Having now plugged the oil vent holes in each rocker head and waited for the massive cold snap to go …and the rains that followed to ease …I’ve finally managed to get out and get 2 ground runs in to check for ongoing oil leaks.
Block and surrounds all degreased and dry I run her up to 50’ then 60’ oil temps .. then ranges from tickover to 2,000 rpm then up to 2,300 before the brakes creak at me that they can’t hold on the wet grass.
Taxy back from the power check area and whip off the now warm cowls to check
3 of the 4 cylinders are showing signs of oil smears and even drips starting at the base of the push rod guide tubes .. on checking the Jabiru maintenance manual these have a double line of rubbers as they locate into the block .. rubbers 11 and 17 in the diagram below as it exits part 13 .. the adaptor
Hi David, I’m a bit puzzled as to why you have blocked the cylinder head vent holes, ?, is there another point at which the oil line pressure will be reduced back to atmospheric pressure on return to the sump, if not, then an increase in crankcase pressure could be forcing the oil past the seals.
I understand that the engine has undergone a complete rebuild, so in theory all T’s and P’s should be on the button, have you checked the max oil pressure relief valve, as any excess should be venting back to the sump (atmospheric pressure).
Hope this helps.
As always .. I really appreciate your engineering input … I’ve been happy tinkering and do8ng the odd air and ground test around this issue but have now pushed back to get this properly checked out.
What I know and have learned to date is that oil is fed from the sump gallery (under pressure) through a smallish bore (2 x 2.5mm) metal feed pipe that runs up between the heads to a T rubber (each side)
The T splits the feed so that each cylinder gets a good supply of oil to lube the rockers.
Oil then flows back down the rocker tube guides (Not under pressure) back into the sump
When I purchased the engine, new, it already had hydraulic lifter valves … on recent reading this would explain why the breather holes had been drilled (as an established MOD for those having hydraulic lifters)
Initial runs after the rebuild, and especially at 3,000 rpm and above , managed to cover the engine and bulkhead and electrics in oil
Trying to locate it proved a challenge as it had simply covered everything in oil !
I fed this back to he rebuild guy as well as my research that perhaps, now switching to solid lifters, we needed to reverse the breather mod … hence the re sealing of these
Following a couple of ground runs this week, I could finally get a much clearer sight of where the oil is coming from.
As you say, are we now in a catch 22 … increased pressure is pushing it back through the pushrod seals OR is the volumetric capacity of all 8 x pushrod tubes sufficient to let flow back down into the sump .. in which case the rubbers – seals are suspect
Hopefully the pushrod tubes can easily slipped up to quickly check rubbers
Thanks again Nigel 😊
Hi David, I was looking at the Jabiru details and noticed a change in the length of the dipstick, to measure oil level max hot, as opposed to originally measuring max level cold, if you have a oil level max hot dipstick, and oil level has been filled cold to the hot level, then it could cause a crankcase over pressure.
I confess to having made a nearly disastrous error when I filled my motorcycle with oil, answered the phone, and promptly put another full can of oil in, realising my error as the oil poured out over my boots, oops !.
I’m not saying this has occurred, but might be worth checking.
Cheers Nigel .. always useful and helpful to get your feedback .. thank you
The Jabiru dipstick is a bit of a pain if I’m honest … they have differing lengths and designs depending on your particular sump design.
So some are straight and short and some are linger and kinked !
This is somewhat compounded by the fact that my particular aircraft sits on the tail …
The only true check I’ve found is to trestle her up into flying position , straight and level, and my particular stick (the short one with a cross hatched mark on the bottom) then reads ‘normally’
If I’m doing a regular pre flight check and the cowl is on, I’ll check for the stick oil mark being approx 1/8” on the bottom of the stick.
I’ve not seen the hot-cold differential mark but will certainly check with the engine engineer when I see him. That said, the Jabiru is a pain when trying to check oil anytime after it’s been running (ie warm) as it climbs the relatively narrow feed tube which makes getting an accurate dip almost impossible.
I’m now waiting on the engine rebuild guy to come and check oil return seals in situ .. I definitely don’t want another year of almost zero flying in ‘23 !