Interesting article on roller tappets

[Steve Melton]

Originally Posted by David-aviator

My engine, which was built from a Lycoming kit by Barrett, has roller tappets.

If you decide to go with a Lycoming kit, they come with the roller tappets. I don't think the old style cam push rod assembly is available with new engines. Also, the Lycoming roller tappets are not available for older engines. It is a different system with a different case. Clone engines, I believe, do not use the Lycoming roller tappets but a different system.

So for better or worse, if you go new Lycoming it is with roller technology (except the 0235). I suspect the change is good but only time will tell. So far, a bit over 100 hours, the engine runs well, uses a quart of oil in 17 hours and the oil remains very clean. I suppose some would say the oil is not doing its job of cleaning the engine, but maybe the engine isn't dirty. No leaks so far, just a smooth running motor rolling along on its rollers.

After changing black oil at 50 hrs on other Lycomings without roller tappets, I am amazed how clean the oil is on my roller tappet engine at the 50 hr oil change. I am asking myself, why am I changing this oil at 50 hrs. I am wasting money. I could cut my oil changes in half. Is the 50 hr oil change a hard requirement or suggested? How about changing at 100 hrs.? Anything wrong with that?

[RVbySDI]

Originally Posted by Steve Melton

After changing black oil at 50 hrs on other Lycomings without roller tappets, I am amazed how clean the oil is on my roller tappet engine at the 50 hr oil change. I am asking myself, why am I changing this oil at 50 hrs. I am wasting money. I could cut my oil changes in half. Is the 50 hr oil change a hard requirement or suggested? How about changing at 100 hrs.? Anything wrong with that?

Getting the popcorn ready. Looking forward to the response you get on this one. I am thinking primer wars, tip-up/slider, etc. but with perhaps more passion!

[chrishalfman]

Originally Posted by Steve Melton

After changing black oil at 50 hrs on other Lycomings without roller tappets, I am amazed how clean the oil is on my roller tappet engine at the 50 hr oil change. I am asking myself, why am I changing this oil at 50 hrs. I am wasting money. I could cut my oil changes in half. Is the 50 hr oil change a hard requirement or suggested? How about changing at 100 hrs.? Anything wrong with that?

So, the cam makes the oil dirty? I always thought it was blow-by, unburnt fuel/carbon and stuff like that. I think you just have a really good cylinder/ring seal with this engine vs. your previous engine.

[Neal@F14]

The real reason to change the oil at 50 hrs is because it's getting loaded full of combustion gas byproducts. Even if it still looks clean, it might contain corrosive stuff dissolved/suspended in it. Oil is cheap. Change it.

[Steve Melton]

Originally Posted by Neal@F14

The real reason to change the oil at 50 hrs is because it's getting loaded full of combustion gas byproducts. Even if it still looks clean, it might contain corrosive stuff dissolved/suspended in it. Oil is cheap. Change it.

but if the stuff is dissolved/suspended, wouldn't you see it?

[Steve Melton]

Originally Posted by chrishalfman

So, the cam makes the oil dirty? I always thought it was blow-by, unburnt fuel/carbon and stuff like that. I think you just have a really good cylinder/ring seal with this engine vs. your previous engine.

I think the sliding cam causes the oil to become black.

[1001001]

Originally Posted by Steve Melton

I think the sliding cam causes the oil to become black.

Can you elaborate on the specific mechanism for your hypothesis?

[Steve Melton]

Originally Posted by 1001001

Can you elaborate on the specific mechanism for your hypothesis?

cam friction (or oil compression) and the resulting heat causes the oil to form micro coke and become black color

[1001001]

Originally Posted by Steve Melton

cam friction and the resulting heat causes the oil to form micro coke and become black color

I guess it would require a spectrographic or micrographic analysis to confirm that coke particles are indeed the source of the black color. I'm not sure about the chemical mechanisms for petroleum coke formation. I am somewhat familiar with coke production in thermal processes involving coal, but not petcoke formation. I'd be interested in seeing any references you might be able to cite.

Are you proposing that high local temperatures and pressures at the cam/lifter interface could cause coking? I have been involved with operation of high pressure reciprocating pumps and haven't seen coking in the lubrication oil, but of course the crankcase temperatures in my application were considerably lower than an IC engine, however, at a bulk oil temperature of less than 200 degF i wouldn't expect thermal degradation. I'd imagine that local temps would have to exceed 500-600 degF with a very rapid ramp rate to cause formation of coke in the time frame encompassed by cam/tappet contact.

Typically formation of coke from coking coals can be prevented in thermal processes by pre-oxidizing the coke forming constituents. Don't know about coke forming in lube oil.

[Steve Melton]

a cam is a wedge. the oil film is moving in three directions as it compresses. in a perfect condition the oil film would be a constant thickness over the surface but it's never perfect and very small localized area could actually see a very high pressure as the oil distributes causing high temperature, 600F or so, causing oil micro coking. I don't have any references to cite.