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Diagnosis?


geoffreywh

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Servicing a 230. 600 hrs. 350 since top-end rebuild and update. All has been going well. Doing a comp test I get all 132-135 (Hot motor) except #2 which reads 125.... a little oil squirted down the bore changes nothing. So, Valve Leakage. Leakdown confirms exhaust. Pop the head off and check, some carbon on the piston, quite a lot the back of the inlet valve. None on the exhaust, very minor signs of burning or overheating on the exhaust valve seat. I pull out the rocker shaft and find the exhaust rocker bush totally rooted. Small signs of wear on the shaft. Almost no wear on the inlet side. Valve guides have some wear, but don't they always?....Why would one bush have worn 0.5mm and the inlet none? Have others worn too? Any ideas???

 

 

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Could be any of the following:

 

Lack of lubrication,

 

wrong oil,

 

overheated head,

 

poor design of valve gear geometry,

 

faulty bush material.

 

I think top end overhauls are typically around 300 to 350 hrs on these engines. Cheap enough to do if you know it needs to be done.

 

 

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Why should one wear and others not. ? Rocker bush wear and/or valve geometry wrong. At the half open valve position the rocker should be at right angles to the axis of the valve stem. If you want to finess it a little before half way is permitted. Carbon on the back of inlet valve means the thing is running pretty hot, or crappy fuel. Nev

 

 

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I see camit sell a different type, might indicate a known issue

 

Some time ago this was a problem and some different materials were tried with terrible result

 

No idea why one would wear, what about regulalrly sticking valve slamming shut, maybe check vale guide clearance

 

 

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I think top end overhauls are typically around 300 to 350 hrs on these engines.

Many (but not all) are getting 1000 hrs+ before top overhaul with the current Jab engines David, they have improved quite a lot I reckon.

 

 

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NOT valve geometry of course otherwise all would be worn........Avgas used, exclusively, 100w shell, no overheating. But, the only reason for one bush to wear out and the other not is no oil feed, so, blocked pushrod or crook lifter....Thanks anyway...BTW.....(oil/carbon flakes on the back of the inlet valve is wear in the guides, NOT overheating) .....

 

 

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NOT valve geometry of course otherwise all would be worn......

Accumulated differences such as pushrod length, rocker post height, valve length, seat depth, head thickness, barrel height, head gasket and rocker variations can all add up to what Facthunter says, doesn't mean it is in your case but it most certainly can be just one by itself.

 

Me, I would just put it down to a bad bush, check oil flow and randomly mix the pushrods and rockers and put it back together and check it a couple of times later.

 

 

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I hear what you are saying. BUT the valve geometry will not cause premature bush wear. It's just a pivot point. It's got to be lack of lube. I'll swap inlet for exhaust pushrod and lifter and check later....Thanks Bex

 

 

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CAMit sell a much-improved rocker bush with grooving designed to ensure the oil gets to the right places: http://camitaeroengines.myshopify.com/collections/engine-components/products/rocker-bush

 

Obviously, even they won't help if there's no oil getting to the rocker in the first place, but if you looking at having to replace a bush, at the cost of a set, I'd go the lot. If you want to really do the job nicely, add the new CAMit rocker arms with the better geometry. If that exhaust valve has been running hot, as you suggest, then I'd be suspicious of a build-up of fried oil in the rocker supply.

 

 

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Just looking at the effort that CAMit are putting into these improvements is very promising and if they have got it right, it will make the Jabiru aircraft a winning Australian Product all around and one worthy of an Australian label.

 

 

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Not necessarily applying to this situation,,but grooving a bearing can reduce it's load carrying capacity. Where to supply a bearing with oil is critical. Nev

 

 

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the valve head and piston are as clean as a whistle. BUT they have been ceramic coated... ( oil "flakes" on the back of the inlet) It's got 2011 lifters and "up the pushrod" lube..Looking at records what I did find is:- when I fitted the new lifters, hollow pushrods and improved rockers I had to shorten some valves (one, where the worn bush is) to provide sufficient clearance for the lifter. (Lifter stroke is 1.8-2.0mm) . I may not have provided enough clearance? ( shorter pushrods were not available at that time..) The "insufficient clearance" may have led to premature bush wear...> Sounds complicated, But it's the only scenario that fits the facts. ............Low comp on a cylinder that has a shortened valve...The low comp cylinder has a worn bush..No other low comps, no other worn bushes. Cure ? Fit shorter pushrod 215 not 216mm rebuild and test.... ...Call me Sherlock? Thanks for the feedback everybody............

 

 

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Nev Said " Where to supply a bearing is critical" .............................. Absobloodylutely. The original had "Splash Fed" (Pause for the laughter to die down" ) rocker bushes , no grooves (and no oil either)

 

 

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I had the view that some of the rocker bushes had some Teflon or similar coating. I would prefer something more "normal" in the way of a bush material. Feeding oil through hollow pushrods is common and satisfactory. There was also a time when the oil was remaining in the rocker boxes in too large a quantity and small holes were drilled to allow air in to allow them to drain better. through the pushrod tubes. Nev

 

 

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