Jabiru engine overhaul

[Old Koreelah]

I’m just doing some maintenance on my Jab 2.2 engine. It’s SN 3178, hydraulic tappets and has been reliable for 15 years. 
Done less than 300hrs in that time, so lots of sitting idle. 
 

Head bolts torques all okey, except 6 of the 24 needed to be turned 10-30 degrees. All valve springs still have at least 300 micron clearance when fully compressed.
 

Leakdowns all 73/80 or better, with most of the air getting past rings. (When the heads were off at c.200hrs, the bores showed some glazing.)

 

When removing distributor covers, I discovered three of the retaining cap screws were loose, the fourth missing!
Something I never checked. Why didn’t Jabiru use retaining clips like everyone else?

[RFguy]

were the head screws adjust backed off half a turn CCW , then CW again ? or just from where it was at ?

Which head screws exactly ?

 

with hydra lifters. the only way you will really know the 'lash' is to have them bled down.  However, another the way that is useful, at TDC, you should be able to wiggle the rockers back and forth left to right on the valve head. No vertical  up down (opening) movement will usually be detectable IE NO play---, but you can feel whether there is heaps of pressure or little pressure ('rest pressure') by pulling the rockers each way on the valve heads.  (Mark Dunstone is credited with that one) .  

I used a little thread gunk to stop the cover screws unwinding... I prefer screws than clips. 

 

Leakdowns sound good. 

 

[Old Koreelah]

Glen I read somewhere recently that Jabiru advise to NOT back off the headbolts before torquing them up. Based on that, I just put the recommended torque on a long Allen key, via a pipe mounted over it and a recently calibrated spring balance pulling at the appropriate distance along that. 
 

Which bolts? The six Allen bolts on each head, including the one hidden under the plug in the rocker box.

[RFguy]

I mean WHICH screws out of the 6 moved ?
I cant beleive that is Jabiru's advice as it is bollocks in IMO if that is so. 

Problem is dynamic version static friction influencing torque.  Hence the reason to back them off to they are moving when you are measuring torque. Otherwise resultant tension could be out by perhaps 2:1... as you are overcoming static friction, corrosion etc etc lots of errors.

 

 

 

Edited April 13, 2023 by RFguy

[440032]

On 21/02/2023 at 5:00 PM, facthunter said:

The engine building part of Jabiru went broke..  Nev

CAMiT built the engines, not Jabiru. CAMiT folded. Totally separate company.

[facthunter]

I didn't say otherwise, really. Camit's business was building Jabiru engines and that was the only engine you got in a Jabiru.. Stiff and Bent didn't see eye to eye on the engine. Nev

[tillmanr]

Camit went broke because they upscaled their production capacity with new huge cnc machines a couple of months before the global financial crisis. 
Their production was good quality but they couldn’t sell the product particularly into the US.
As an aside, they improved their version of the engine which was sold under their name but Jabiru did not want the changes. 

[Old Koreelah]

8 hours ago, RFguy said:

I mean WHICH screws out of the 6 moved ?
I cant beleive that is Jabiru's advice as it is bollocks in IMO if that is so. 

Problem is dynamic version static friction influencing torque.  Hence the reason to back them off to they are moving when you are measuring torque. Otherwise resultant tension could be out by perhaps 2:1... as you are overcoming static friction, corrosion etc etc lots of errors.

Thanks for your feedback Glen. 

Which head screws moved? 

 

On Head 1, Number 6 moved c.30 degrees.

On Head 2, bolt 2 moved 10 degrees, 3 moved 20 degrees and 6 moved 30 degrees.

On Head 3 no bolts moved.

On Head 4, 2 moved 20, 3 moved 10 and 6 moved 35 degrees.

 

Thus, mostly the bottom bolt, not the top three. Perhaps because the bottom of the head gets less cooling air and therefore hotter and more likely to soften?

Inside my tappet covers is not as shiny as it was at 100hrs. Since then, CHTs  have exceeded 160 a couple of times (but none have ever approached the 180C often quoted as max.)Testing with two other instruments have convinced me the CHT readings are within 10 degrees of true.

What you say about backing offhead bolts makes more sense to me than the factory advice; being a 19-reg homebuilt, I have cast a wide net for maintenance ideas. Despite not having always complied with some of the factory’s earlier suggestions, the engine has given years of trouble-free service.

 

 

[facthunter]

Loosening and retightening will stress the head but some of the bolts are nearly seized at times. Suspect the ones near the exhaust ports. Nev

[Thruster88]

Thread pitch on those Jabiru head bolts 1.25mm? So 30° turn = 0.104mm of head recession . Doesn't sound like much unless it is happening every 25-50 hours.

[facthunter]

Difference in expansion rate of Al and steel plus head hotter. The surface scrubs. Probably related to heat cycles more than hours. Distortion near  exhaust port of head and cylinder caused by uneven heating.  Nev

[RFguy]

Well the #6 telltale will likely be recession., that's the hottest spot.... Do check that the rockers slide left right at TDC. exactly how much spare clearance you have depends on WHICH lengthof push rod Jabiru gave you. they vary.... most of the original Gen3 were too long and then went down a couple of mm ! (and didnt tell anyone and the manual has errors) 
 

eventually, you'll need to remove the heads , milled down say 0.9mm from the non recessed area so you get a nice flat surface again, and then drop in a 0.9mm shim. like below : 

 

 

 

Edited April 14, 2023 by RFguy

[facthunter]

The choice of material there might be something to consider. IF it cracks the head will  be damaged. You'll probably get home though as long as you're not going to New Zealand.  Nev

[Markdun]

The trouble with those numbers on torque is that what you are seeing is variation from other variables, which are unrelated to bolt tension which torque is merely a proxy for.  I doubt the head  cap screws are the same alloy as the cylinders and the variation are probably more to do with varying electrolysis / corrosion of the threads of the cap screws in the cylinder.  Most likely the lower head screw has less corrosion and moves easiest.

 

FWIW I think the purpose of head bolt tension checks is to assess whether there is recession, except perhaps a check at 5-10 hours after the head has been replaced.  As such I think it is better to do head bolt tension checks far less frequently than specified in the maintenance manual, and to do the torquing properly, ie dynamic by backing off and then re-torquing.

 

 And given how easy it is to pop off the head on the Jab to have a squiz, why not do that at 300 hours?