slartibartfast Posted January 22, 2011 Posted January 22, 2011 I took my heads, barrels and pistons to an engine rebuilder to fit K-Liner valve guides (the guides were well stuffed after only 300 hours - grumble, grumble). In the process of cleaning everything up, whatever was lining the bores dissolved, and now there's only bare chrome-moly steel. The Jabiru manual says the pistons and rings run directly in the steel bore. Is this correct, or do I need to re-line the bores somehow? Doesn't seem likely, just thought I'd ask you experienced guys because the factory is closed today and I want to put it back together tomorrow. Cheers, Ross
facthunter Posted January 22, 2011 Posted January 22, 2011 You probably only removed some oil "varnish". The pistons run directly in the steel. Some cylinders are nitrided but Jabiru are not. If the bores are "glazed" they should be rehoned to give a surface that will hold oil. Remove a minimum of metal. You are only trying to put fine scratches on the surface. ( not up and down but at about 30 degrees to the axis with a criss cross pattern ) Nev.
slartibartfast Posted January 22, 2011 Author Posted January 22, 2011 Thanks Nev - your usual sage and succinct advice. That's what I figured, but I feel better for hearing it from you. The engine guy did give it a light hone on my instructions. It looks better than original. The valve seats were oval and the valves were not seated well. There was heaps of play in the stems inside their guides. That'll be where the oil went. I'm putting it back together with the new hollow rods and rockers and sealing off the external oil feeds. The compressions should be much better with new rings, guides and beautifully seated valves, even though I'm shimming the barrels to reduce compression a bit (after chatting with the engine guys at Jab). The cleaning process even made the ports much bigger. Because of the badly leaking guides, there was a lot of build-up beyond them. We'll see how it runs now. Hopefully I'll get more than 300 hours out of this lot. Thanks again.
facthunter Posted January 22, 2011 Posted January 22, 2011 Yes, Worn guides are a disaster. You cannot keep the valves seated when that happens, and a heap of oil can go down the inlet guides. What is not generally realised is that most engine oils will LOWER the fuel's octane rating and THAT could be a factor in detonation in some engines. Any build up of carbon can cause it too, if it is charp edged and can incandesce. Good luck and happy flying. Nev
Relfy Posted January 22, 2011 Posted January 22, 2011 Just out of interest Slarti, what fuel do you use and what temps were you getting in cruise/climb? Has overheating ever been a problem with the engine/setup? How much oil were you using before the rebuild and what oil were you using? I've been wondering with my engine as it doesn't seem to get hot at all and only ever reaches around 90deg oil temp in the climb on a hot day (33+ deg). Coming back from St George yesterday I was only getting about 60 degrees on the oil temp in cruise and I was wondering if running too cool may cause issues with reduced scavaging of deposits etc.
slartibartfast Posted January 22, 2011 Author Posted January 22, 2011 Relfy, early on I had some cooling issues during summer on long climbs. Sometimes I had to level off for a while because the oil was getting to 100C. It didn't stay set up that way for long. I built a scoop to create negative pressure and drag more air through the cooler. Now it rarely gets over 80C. I have always used Avgas and run on 15W/50 Aeroshell (the first year or so I sometimes used Aeroshell 100+). The last few flights it was using more than 100ml an hour. I concede that the early cooling problems may have caused the issue. All part of sorting out a new setup. Hopefully, it will go long and hard now. The engine too.
facthunter Posted January 22, 2011 Posted January 22, 2011 relfy, running too cool (Oil temp) will cause issues. 60 is too cool. You need about 85 (ideal). What happens? The oil accumulates water and the oil goes cloudy. Water in the oil causes internal corrosion and sludge build up. The water is a product of the combustion of hydrocarbons and gets into the oil past the piston rings. (Blowby)
Relfy Posted January 22, 2011 Posted January 22, 2011 That's what i think also Nev. I always like to work the engine and get the temps up to 80+ every flight for that reason through a good climb etc. Doing some circuit work also brings the temps up a bit also. It will be interesting Slarti to see how it goes now. Will you do a leak down before starting?
slartibartfast Posted January 22, 2011 Author Posted January 22, 2011 Will you do a leak down before starting? I don't think that would be too useful. I'll do one after the first run, but I wouldn't expect anything meaningful until 25 hours on run-in oil. I suppose I could do a pre-start leak-down just for info.
Relfy Posted January 22, 2011 Posted January 22, 2011 Sorry Slarti, that's what I meant to say ie after starting/run etc to compare pre and post compressions.
Guest DJH Posted January 22, 2011 Posted January 22, 2011 I don't own an aircraft, but, I'm interested to know what piston clearance is optimal for the 4 cylinder Jabiru engine, when honing the cylinder barrel.
facthunter Posted January 22, 2011 Posted January 22, 2011 it's normal with any engine to specify MINIMUM clearances and "wear" limits. which is when the part should be replaced. If you remove too much metal by honing you run the risk of rendering the part unuseable. The honing should just put light marks all over the surface, so that an oil film will be maintained, and not remove any real measurable amount of metal. Don't send this work to just anyone, because it is quite a precise process. A cylinder will also have out of round and taper limits and they are not large allowances, usually around ONE thou . Far finer tolerances than for cars etc Nev
Guest DJH Posted January 22, 2011 Posted January 22, 2011 I've honed countless cylinder barrels while building racing kart engines, it was my job. I'm familiar with bore gauges and micrometers for setting piston clearance, sizing pistons and honing to cross hatch and parallel the bore from top to bottom, or whatever shape was required. Are these Jabiru pistons size graded? How much piston to bore clearance is needed to avoid a piston grab, 0.001", 0.0015" etc is my question. I have seen a few examples on Youtube of Jabiru engines which have had a piston nip.
