Roller Tappets: Coming soon to an engine near you

Recip Technology

Roller Tappets

Coming soon to an engine near you

By Joe Escobar

October 2004

October 2004 coverAt this year's EAA AirVenture in Oshkosh, a topic that was creating plenty of buzz was roller tappets. Announcements were made by both Superior Air Parts and Lycoming regarding the introduction of roller tappets to their engines. Superior announced that it would be introducing hydraulic roller tappets on it's experimental XP-360 engine. Lycoming also announced at the show that it was pursuing roller tappet certification for its engines. Roller tappets have been available on automotive engines for many years, and aircraft engine manufacturers are now making the leap to introduce this technology to the aviation community. We will take a look at what the introduction of roller tappets will mean for you and how it will affect the engines you work on.

Why roller tappets?
You usually hear of better performance and more power in relation to roller tappets on automobile engines. Tim Archer, vice president of sales and marketing for Superior, shares whether this will be the case with its roller lifters. ''Increased performance' and 'more power' are several buzzwords you usually hear in reference to roller tappets on automotive engines. And that is true when you are talking about automotive engines that are operating in the 5,000- to 6,000-rpm range. In the range that aircraft piston engines operate, any increase in performance will be minimal ' some operators may see a 1 or 2 percent increase but for the most part, increased performance is not significant.'

engineOliver Leber, senior product engineer for Lycoming, shared the same thought. 'As far as performance and horsepower, it is not going to change any of that. Not significantly enough to make a noticeable difference.'

So what is the benefit of roller tappets? Well, the main benefit to customers is decreased friction. This results in a significant decrease in cam wear, a smoother running engine, and lower operating temperatures. 'Essentially we are improving our system by going from a sliding to a rolling mechanism,' says Leber. 'With a sliding system there is more friction and more wear involved from a design standpoint,' Leber shares. 'The advantage is in the interface between the cam lobes and the tappets. There is less friction and that is the primary benefit to the system.'

What does this mean at overhaul?
What can you do if you receive an engine at overhaul that has flat tappets? Will you be able to change them to the new roller tappets? The answer is yes, but there are additional items that need to be addressed. Roller tappets will eventually replace current flat tappets.

roller tappets

In the case of the Lycoming engine, the camshaft must be replaced at the same time roller lifters are installed. 'Because the interface between the tappet and camshaft is different from a flat tappet to a roller tappet, the profile of the camshaft is different,' explains Leber. 'The overall lift will be the same. So from a valve action standpoint, that's the same.' Basically, the camshaft is designed to match the same output with the roller application. So if someone wants to change to roller tappets, the camshaft needs to be replaced as well with the proper part. The new tappets and camshafts will have unique part numbers and mechanics need to ensure that if there are roller tappets installed on the engine then the proper camshaft is installed as well.

In addition to making sure the proper camshaft is installed, the crankcase will need to be sent to the factory. 'Initially, the crankcase needs to be sent to us for modification,' explains Leber. 'This is to install an anti-rotation mechanism for the tappets.' He explained that while flat tappets rotate freely, roller tappets need to be installed in a manner so as to prevent rotation and ensure the roller is in correct alignment with the camshaft. Once the modification is done, only roller tappets can be installed in the crankcase. Flat tappets will no longer fit. However, one thing to keep in mind is that even though the crankcase is basically idiot-proofed to prevent inadvertently installing flat tappets, either type of camshaft can still be installed. It is imperative to check for proper installation.

Cam walk (axial thrust)

As roller cams became more popular in the 1950s and '60s, auto racers removed their flat lifter camshafts in favor of roller cams. A problem that some of them encountered was cam walk (either forward or backward cam movement in the block). When engineers looked into the problem they discovered that the cause of this problem was the roller tappet bosses. In some cases, the tappet bosses were bored out of square with the cam bearing bore. In effect, the roller tappets would thread the camshaft in or out depending on the tappet alignment and camshaft rotation. Even a fraction of a degree of misalignment would cause the rollers to skid laterally over the cam instead of rolling smoothly over it. On a V-8, 16 tappets out of alignment could easily create several hundred pounds of cam thrust force. It is easy to see that tappet alignment is critical.
Source: Isky Racing Cams, www.iskycams.com

Time frame
So, how soon will you be seeing roller tappets on the piston engines you are working on? It may be sooner than you think. Both Superior and Lycoming are expecting to have roller tappets installed on their engines in the near future. Superior will offer the roller tappets on its experimental XP-360 engine later this year. After that, the company will likely work through the certification process and incorporate the roller lifters on its certificated Vantage engine.

Lycoming is in the final stages of certification and expects the tappets to be available on some models early next year. The tappets will be introduced by model series. They will first be available on 360 engines from the factory with the aftermarket introduction shortly thereafter. After that, it will be introduced to the factory 540 engines with the 540 aftermarket introduction following. Eventually roller tappets will be available for all engines that currently use flat tappets, which are the majority of Lycoming engine models.

The bottom line is that roller tappets will offer the benefit of less wear and smoother running engines. It may not be long before your shop or FBO sees an engine with roller tappets. Stay tuned to news from your engine manufacturer and to AMT to ensure you are up to date on this latest trend in engine technology.

Additional ReSources
Lycoming
(570) 323-6181
www.lycoming.com

Superior Air Parts
(972) 829-4600
www.superiorairparts.com

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