Reducing the weight of the Lycoming IO 540

Well I finally received the standard flywheel to replace the original hot prop flywheel. As you may recall that one weighed 8.86 pounds. The standard flywheel/ring gear assembly weighs 6.9 pounds. For a whopping weight savings of 1.96 pounds. Probably not worth the cost of buying a new one, but mine was a straight across exchange so there were only some shipping costs involved.

In a future installment I hope to have some additional weight saving tip centered around trading out the stock starter and alternator. Until then it's time for me to start hitting the cottage cheese and rice cakes.)

Lycoming Oil Sump Clearance on the RV 10 Engine Mount

A concern that comes up with different variations of the Lycoming IO 540 is will the oil sump clear the lower cross tube on the Van's Aircraft motor mount? The tube in question is pictured to the right and is the one that runs between where the two lower vibration mounts bolt on. This has become a big enough issue that Van's wrote a service letter on it: RV-10 Engine mount/Engine interference.

As you may recall the Lycoming IO 540 C4B5 I'm working with is from a Piper Aztec. The good news is there is no interference issues with this engine. The clearance was right at 3/8". I'm not sure if all C4B5 use the same oil sump, but the right engine on a 1974 Aztec works just fine:)

If there is interference there are two fixes I have heard about primarily through the Matronics RV 10 e group and these are: 1. get a different sump or 2. cut out the tube and weld in a new one that is bent to clear the oil sump. I'm just glad it was a non issue for me! If you are not familiar with the Matronics RV-10 Aircraft group check it out. It is a great resource: Matronics sign up

Lycoming IO 540 Ring Gear Assembly (flywheel)

Another item that is an optional change is the Ring Gear Assembly. Why would anyone want to change it? Well, the one that came with my engine was setup for a Hot Prop or deicing propeller. As you can see in the first picture the hot prop gear assembly has greater mass than the standard ring gear assembly shown in the second picture. In short it's a way to save some weight in the nose of the RV-10. The hot prop assembly weighs 8.86 pounds. Were as the standard ring gear assembly weighs 6.9 pounds.

A couple of things to keep in mind about this change out is to use the same tooth count on the starter ring gear that matches the starter being used. They come in 122 and 149 count. Also, you will probably want to stay with the same alternate gear pulley size. The standard Lycoming size is 9 3/4". This is the size recommend for use with Plane Power alternators.

While changes such as this are legal on homebuilt aircraft, it can change a certified engine to an experimental engine depending on the part used. Also, it's important to document changes such as this in you engine log book for later reference.

Manifold pressure tap on the Lycoming IO 540 C4B5

If you are planning on using an IO 540 out of a Piper Aztec keep in mind the manifold pressure tap is on the number 6 cylinder (this the left rear cylinder from the pilots view) whereas Van's has used a tap off of the number 5 cylinder (right rear.) I mention this as a heads up to make sure the old pressure tap off the number 6 cylinder gets properly plugged, otherwise that cylinder would run really lean if at all!



In the first picture you can see the number 6 cylinder with a new plug installed with fuel lube and torqued to the correct setting.










The second picture shows the newly installed manifold tap as used on the RV 10 aircraft.

Lycoming C4B5 Power Rating for the RV10

As we discussed earlier the IO540 C4B5 has a different power rating than the IO-540 D4A5 recommenced by Van's Aircraft. These are 250 HP at 2575 rpm for the C4B5 and 260 HP at 2700 rpm for the D4A5. Everything I have read points to the fact that they are the same basic engine and capable of running at either rpm/power rating. It's just Piper derated the C4B5 by running it at a lower rpm for the Aztec. If you would like to explore more about the different Lycoming 540 series aircraft engine try this link Lycoming 540 reference.

Lycoming engine mounting ears

To ready the Lycoming IO-540 C4B5 for the RV 10 the engine mounting ears must be changed. The C4B5 comes with the large diameter mounts. Where the Van's engine mount requires the small diameter ears. The accompanying picture is of a used example of the correct part. The required mounting ears are Lycoming's part # 70456. A none PMA'D experimental part is available from A.E.R.O. Here is a link to their website www.aeroinstock.com Also, they are available second hand or from Lycoming.

Changing the Propeller Governor

As we mentioned before there are some things that need to be changed to get the Lycoming IO-540 C4B5 to fit our use on the RV 10. One of these is the propeller governor. The original unit for the Piper Aztec won't work for several reasons. First it's too bulky to fit under the RV cowl, but more importantly being it is used on a twin engine aircraft it feathers the opposite direction from what we need.

In the picture you can see the difference between the original on the right and the MT governor on the left, which is available through Van's Aircraft.


Of course the fun doesn't stop here! As you can see in the photo the mounting flange on the MT unit is thicker. So the mounting studs will need to be replaced with the longer ones. These are available from Lycoming, however I purchased mine through ECI (Engine Components, Inc.) at about half the cost. Yes, they will deal with us builders. The ECI part number for the new studs is AEL31C16. If ECI sounds familiar it's because they make Titan engine cylinders and other aircraft engine parts. Their website is www.eci.aero.

Make sure you get you Lycoming engine manual out and install the new bolt studs according to the manual. If this is new to you ask your EAA technical conselor or and A&P. Engine cases are too expensive to mess-up!

Fitting a Lycoming IO540 C4B5

The prototype RV-10 aircraft used a Lycoming IO540 D4A5. The second factory plane used a Continental IO360. To save a few shekels we used a rebuilt variation on the aircraft engine the Lycoming IO540 C4B5 out of a Piper Aztec airplane. Of course there are some difference between the two engines such as:
Power rating

Engine mounting ears

Hot Prop Flywheel

Prop Governor

Prop Governor mounting studs

Manifold pressure source