Guest JRMobile Posted January 22, 2011 Posted January 22, 2011 even though I'm shimming the barrels to reduce compression a bit (after chatting with the engine guys at Jab). Thanks again. HI Ross What was the main idea behind reducing the compression on your jab engine. I'm 22 hours into a top-end overhaul after 375hrs so you have got me interested. Cheers John
slartibartfast Posted January 22, 2011 Author Posted January 22, 2011 What was the main idea behind reducing the compression on your jab engine. I'm 22 hours into a top-end overhaul after 375hrs so you have got me interested. Hi John. I was talking to an engine guy at Jabiru (I forget who - not Don) about my plans. Forum member Rodknee very helpfully offered to chat on the phone about K-Liners (that's what makes this forum so great) . He mentioned shimming to reduce compression so I asked the Jab guy. He said it was worth doing because over the years the compression had crept up as they made engine changes. Mine is serial 2659, so fairly recent (about 4 years - it didn't go straight into the plane). He says the compressions had got a little higher than necessary, and lowering them through shimming would help with longevity. DJH, I don't remember the clearance we ended up with. About a thou. There are no oversized pistons for Jabiru because you can't take any more material out of the bores. The pistons are actually oversized Commodore pistons with a few mods, but only one size goes into the Jab. I got new rings, but the same as the originals.
Guest DJH Posted January 23, 2011 Posted January 23, 2011 Thanks for your reply, I was thinking 0.0015" or even a 0.002", as the engines are air and oil cooled, a generous clearance would be safer IMO. In days gone by, pistons were graded, to indicate their real size. Two 3.00" pistons usually didn't measure up with the micrometer as being the same size. Any engine builder worth his salt will measure the piston and bore size, then adjust the clearance with the hone or select another piston, instead of simply trusting to whatever size is stamped on the box. Today's very good CNC equipment probably makes all pistons identical sizes. I'm sure all you guys are on top of all this stuff anyway. I am in no way qualified to talk about aero engines, just interested in all types of IC engines.
facthunter Posted January 23, 2011 Posted January 23, 2011 I don't know the safe running clearance for the Jabiru pistons but I would reckon they are much more tham .001". That's as far as I will comment on the Jab. Anyhow it's not a matter of speculation there will be a minimum figure. If you are re-using pistons that have not grabbed, they are unlikely to do it after a slight hone, and re ring. Holden stopped using "graded" pistons in the cast iron V6 in the last model of that engine. They normally ran colour coded (or letter stamped).pistons that took account of the "actual" as distinct from the nominal size, of the cylinder, so you get a more consistent "running" fit. IF you don't selectively fit each one, The clearance has to be enough on the most likely combination of smallest cylinder and largest piston to still run without seizing. These later engines are noticeably more noisy mechanically. I believe Jabiru chose the largest oversize available so that people won't overbore their cylinders. These are NOMINAL oversizes to which the original engine can be REBORED . We are NOT talking about the GRADED pistons mentioned earlier. The graded ones will be portions of ONE thou, oversize, whereas the overbore options will be portions of 1 mm. (approx .040",or 40 thou.) Usually .040 is the max oversize supplied due cylinder wall thickness, core shift etc. IF Jabiru made the cylinders thick enough to rebore they would have to be more heavy. They are not expensive anyhow, and everything has a service life. High hours cylinders in air cooled aero engines are a source of unreliability . Nev
eightyknots Posted January 24, 2011 Posted January 24, 2011 The cleaning process even made the ports much bigger. With the more voluminous airflow, you may get 81 horsepower now!
slartibartfast Posted January 24, 2011 Author Posted January 24, 2011 Nup. Mine's a 4. The 1 Hp gained will be lost in lowering the compression. Yes DJH, I'm reusing the pistons, so they won't be any tighter than they were. I couldn't go out to the hangar yesterday, so I'll do it on Wed.
Guest Pioneer200 Posted January 25, 2011 Posted January 25, 2011 I have just been advised by e mail that our clubs J160 is out of service for a good while. The engine had developed a clicking noise on start up that would go away when warming up. This noise progressivly got worse during a cross country flight at the weekend and it was decided to investigate. It was found that there is sufficent wear in the cylinders to warrent sending the engine back to Jabiru. This engine has ONLY done 400hrs ( in 20 months) so Jabiru are rightly concerned with this. Not being to mechanically minded, what things are most likely to cause this??? I have noticed when flying it that it struggles to get up to good operating temps, usually stays between 60 - 75 and CHT always near bottom of operating range. Its a bugger, being from NZ we will be without aircraft for a while. If only I had a rotax 4 stoke powered option to fly!!!
facthunter Posted January 25, 2011 Posted January 25, 2011 The temps need to get into the right range. Too cold is not good. Does the engine get started at the end of the day, snd taxied a short distance and then put away for a week? That would cause the bores to surface rust and cause rapid wear. Otherwise do you have abrasive dust in the environment of the Airport? Nev
Guest Pioneer200 Posted January 25, 2011 Posted January 25, 2011 Hi facthunter, the jabiru is our trainer so generally gets used mainly on weekends but also during the week on occasions. 90% of the time it is put away warm after use. I wonder if we should be try a different grade oil of here in NZ to try and get the temps up. Would that work??? In the winter it can take 10 mins to get anywhere near the recommended 50 degrees oil temp before run up. We operate in winter temps of 0 - 15 degrees and summer temps of 15 - 30 degrees.
facthunter Posted January 26, 2011 Posted January 26, 2011 You are not so different (temp wise) to some parts of Victoria and Tasmania. The oils should be the multi-grade recommended by Jabiru. (aeroshell or what ever.) I would not change that. You might just have ro reduce the airflow over the engine/oil cooler. Nev
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