'Head crush' on the 3300

[gregv]

My 3300 (33A-391 - 402 hr TTIS) started throwing excessive amounts of oil out the breather recently, so out with the leak down tester (thanks for all the helpful tips from forum members on getting this right) to discover a significant drop in cyl 5 compared to 4 months ago. Broken second compression ring found stuck in its groove in cylinder 5, with mild barrel scoring and a bronze coloured piston skirt presumably from excessive blow by. On inspection, every other cylinder also had at least one stuck ring (no others broken), with plenty of hard carbon deposits in the ring grooves. The engine is in a J200 airframe (#60) with standard air ducts and gull wing deflectors in between cylinders. Assuming the rings stuck primarily because of overheating, it looks like I'll need to do some work to increase airflow to the heads. CHTs on #6 and #5 (the only two I monitor) were generally running around 280°F or less (measured between the sparkplug washer and head). Less than manual limits, but too high for engine top end longevity it seems.

 

I sent my cylinders away for check of wear limits and a hone as per the manual specs, and the guy doing the job has noted that 'the cylinders seem to be working themselves into the aluminium head'. He has asked for the Leeb hardness number for the heads, which I have duly asked the factory for. Is this what I have seen mentioned before as head crush? Is this common in 3300 engines? I have to admit I didn't notice this on disassembly but then I wasn't looking for it either. Head bolts were checked at 24ft/lbs cold at oil change, though I guess they could have been tighter and I wouldn't have known.

 

Any comments or previous experiences with this would be gratefully received. I'll get some photos up, and post how things turn out. Anyone know whether there is an acceptable wear limit with respect to this?

 

Greg v

 

 

[deadstick]

Hey Greg, do it all whilst you have it apart, have the head serviced and the lip removed it is standard for these cylinders to pull into the head a little, I have had much the same problem at 230 hrs I had number 2 comp rings stick allowing a lot of blowby into the sump (carboned up). Never let anyone torque the head bolts when its hot and yes there is always one cyl that is significantly hotter than the rest and I suspect on mine one head went very close to elastic given the way the cylinder pulled into it.

 

 

[Vev]

Hi Greg,

 

It certainly sounds like you have an over heating problem, stuck and broken rings is a clear sign of heat in the Jab ... interesting to know what jetting you have in your Bring?? Also useful to have EGT's fitted in the jab as anything over 1250 F is death and jetting is the best way to manage temps.

 

Head crush is common problem if one is too heavy handed on a torque wrench. They really shouldn't be torqued down beyond 20ft lb (as specified in the manual) after the initial 24ft lb assemble torque value.... any more than 20ft lb and you begin to crush the heads.

 

Cheers

 

Vev

 

 

[Bruce Tuncks]

Hi Greg, 280F is not too high a temperature, (137 c), although you did not say what the highest temperature you see on climb is.

 

I do think that the heads are made of a relatively soft alloy, so in general the cooler the better. If the front cylinders were in better condition than 5 and 6, it would be interesting to temporarily move a sender to no 1 to see what it runs at. If the front cylinders were cooler and they were ok then that is good evidence that heat is the problem.

 

It may pay to check the temperature gauge by dropping the senders into boiling water to see if the gauge shows 212 F.

 

Bruce

 

 

[jetjr]

Be aware you most likely have "thick finned" heads. My engine is very similar to yours No. 451 (similar AC too . No 85 J200)

 

I just changed to new engine which came with new heads and theres a huge temp reduction, even with same cowls etc

 

Before spending too much cash on old heads, ID consider upgrading as theres other benefits too with rocker geometery etc Im told

 

Also could look at liquid cooled if you wanted to.

 

Im also a believer in individual head monitoring, theres big differences accross the engine in both CHT and EGT. I agree heat is a major problem and needs watching

 

 

[bushpilot]

Hi Greg,It certainly sounds like you have an over heating problem, stuck and broken rings is a clear sign of heat in the Jab ... interesting to know what jetting you have in your Bring?? Also useful to have EGT's fitted in the jab as anything over 1250 F is death and jetting is the best way to manage temps.

 

Head crush is common problem if one is too heavy handed on a torque wrench. They really shouldn't be torqued down beyond 20ft lb (as specified in the manual) after the initial 24ft lb assemble torque value.... any more than 20ft lb and you begin to crush the heads.

 

Cheers

 

Vev

I agree. In fact each time you torque the head bolts, it's best to take them out one at a time and brush a bit of anti-seize on the thread, then reinsert. Otherwise they tend to grab and over-tightening is more likely.

 

 

[jetjr]

Bushpilot, there was a discussion on this elsewhere, Jabiru specifically say DONT loosen bolts before tightening, (not sure why) and as correctly said tension to 22ft lb only

 

Assembly tension is to 24ft lb

 

Biggest error is likely to be doing it when engine is hot

 

I think by not loosening and just testing with 22ft lb, you get a good indicator if tension has changed, ie if it yields then theres an problem.

 

 

[bushpilot]

Bushpilot, there was a discussion on this elsewhere, Jabiru specifically say DONT loosen bolts before tightening, (not sure why) and as correctly said tension to 22ft lb onlyAssembly tension is to 24ft lb

Biggest error is likely to be doing it when engine is hot

 

I think by not loosening and just testing with 22ft lb, you get a good indicator if tension has changed, ie if it yields then theres an problem.

I should have been clearer: The approach is to loosen one at a time with the others left firmly in; that way the head wont move whilst lubricating the thread. Then screw that bolt back in and pull up to 20ft lb and repeat with the next bolt. That's what we do - but others should check with their L2..

 

 

[ianboag]

We have a one-year-old 160 hr J230 on the field here that just burned an exhaust valve. The (fine finned) heads also show signs of crush. It has been maintained properly. Go figure.

