SonexBuilders.net

I recently purchased a used sonex, and during last year's condition inspection an invisible crack was detected in front of cylinder 4. The rest of the engine had 550h on it, but compression tests and oil pressure were still ok, so i decided to attempt to fix the crack instead of rebuild. Sonex suggested putting jbweld on the crack and monitoring it. I found an experienced magnesium welder to weld the crack without disassembling the engine.

The weld held for about 6 hours, then the crack was larger and leakier than before. I sent the engine (and a new case) to Scott Casler at Hummel Engines for a rebuild (I never assembled an aerovee so I figured it'd be safer to let someone with experience rebuild it) and now it runs excellently and the only oil leaks are from the breather tube :)

If you have a crack in this area... perhaps just cover with JBweld and monitor like sonex suggests, but prepare for rebuilding.

Well, your 'experienced magnesium welder' should have known better. I'm just an all-round mechanical guy and know for sure that welding on an assembled engine case isn't an option. As it shows the problem got worse and ruined the case. Cracks in engine cases are fairly common. Even Rotax has them. You should have followed the Sonex suggestion.

Having run Lycomings most of my life I grew up thinking that motoring around with a cracked case was a failure waiting to happen. So I'm a bit curious why a cracked case is not a problem (with a bit of JB). Is there little danger of a catostrophic failure with a cracke case?

Sonex's suggestion would likely have been better for the case and I have learned not to attempt to weld crank case cracks in place :) Apparently the heads and some valves weren't in great condition either so a top-end rebuild was at least a good idea. Even during break in, the cht's are 30 degrees cooler than before and there aren't occasional egt/cht temp spikes accompanied by drops in rpm. (maybe some pre-ignition or detonations ?)

Sonex said that area of the case isn't a high stress location and to put on JB weld AND monitor the crack for growth and replace the case (or do a proper weld) if it grows. The crack in that location looks like it is also compressed by the Cylinder head bolts which might help slow crack growth.

I'm with those that are of a mid-set to say, if the engine case is cracked, the airplane is not airworthy.

Others certainly have every right to make their own judgement call ...

daleandee wrote:I'm with those that are of a mid-set to say, if the engine case is cracked, the airplane is not airworthy.

Others certainly have every right to make their own judgement call ...

Same here FWIW

Even certified engines have allowable crack limits. pretty much all Continentals allow for 2" or less case cracks in non critical locations, see SB M90-17. If it's in a non stress area and not near an oil passage then the worst thing that can happen is a small oil weep. In my experience all Aerovees weep somewhere, most leak from several places
The case isn't going to just come apart if its cracked in a non stress area.
Now..... I would keep a very close eye on it. If it's growing between annuals or anywhere near an oil passage, I'm pulling it!

I personally wouldn't do JB weld, the amount of oil that weeps is insignificant if your case is vented properly and you are covering it up so you can't keep track of any growth that occurs.
IMO