Hot rear cylinder example Jabiru 3300A-2575 and plenums

[RFguy]

Replacing rocker bushes...

rocker shafts measure well. bushes measure 0.2mm beyond spec (in one axis - the wearing axis.) 

The rocker shafts at the front of the engine have little/no deposited bush material and no scratches.

 

#6 is the worst, they get worse steadily toward the rear (last to get oil supply on start ? lowest oil pressure ? hottest ? ( a bit)  ?

 

very soft material, scartches flat with a fingernail. looks like copper. might do a chem test on it. to find out. 

will inspect push rod ends and oil supply shaft etc , and return .  the very rear is the worst.

 

attached : 

 

Edited March 11, 2021 by RFguy

[Thruster88]

Looks like the surface finish on the shaft is a little rough.

[RFguy]

and smooth on the other side.  I think material has come away and  scratched.

they'll all get polished flat. I will leave the copper in the grooves.

The ones at the front of the engines are near unworn.  progressively gets worst toward the rear.

 

The binocular microscope is always handy... There are plenty of good 10x to 50x monocular hand held tube scopes that are very useful to have during aircraft maintenance- particularly a type that has a built in light source (essential) . Even a handful of loupes of different magnification sis excellent for examination in situ.

[Bruce Tuncks]

Great pic glen. It looks to me as if there was something scratching the surface. But it also looks like the scratches are longer than the rockers would make.

[RFguy]

Bruce well spotted.

 

#6 was the first head I did - I was figuring how to get the shaft out (turns out just drive it out...) and I had rotated the shaft with the slotted end driver. That of course is only to get the keeper notch in the middle aligned, not for anything else. so I would say some of that scratching I did with I rotated the shaft .

 

Of course there is still quite alot of bush material in there, and much more at the rear as I said. at the front, almost imperceptible .... (and the bush wear is 50% less at the front ) 

 

Edited March 11, 2021 by RFguy

[facthunter]

It's a crook (ground) finish and I'd do a hardness check..Nev

[RFguy]

Hi Nev. OK, yeah.

what's the bushing material- a bronze ?   although, I could swear it looks like some sort of aluminium bronze allot that I have seen before. Can do a chem test on it I guess.

glen

 

[facthunter]

It's not plating. It's there because the oil film has broken down or the bronze is too soft. How strong are the valve springs at full lift?. Aluminium bronze is critical to lubricate and difficult to machine. Best small end boring is by a diamond tipped boring bar.  The best  small end bushes I've used are steel backed copper lead and rolled strip. Nev

[RFguy]

single spring setup : 6kg from memory.   double spring is 5kg + (inner small) 1kg

I will get  a bush out and have a good look at it. they all have to come out anyway and get back to you.

[RFguy]

also, there are NO  longitudinal (lengthways) marks present due to driving the pins out (wooden dowel , hard mallet) .

[facthunter]

The springs aren't exerting any pressure either. Those valve spring figures seem far too low. Don't have the bushes with too little clearance. There must be no chance of any seizure. They only wear on the bottom  and a bit of slop doesn't matter. Nev

[RFguy]

thanks Nev

Yes I did ask myself why a bit of slop isnt a bad thing. the hydraulic lifters will take out the play.  

We'll see that the new bushes measure in it when they arrive tomorrow. $5 bush. not much.

[RFguy]

-------------------------------------------------------------------

UPDATE, POST ARP ULTRA TORQUE (ARP-UT) LUBE  !!!

A week ago, I tightened up the thru bolts to 20 ft with W100 while I waited for the fancy lube to arrive.

Yesterday I went to the plane and applied the ARP-UT  lube and measured stretch for tensioning

WOW what a difference. With ARP UT,    The increase in torque is very much progressive, almost linear with rotation. With W100 it was rather vague and the torque vs rotation didnt not change much untl it felt like there was a binding point, where it would hit torque.

W100 : 20ft - 36 ft : 20 to 30 degrees rotation, +/- a bit

stretch at 36 ft : avg 0.25mm .

ARP-UT : 60 deg

Stretch at 36 ft ~ 0.4mm

NOW !!- the jab book says 43 ft for 7/16 thru and 35 ft for 3/8 thru bolts, even though the section between the threads is about the same size, and the short necked up regions and lengths  are about the same size. Did Jab just up the torque because it sounded right  and was approx the ratio of the new thread sizes ???

I seem to have reached  and gone slightly beyond the required stretch (about 0.35 mm by S Berger ) at 36 ftlbs.

 

Using ARP-UTGoing from 36 ft to 43ft, the distance was about 15-20 deg and the stretch increased about 0.07mm , out to almost 0.47mm, inline with the expected elongation for that much rotation of a 7/16-20 thread. 

When I get back there next week I will measure and see if anything has relaxed.

[RFguy]

Saturday, reinstalled the heads. then did leakdown. after sorting misc problems, all cylinders good except a couple hanging valves open - the rear 5 and 6.....

1) over the  whole engine, some lifters were pumped up a little. sorted that.

2) I removed the lifter cover port and removed the lifters on #6 EX and compared to others, found no operation difference. At least now I know what a roller lifter looks like (I am a overhead cam trained person). They would have the ability to get sludged up though, small orifices etc.

3) The push rods were pretty much all identical length . they've all been cleaned thoroughly.

4) The rockers spherical socket depth varied 0.1 to 0.21mm in depth (!). Is this wear or mfr defficiencies/limits ?

I determined that apart from some variation in rocker socket depth, the reason for the trouble on #5 ans #6 was all the head recession. 99% of the problem - and this is a failure mode I can see . - head recesses, operating lifter headroom falls to do and eventually negative so the valve does not close fully.

