SonexBuilders.net

I am looking at a Sonex with a 3300A engine. It was installed in 2002 and has a total of 300 hours on it now. The compressions are 60-60-60-60-69-69. I'm speculating it is not long before the need for a top overhaul if not already. Could I be wrong about that? Once compressions get down that low, how much longer before they dip below the 60 mark?

Tom Murrell

don't have any idea on life span but I bet is is still running strong at those compressions. I know that there is a push by some A&p not to place a big issue on compressions # as absolutes. (although I believe it is still listed in FAA literature as required-I am no a+p so take this with a grain of salt) They are more looking at differences in numbers etc. I would bore scope the engine and that will tell you a lot about the inside of the cyl. If you can get someone to do that. If you try it on your own just be prepared for everything to look HUGE and every scuff or scrape will look like a canyon. I would look at the valves especially for burned exhaust valves. Thing about the numbers, an engine will not just "quit running" if your compressions get low. They may get harder to start but the compressions in and of themselves will not cause a plane to fall out of the sky. It may make it down a bit on power, but it will still put out a lot of power at those numbers. Now if you have 6 broken or stuck rings causing the numbers to be lower than there may be room to worry a bit, but that is highly unlikely. a leak down test will tell you more than simple compression numbers.
Mark

Stuck Jab piston rings is nothing new. I have been as low as 105, and found I had no choice but to take things apart. Things were stuck solid. 60 to 69 sounds extreme- are you sure your numbers are correct?

The engine is sensitive to running with a mixture too rich- the egt-values imposed by Jab are already well on the fat side. Soot will form in the ring grooves, and after a while the rings are stuck.

In addition, the mineral based oil they recommend is none too happy with the very high local temperatures generated in the pistons due to almost non existent barrel cooling under certain conditions, and oil coking in the grooves is a real possibility.

Finally, if the engine is run on 100LL, lead deposits will accumulate in the grooves- again causing messed up rings.

On the other hand, the engine may also have been run with too much cooling, causing glazed barrels. Down goes your compression.

If you decide to take things apart, make sure the pistons are in tdc when you lift off the barrels. If not- should you have a loose, broken ring, it might fall out- and down ino the sump. Guess how I found out.

Do not take off the barrels before you have laid your hands on a good piston ring groove restoring tool- coke is unbelievable hard to get out; you can forget about using a file.

Thanks
Kai

Let me clarify, I'm looking at a Sonex for purchase. Those are the stated compressions. I'm trying to get an idea of what I would be getting myself into if I purchased an engine with compressions that low and how soon I'd be looking at a top overhaul. I know the debate about compressions on Lycoming and Continental engines, what low is too low. I know Continental says you can go as low at 40 on a C-85. But I know little about the Jabiru other than what I've read in the service manual which says anything below 60 should be addressed. Based on that, my thinking is that I could be faced with a top overhaul sooner rather than later. It certainly wouldn't make it to the 1000 hour mark which I understand isn't uncommon.

Tom Murrell

I don't know what the price of said sonex is but you are probably right about it not making another 700 hrs. will it make another 300? maybe maybe not. If the plane is currently building hrs that is a good sign. If it has sat for years and these are the numbers I would be concerned. Also are these stone cold numbers or are these after it has run and warmed up? I would probably have another compression test done and if these are true I would be looking to get a lower price than the going rate for the engine based on poss repairs. If you do it yourself, used replacement barrels are going for just under $400 and ring sets about $30. That was as of Oct '18.

It is building time now but it wasn't used much for a number of years prior to the last couple.

Good reading: https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2013-07_compression-in-context.pdf

Interesting article. The Jabiru manual perpetuates the 60/80 compression test myth. The manual states "Maximum allowable pressure loss is 25% - therefore a differential
of lower than 80/60 indicates a problem which must be addressed and corrected before an aircraft can be brought back into service."

Well, that might be Jabiru's opinion but that's all it is. Luckily you're free to use your best judgement. If the valves look OK with a borescope I would personally not be too worried, especially with what that article says about the measurement being really unreliable. If it was an Aerovee, I wouldn't be worried since a "top overhaul" costs less than a grand. Don't know how expensive it is to do work on a Jab. (But that's just my opinion, and I'm no expert, so take that for what its' worth. ;-)

Pulling the heads of a Jabiru is not difficult. Assuming the price asked for the plane you are looking at is reasonable, If it were mine I would want to pull the heads for a look see in there just to be sure that what I was counting on to keep me out of the trees looked okay. If you have no experience working on engines, should not be hard to find some local help for removing the heads. If then see that the cylinder barrels should be removed and honed, make sure you have some help from someone knowledgeable of Jabiru / Camit engines. Also strongly recommend joining the Jabiru/Camit forum.

Do you know the serial number of that engine ? Early solid lifter engines have a good reputation, early hydraulic lifter engines can be problematic.

David A. Sonex TD with CAMit 3300. (and a Jabiru 3300 in storage.)