In the discussion of rocker arms, the subject of roller rockers comes up occasionally as an alternative to the stock ball type. While they are made in America and very fine quality, there are actually some pros and cons to using them in a flight engine.
First, a bit of history: Roller rockers were developed to replace ball types so V-8s could use 7,500 rpm and cams with .650″ lift. They were never designed with simplicity and longevity in mind. Back more than 12 years ago, there were several Corvair car parts outfits like SC performance and Clarks selling roller rockers, and most of the literature implied that they were developed by these companies. This all seemed reasonable in a black and white photo. However, the first time I saw an SC performance rocker in person, I saw it was orange in color. Because I spend my youth on NJ drag strips like Englishtown, Atco and McCarter highway, I instantly knew they were made by a company in the middle of America called Harland-Sharp. H-S didn’t have a website as late as 2003, but they directly sold to builders and they were a lot cheaper than SC Performance, which carefully trimmed the H-S name off the packaging before marking them up for resale.
Part of the internet hype at the time was roller rockers lowering oil temps and boosting power in Corvair engines. Neither of these are vaguely true. I bought a set just to test, and when our 601XL flew in early 2004, I am pretty sure it was the first Corvair powered plane to fly with roller rockers. We flew it several hundred hours and checked the valve train intermittently. They worked, but just as I predicted, no change in power nor oil temp. Other builders followed this with even more hours, notably Mark Langford who eventually flew more than 1,000 hours on the same set without issue.
When installing roller Rockers several other items must be changed. They need to be mounted on longer rocker studs, commonly sold by Clarks as #9295. (The studs that Langford and I used were made by ARP in California, and the current Clarks item looks visibly different, but I don’t know their origin.) They must have deeper than stock valve covers, custom length pushrods and Poly-locks.
Roller rockers have their own adjustment nuts called “poly-locks” It is basically a threaded tube with an Allen set screw up the middle that jams on the top of the stud. Most builders and car people don’t understand the two reasons for the existence of Ploy-locks are very rapid adjustment of the clearance on mechanical lifer cams on V-8s (This is not for maintenance, it is to alter the power delivery on the engine, often to suit traction conditions in drag racing. These went with the little T-handle hold downs bolts on valve covers) and second was to allow the use of a device called a ‘Stud Girdle’ that clamped the tops of all the Poly-locks rigidly together to prevent the from flexing when using combination of very high lift, very high spring pressure and astronomical rpm limits, none of which is ever remotely seen in Corvair flight engines.
PRO: Dan Weseman, Florida, 400 hrs on 3,100cc Cleanex , 125 hrs on 3,000 cc Panther.
Above, Dan Weseman and I stand in our front yard. This was the first run of the Panther’s engine.
I spoke with Panther designer and builder Dan Weseman on the phone yesterday. When his Corvair was assembled we put it together with new rockers, which turned out to be Chinese ones. He is going to replace them before flying again. Dan said he is thinking about a set of H-S roller rockers. His engine was already built with longer studs, so all he needs are the rockers, a new set of pushrods, and perhaps doubling up valve cover gaskets. Dan was a hard core hot rodding guy before getting into planes, mostly working with small block Fords in Mustangs. He has had many sets of roller rockers and is pretty confident that he isn’t going to have a reliability issue. He points to our experience and that of Langford. It isn’t a guarantee, but he finds it it be a good indicator. He is well aware of the life-span limitations on roller rockers at very high loads, but judges that operating them on flight engines are well below this threshold.
CON: Woody Harris, California, 440 hours on 2,850cc 601XL
Woody Harris, above left, and his friend Steve celebrate with cigars and Piper Heidsieck champagne after the first flight of his 601.
I spoke with Woody today. He is the guy who just broke the exhaust rocker on 9 August. He is going to change all his all his rockers out before flying again. He took a moment to amend my notes saying he had 160 hours on his rockers; after that first guess, he checked his records and found out that he actually had 350 hours on the Chinese rockers. He strongly suspects that he got that far because before installing them he did a very careful job of meticulously de-burring all the surfaces in the ball area. I include this because if anyone suggests that the issue with Chinese rockers was improper installation, we can just put that to rest now. The issue with them is poor quality control in manufacturing, period.