#5 cylinder made closed spec after bleeding out the lifters JUST JUST.  The good cylinders, 1,2,3,4 have above 0.2 to 0.5 mm of room if you push hard on the push rod end of the oil-empty rockers

I swapped the best clearance rocker for the #6 exhaust (the worst and open) and #6 now make leakdown. the one I swapped it for, #1 inlet JUST JUST makes shut. So the rocker spherical socket depth is a issue.

Originally I did keep all the rockers and pushrods with their cylinders, but during cleaning and rocker bush replacement, I got lazy. Valves and valve collets stayed with their cylinders and valves, I was warned and was careful about that. Picture 6 break loaf trays with their owners.

The #6 EX valve had been lapped 30 hours ago- this was the only work the engine had done done over 400 hours. This lapping would have increased the sensitivity to the above problem as a small amount of the valve material would have been removed

So, this is a failure mode to be aware of- head recession causes valves not to fully close and valve seat/valve damage.

I am not very pleased with the outcome of 'just' . The recessed cylinders at the bottom, perhaps 0.1 to 0.15mm equates to about 2x at the valve tip  due to the the mechanical advantage of the rocket geometry. I think the head -cylinder interface surface needs to be machined square again and shimmed. The recession causes the head bolt tension  and fit to get a good seal to be more critical.

All cylinders now making good 80/80 depending on technique leakdowns. Pullover compression feels strong .  managed not to get smacked in the head my the big prop during leakdowns.  just.
 

Edited April 25, 2021 by RFguy

[RFguy]

For those that the head recession causing  incidental valve opening is not quite clear  -  the effect of head recession is to bring the head closer to the crank case.  - or - the same as increasing the length of the push rods -- thus the rockers are partially actuated (valve opening) even when the lifters are fully relaxed.. 

[Blueadventures]

32 minutes ago, RFguy said:

For those that the head recession causing  incidental valve opening is not quite clear  -  the effect of head recession is to bring the head closer to the crank case.  - or - the same as increasing the length of the push rods -- thus the rockers are partially actuated (valve opening) even when the lifters are fully relaxed.. 

can you take the required thou off the end of the valve stem?

[RFguy]

no.   the top collet ring needs meat above it .

best solution is

1) machine and shim head-bore interface, or just shim.

2) increase spherical socket depth in rocker. I imagine some sort of hardened grinding ball is used. dunno. 

 

leaving it is asking for trouble, I like things with some allowance for situations I have not yet thought of.

[Blueadventures]

14 minutes ago, RFguy said:

no.   the top collet ring needs meat above it .

best solution is

1) machine and shim head-bore interface, or just shim.

2) increase spherical socket depth in rocker. I imagine some sort of hardened grinding ball is used. dunno. 

 

leaving it is asking for trouble, I like things with some allowance for situations I have not yet thought of.

Bugger, what about reface / grind the top end of pushrod? 

[RFguy]

The pushrods are all identical to the 0.01mm, I'd rather not vary that. I think that is it best to modify what is already in variation- the spherical socket depth. the problem children are the shallow ones .

 

again, best solution is a shim/ machine and shim.

However , yes it would be good to get the rocker spherical sockets to be more consistent.

 I guess during engine manufacture the parts may have been matched.

 

Note - this is no issue if there was no head recession.

 

 

[Blueadventures]

My stupid question, for the day - can you shim the head? Gasket type material?

Edited April 25, 2021 by Blueadventures

[RFguy]

shimming is done.

[RFguy]

different length push rods might be easiest. or grinding the shallow rocket sockets a little deeper.-bore interface would do it.  but then compression on that cylinder would change .  (but it has already with the recessed material.).

deepening the rocker sockets would seem best- as they are the parts in variation. 

Edited April 26, 2021 by RFguy

[facthunter]

Normally hydraulic lifters have much more clearance when empty and bottomed Out than your figures. It would be some thing like 1 mm. The best way to allow for seat and valve recession is to shorten the valve stem. That keeps geometry  as it was. It's normal for non pumped up hydraulic lifters to have a distinct clatter till the oil pressure fills them. Oil galleries should be pressurised before start up for this and other reasons. Deepening the ball ends may go through hardness.. You could use eccentric rocker bushes. installed appropriately I'd also "stake" the bush in a formed indent  at the top under those circumstances.

 " Riding" Valves will certainly cause severe  problems. Will the adjustable rockers retro fit?  (still using the hydraulic roller followers Nev

[RFguy]

Hi Nev

good point about the hardening of the rocket sockets/ball ends  ! mmmm

good idea on rocker bushes

Has roller hydraulic (late 2013 Gen3) .

I could take 0.25mm off the top of the valve stem .  Not a big job. Maybe that is the simplist . then I would have 0.25mm clearance with lifters pushed in . not great but better than zero.

 

BLUEDADVENTURES yes, I know you suggested it also. My concern was having enough meat above the top collet rings. I will measure it and see what the meat is there.  at least 1mm. maybe 1.5mm. 

 

Wonder what happens when the engine  warms up, expansions etc. push rods I think are aluminium with steel ball ends.  maybe they are steel all the way, someone would know. cylinder doesnt expand too much.  push rod tubes are aluminium, they are going to lengthen. putting compression on the clips in the head and the crank case (as the cylinders are steel).

Edited April 26, 2021 by RFguy

[facthunter]

You can't tell with the hydraulic lifters but if the push rods are the same material someone with that kind of engine may have  compared the hot and cold variations. With steel cyls I think they probably close up a bit but that's a guess. An engine on load will have a much hotter exhaust valves. I wouldn't be adjusting valves near a live prop and would rather work to a cold clearances figure provided it's properly arrived at. as the only way to go.. Just in passing  running most motors with the cowl off should be limited to quite short periods. Nev

Edited April 26, 2021 by facthunter