 

 

[bushpilot]

Our J160 has 720 hours with no issues -and good compressions; a J230 with 370 hours and another J230 with 290 hours - both good as new on comp's.

 

 

[gregv]

Thanks for the helpful advice so far. I'll try and attach some photos from disassembly of the heads hopefully showing the problem area- I'll take some better images when I get the heads back later this week. Connection is patchy what with the earthquake today...and that was just in Wellington Airport! Who knew Christchurch Information directed all jet traffic in NZ...

 

Jabiru Engine support have emailed telling me there is no Leeb hardness number available for the aluminium heads used in my engine. Moot point really - the cylinder has crushed the heads so they are too soft for whatever torque I had them at (24ftlb cold, but maybe I need to calibrate the torque wrench again!)

 

[ATTACH]13180.vB[/ATTACH]

 

[ATTACH]13179.vB[/ATTACH]

 

Greg v

 

 

 

 

[ianboag]

Don't get me wrong. I'm not trying to bag the Jab.

 

I fly a 160 that had a valve grind at about 800 hours which isn't too excessive IMO. There's something about all this that we don't understand.

 

 

[jetjr]

There has been some assessment that the material the heads are made from is a compromise between soft enough to machine easily, and hard enough to do the job.

 

Seems they are a bit on the soft side so torques must be carefully applied, the difference between 22 and 24 is difficult to even adjust on my wrench, let alone if its out of adjustment

 

This material is also prone to extended expanding and shrinking with heat so it could be a core issue some of the issues we see.

 

Bottom line is the heat (CHT and EGT) needs to be carefully managed and monitored. They can vary from below limits to well above at the one time. It can even change with rpm.

 

Ianboag, the one with burnt valves, was he watching EGT on that cylinder?

 

 

[gregv]

As promised, some more images as things slowly progress.

 

I guess the question comes down to this - with the cylinder head crush at the level in the photos below, what would be the potential issues with honing the cylinders, replacing rings and valves, and putting the thing back together?

 

It seems clear I need to get my temps down given the amount of carbon / oil residue that bound the rings up, so looking at 6 cyl CHT monitoring as a minimum. My temps have been CHT max at climbout 280°F, usually 265-70°F cyls 5 and 6. EGT at factory position right exhaust manifold 670-690°F cruise, 620-630°F at wide open throttle, oil temps between 70-95°C depending on outside air temp.

 

Thanks again for the helpful comments so far.

 

[ATTACH]13235.vB[/ATTACH]

 

[ATTACH]13232.vB[/ATTACH]

 

Cylinder 1 exhaust side

 

[ATTACH]13234.vB[/ATTACH]

 

Cylinder 6 plug side

 

[ATTACH]13233.vB[/ATTACH]

 

Cylinder 6 exhaust side

 

 

 

 

 

 

[Bruce Tuncks]

Sorry Greg but I can't tell from the photos, the heads look like they were made that way to me. The crush I have seen was on an older 4 cylinder engine and it took the form of distortion in the zone of the head-bolts near the exhaust valve. Plastic deformation had taken place and the head material near these bolts had compressed a mm or so. This engine had often run at 350F on climb, which was less than the red-line of 390 in those days. There have been real changes since then in the red-zone temps and the head design, so I would have said the problem is mainly in the past. But your carbon buildup sure does seem like heating, so I'm puzzled.

 

My 2200 now runs similar temps to your figures and has not shown problems in 320 hours. Before this, it ran about 40F hotter and an exhaust-valve seat wore prematurely.I changed the rear cylinder heads to later ones with more finning ( not the current ones with even more again ) and tweaked the ducts etc a bit to increase the cooling.

 

So I don't think 280 is too hot, in fact there are authorities who say that less than 250F can give problems with lead deposits building up.

 

I understand your reluctance to put things back just as before... why wouldn't the same thing happen again?

 

 

[jetboy]

Hi Greg, I cant see anything bad in the photos in fact the heads look much cleaner than mine @ 300 hrs

 

I think the machined ridges are still showing in your head - cylinder contact area these will become flat in the area around the bolts anytime cylinders are checked above 20 ft-lbs too often.

 

When you put it back be sure to have the clearance honed out to the latest specs, and the cylinder base nuts bevelled per AVDAL SR050 if not already. The workshop in UK also recommends heating the cylinders around the bases before tightening them down to make sure they don't set oval too. Wish I'd known about that before putting mine back. But I did get + 200 rpm extra in the climb from the fixed base nuts, so it has been worthwhile.

 

Ralph

 

 

[gregv]

When you put it back be sure to have the clearance honed out to the latest specs, and the cylinder base nuts bevelled per AVDAL SR050 if not already. The workshop in UK also recommends heating the cylinders around the bases before tightening them down to make sure they don't set oval too. Wish I'd known about that before putting mine back. But I did get + 200 rpm extra in the climb from the fixed base nuts, so it has been worthwhile.

Ralph

Thanks Ralph. The factory now provides ARP nuts as standard with new throughbolts, and they have a smaller diameter, hopefully avoiding the cylinder distortion caused by larger diameter nuts. I have new ones here ready to go in. Stan Hyde at Feilding reportedly had two lots of through bolts supplied with undersized threads that stripped when torqued up, so replaced those throughbolts with ARP units, though they would have been metric, I think there was some machining involved.

 

Greg v

 

 

[slartibartfast]

My through bolts looked fine, so I got the factory to send new nuts. I torqued them all up fine, then backed one of them off because I thought another thread on one side would be wise. On re-torquing that one, the thread on the bolt stripped - not the nut. Jab want me to send it to them to check it out.

 

They sent a new bolt, but not new nuts. So they seem to be all the same as original (4 year old engine).