Interesting article on roller tappets
 

Slightly off topic, but I figured the airforce crew might be interested. A new article came out today that talks about roller tappets vs. sliding tappets. It's about cars, not airplanes, but gave me at least some insight into what's going on in the engine http://www.economist.com/blogs/babba...0/11/tribology

Getting close to engine ordering time, so learning more about engines has been a top priority. Whether or not roller tappets are worth it has been confusing. One engine guy told me that they're more corrosion prone than sliding tappets, and a waste of money...

Interesting article. While my (yet-to-be-flown) Superior IO-360 has roller tappets, I don't know too much about them. Anecdotal story: A tech at Superior did comment to me several years ago when they were still making engines that when they switched to roller tappets their warranty claims on cams went way down.

I do have some experience with roller tappets in small block Ford V8's. In that case they are considered to be nearly indestructible, at least for street use. Half a million miles or more on cam and lifters is pretty normal. As for the oiling thing, I'm not sure I buy it, or even that it matters. Because there is no sliding contact, they are much less sensitive to the wear that can take place on initial startup, before oil has a chance to bathe the cam lobes.

my first engine wore out due to cam lifter making metal at 3,150 hours. ill take my chances with a roller lifter this time. will let you know in about 8 years what happens. fly often. change the oil. keep out the rust.

While I don't profess to know any spectacular details between roller and flat on aircraft, I've done more than a few engine rebuilds and modifications on many a hot rod. I could care less about body work, but I'll do engine work all day until the cows come home.

I do like roller tappets, but not for longevity. I think they will help, but frankly, I've got a 200,000 mile car that's certainly got flat tappet and never been torn down. It's all about RCM (Reliability Centered Maintenance). Excellent set of articles in recent Sport Aviation mag. Roller tappets allow me to have a much smoother running high HP hot rod engine. The profile of the roller allows it to follow the cam perfectly. Where the flat version sort of "falls" off the top of the lobe. This makes the cam sort of ride on the edge of the tappet at bit.

At the same time, I can only describe the spectacular failure if a roller bearing fails to be well lubricated. I've pulled out a tappet that looked more like an oval as the cam just ground away the roller.

I can see where both have their pros and cons with all in life. In no doubt it will help with friction, and if you pit a lobe of the cam, you'll likely be able to keep going. The problem with that pit is that's where everything bad will always settle. So the cam can/will still become a point of failure, regardless of tappet.

My plan (with my flat tappet IO-320 rebuild) is to change the oil often, fly the airplane often and keep up with oil analysis. Ignored little problems become big problems, and big problems become expensive. That $100 oil change doesn't look so bad any more, heck it costs more than that for one of my sports cars these day.

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. :)

I'd like to know why the mechanic thinks roller tappets are more corrosion prone. They might have more parts to corrode, but once a tappet face begins to spall it's all over for the cam.

I will say that when I started my IO-390 (roller cam) on oil analysis it was flagged for having the iron as too LOW.

not for me
 

Roller tappets are not field-overhaulable. At overhaul they must be replaced and they are $250 apiece...mandatory replacement. That's an extra $2000 at overhaul on a 4-cyl engine. No additional horsepower gain.

Had the valve cover off of my VW TDI at 200K miles. The cam has a very aggressive profile, and I was pleasantly suprised to find zero wear (flat tappets). The engine has had nothing but Mobil 1 Turbo Diesel Truck oil in it, which is high in ZDDP, changes at 10K intervals. I intend to try running this same oil in my Rocket.

http://www.cpgnation.com/forum/new-t...mshaft-10.html

Scroll down to the part about "lubrication".

Roller lifters have been around for many years, the driving force behind their use in automotive applications used to be for high performance engines------they allow a much more aggressive cam profile than a flat faced lifter. As the duration of a cam increases, it gets to a point where the flat lifter will not follow the profile of the lobe without hitting the corner of the lifter. Also, the "ramp" of the lobe, which is basically how fast the valve opens, is another factor in the mix. The benefit of lower friction of a roller in almost nil-----the main power gain is that a roller allows the cam profile to be optimized for more power.

Now when you look at the cam profile in typical air cooled an aircraft engine, the cam is extremely mild in profile than an automotive unit. The need for a roller lifter to follow the cam lobe is just not there. The benefit of a roller lifter in an aircraft engine is two fold, less wear due to rolling action instead of sliding action, and the ability to survive on modern oils that have had the critical wear controlling compounds removed from them.

When you couple this with the issue of infrequent operation, and the accompanying oil drain off of the cam, and surface rusting----even a minor quantity of rust----the roller is able to shine in this environment.

You might want to re-think this! :eek:

Oil additive
 

Off topic, I use BG MOA (motor oil additive) in my oil changes. 1 pt to 5qts Aero Shell 15W-50. At 500 hard acro hours (SC-IO-540-C4B5). I?ve continued to have excellent oil analysis results. Personal results, I had a high time plane with a 0-320, After a TBO overhaul and break in. I start using the BG MOA. The plane was in a flying club and then later sold the plane with 1900 hrs SMOH to another flying club on the field. Last heard was that it had over 3700hrs since SMOH and still had excellent comp. & oil analysis.

Thomas S.