Getting Started in 2013, Part #12, Piston Choices

Builders;

Here is a look at some notes on the 1300 group. I left out the 1302 section on rods because I am revising it now. For right now this is a good overview on piston choices.

Engine kit notes:

Builders purchasing a 2,850 or 3,000 cc large bore kit from us receive the new forged pistons, the wrist pins, Total Seal piston rings, new cylinders, head gaskets (2,850s get base gaskets also, 3,000 cc engines don’t use them), and professionally rebuilt connecting rods with ARP rod bolts with 12 point nuts. The price of the 3,000 cc kit includes the machine work to the case and heads to accept the larger bores. Builders selecting either of these options will be fulfilling the contents of both the Piston and rod group (1300) and the Cylinder group (1400).

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Piston and rod group (1300)

1300- Piston set with wrist pins

1301- Ring set

1302- Connecting rods  -6-

 

Commentary:

In January of 2011, when I still was a part of online discussion groups, I sat down over four nights and wrote the history of pistons and cylinders on Corvair flight engines. It included the who, what, where, when and why of the whole spectrum of possibilities. It was a four-part story that covered nearly 18,000 words. I looked at it before writing this section of the Manual. Reading it now makes me reconsider things. The life of a great person is worthy of such documentation, but is the history of pistons and cylinders on Corvairs a valid subject at this length? Who was I writing it for? Was it to convince skeptics that I really am “The Corvair Authority”? The people who are focused on the Internet discussion liked it, but I doubt it needed that length for people focused on building. I am writing this for people who want to know how to build an engine today, to use it to fly, and a much shorter piece is afforded by this focus. I am just going to cover the pistons we use today, have a short section on what not to use. Please realize that there is no combination that you are likely to hear of that we don’t know about. We probably test ran it, and may have flown it. If it isn’t in here, there is a reason. If you ignore that reason, the odds of your plane ever getting finished just got smaller.

1300- Piston set with wrist pins

There are three displacements of Corvairs that we build today. They are the 2,700 cc series, the 2,850 cc and the 3,000 cc engines. There are two good piston options for the 2700 and they both come in 3.437” standard bore, .020”, .030”, .040” and .060”. The pistons for the 2,850 and 3,000 cc are available from us; they come as part of Piston/ring /cylinder kits, but we will discuss them as a separate part here.

First, a quick mention of all the pistons you should never use in a flight motor: Never use a cast piston of any kind. This includes old GM pistons, the ones sold by Clark’s as “high-tech,” any piston that is labeled as Hypereutectic, and the vast majority of pistons intended for VW applications. All of these cast pistons have flown in Corvairs before, but we have been telling people for 20 years never to fly any kind of cast piston because they are not tolerant of any kind of mistake in engine assembly, tuning nor operation. Their life span in an engine that is detonating hard is about 15 seconds. This will also hurt forged pistons, but they will not break or get holes in the piston tops. The second type of piston never to fly is a forged one made by a company focused on making light weight, high rpm pistons with thin tops. Pistons designed to make the most power at 7,500 RPM are different from ones designed to be ultra-reliable in 4,000 RPM engines.

Traditionally, the most popular Corvair flight piston for a 2,700 cc engine was referred to as the “TRW” piston.  In the past 10 years, a corporate name change on the box led most people to call these “Sealed Power” pistons. Until 2009, these were made in the United States and the quality was very good. About eight years ago, the pistons began coming with a greenish gray anti-scuff coating on the skirts. Approximately 90% of the Corvairs now flying have these pistons in their bores. They were available in standard, .020, .030, .040, and .060. Over the years, I have built more than 100 engines using these pistons, almost all of them .030 and .060. They work very well in flight engines and have an excellent reputation for staying together even in engines that are being detonated heavily. I have never seen a hole punched in the top of one of them.

In 2009 the wonderful world of outsourcing had the manufacturing of these pistons moved to India. The corporate ownership was so proud of this change that they announced it by keeping the box exactly like it was with the sole exception of printing the words “Made in India” on the end of the box in a print size usually reserved for credit card contracts. The pistons manufactured in India look exactly like their U.S. counterparts. There is nothing wrong with them; however there is clear, irrefutable documentation that the manufacturing tolerances of the Indian built pistons are significantly looser than their U.S. built predecessors. I would not seriously worry about this, but it is a reality. These pistons are available from many different sources, including Clark’s.  

1301- Ring set

The kits we sell for the 2,850 and 3,000 cc engines include Total Seal rings. These rings are carefully matched to the bore finish of the cylinders that come with the kits. These rings feature the highest quality chrome steel, from a company known for a quality product. The brand it Total seal, which people thing is ‘gapless’ which is not so. Total seal bakes both gapped and gapless rings, we use the former.

For builders working on a 2,700 cc engine, there are numerous choices when it comes to rings. Over the years we have tried them all. I feel that the best combination for a Corvair flight engine is the Hastings chrome ring mated to a 220 wall finish on the cylinder. There are also moly rings available for the Corvair, and the appropriate finish for these rings is 280. We have built a number of motors around moly rings but I don’t see any particular advantage in our application.  With proper break-in, regular oil changes, and breathing filtered air, the lifespan of chrome rings should easily be 1,500 hours. Moly rings would theoretically last longer, but they are far more difficult to break in and they are less forgiving of the wall finish. We have also flown a number of engines on regular cast iron rings. These break-in literally in minutes and they are surprisingly tough. Their Achilles’ heel is any kind of detonation. Even a few moments of knocking will often crack a top ring if it is only made of cast iron. Although their low price makes them tempting, stick with chrome rings.

The rings that go in your engine must be the same size as the pistons. For example, .030” over bore pistons require a .030” over ring set. Corvair piston rings are expensive by automotive standards. If you shop around you can often save a significant amount of money.