 has a list of recommended torque values. For the Exhaust Pipe Cylinder Head nut with M8 threads has a recommended torque of 16 ft.-lbs.

[mike.smith]

"How to Build a Reliable VW Aero Engine" by Rex Taylor on page 82 has a list of recommended torque values. For the Exhaust Pipe Cylinder Head nut with M8 threads has a recommended torque of 16 ft.-lbs.

Sounds as reasonable as any other answer.