Woody is pretty sure that he is heading back to GM original rockers, not roller rockers. Woody functions as our ‘man on the west coast.’ Many builders have met him this was and have had a glimpse of his racing background. Lots of people have had something to do with ‘race cars.’ On the other hand Woody has run a Ford GT-40 to the lap record at Brands Hatch and was McLaren’s rep in North America. He also has a lot of experience with roller rockers, and he isn’t going to put them in his plane. He ran roller rockers in very demanding situations and thought they required constant attention. He concedes that our application, doesn’t stress them anywhere near that far, but his point is that the original GM rockers have a very long history of working, and he simply wants to move back and tap into that reliability. Nothing wrong with the ball design, it is just a question of who made the parts. To Woody, roller rockers are an answer to a question that our application is not asking. Today he is just looking through his collection of used GM original rockers from core engines to find 12 in good shape. We additional spoke about looking at several different brands of grooved balls to see if they are made differently, but I pointed our that I have been using the ones from Clarks with GM rockers on all of our engines in the last 12-14 years, without issue.
“Tell me what to do.”: Obviously I make recommendations about how to build Corvair engines, but I always first try to lay out the background information. I am here to share what we know, not simply tell people what to do, and I thought this was an ideal question to highlight this on.
There will be plenty of people who chime in with no personal experience and tell others what to do about rockers. I try to be polite, but that kind of info doesn’t help anyone. Second there are people who will point out one other person’s experience, like Mark Langford’s 1,000 hours on roller rockers. Information like that doesn’t help either, because in many cases the person bringing it up doesn’t know many of the important difference in assembly or operation that may be a factor. It is my business to understand these, and I politely point out that many comments chimed in often miss details or are off the issue and outside the cause-effect-solution chain. Last let me point out that even one guy point out what has worked for him for 1,000 hours is just a good data point. To have the complete picture, one must have the global view, and include all data points (with their details and conditions) including all the parts that never broke. I am in a good position to provide that perspective on Corvair engines.
To me, the best solution for most builders is the path that Woody is taking, to go back to having GM original rockers on flight engines. These have a very long track record of working, they are very cheap, and they can be retrofitted in a few hours, end of story. However, there are a number of builders like Dan who will consider roller rockers, and for those builders I wanted to provide the pros and cons here, to have them make a far more informed choice. We have our own 3,000cc Corvair going together for our Wagabond, and I have both a set of Harland-Sharp rockers and plenty of GM ones. I would not be reluctant to fly it either way, but in the next weak or two I am going to give some consideration to which to do the final assembly with.
Below, some notes and photos from the archives:
Below, a picture from the Summit Racing website. There are different sets for 140 hp heads and another for 110/95 hp heads. You can not mix them because the splay angle of the valves in the heads are different. The ball design of the original rockes negates this, because the axis is free to float on a ball rocker and it is rigidly set by the trunion angle on a roller rocker. Most sets sold to car people are the 140 hp sets, the difference is so fine that it can’t be seen holding it in your hand. Keep this in mind before buying a cheap used set off ebay.
Rocker Arms, Stud, Full Roller, 1.58 Ratio, Aluminum, Orange Anodized, Chevy, 2.7L, Set of 12
Part Number: CSP-SC110
Above, a 2005 photo of an engine we built in our old Edgewater hangar, sitting on the old dynamometer, showing the roller rockers. We built about eight engines like this. All the pushrods we used came from the Smith Brothers on the west coast. Every engine with roller rockers requires non-stock length pushrods to have correct valve geometry. It is not tough to measure, but we met many builders who guessed wrong on their first try. Old 3,100 engines all required custom length pushrods, and this was an Achilles heel of the engine for first time builders. We eliminated the custom length pushrod issue when we went to the 3,000 cc engines six years ago.
Above, a 2003 photo of the 2,700 engine we assembled and flew in our 601XL. This was the first Corvair powered plane to fly roller rockers. The longer studs required by these rockers, and their poly-locks (the locking nut system for a roller rocker) dictate deeper valve covers than stock. Traditionally, car people used heavy cast aluminum valve covers. Above is my solution: I milled away the center flat portion of the valve cover, folded up two boxes which were 3/8″ deep, out of .020 steel. I welded these on in place of the removed flat spot. This was not a particularly easy weld bead.
Here is the modified valve cover installed, above. Also visible in this shot is titanium-ceramic exhaust coated by the Moore brothers, a famous shop which does STC’d coating on aircraft parts. This design and method was superseded by all of our 304 stainless steel exhaust systems