Re: q-1 with rotax 503

Isaksson Roger <scratchdeeper@...>

Correct, its about 1 3/4 inch wider, they come in different shaft diameters.

If you are unsure of what to use, sort them out first, by horisontal or vertical, shaft, go with the horisontal shaft ( well that was easy)

Take a magnet and touch the engine casing, it shoud not be magnetic, if it is, you're looking at a cast iron anchor. Get the aluminun casing only.

To be absolutely sure what you have, pop a head and measure the bore and stroke.

Measure the Diameter, and divide it in 2. ( This is the same thing as the Radius)

Measure the stroke from the cyliners top position to it's bottom position.

Ok toss in the numbers in this formula .

R X R X 3.14 x stroke x 2.

With other words.

Radius times Radius, times 3.14 (Pi), times stroke, times 2 ( number of cylinders).

If you get 48 cu inch or close to it, you have the smaller Onan. Same, if you get 60 or thereabout, you have the bigger Onan you want.

Shave the flywheel off in the same manners as the original was done. Here is where a lot of weight is shaved.

A counter weight is needed on the propflange, check old litterture on how it looked and where it was located. Probably some trial and error here until you get it right.

For power, there are a few fairly easy things to do here.

Compression, shave the heads, but be very conservative, dont go overboard, but do just a touch, as this is a flat head, and a lot of pockets crannies and cavities are present, that can produce "knock". I would probably dont go more than 8 to 1 in compression ratio, but would rather stay happy with 7,75 to 1 in comp ratio.

Porting, the ports are in the engie block ( as this is a flat head), they were designed by a drunk plumber, and is almost a 90 degree knee turn.

They can be ported very nicely for much better flow. Do some hours with a Dremel tool.

Top of piston and inside the head, you can apply a thermal coating, that will trap as much of the heat as possible, increasing the efficiancy a few ( but important) percent.

Regrind the cam, however, the camlobes backside are almost flush with the camshaft, you must put the camshaft in the lathe and get a slightly slimmer shaft before you go to the camgrinder. Ask for a modern profile where you get the power around 2500 and 3500 RPM, give him the data that this is a flat head as cams do different things to an over head and a flat head engine.

You should conservatively get 26-27 HP with this set up, maybe more if you're lucky.

Prop , you can go through a lot of props especially to determine a stall speed for a specific engine that is an unknown. I would get a slightly longer prop, that you can cut, if needed, and adjustable. These are the composite props. IVO prop can be cut in a band saw.

The original prop, on the Onan that came with the kit, and engine assembly was carved to stall at around 3000 RPM static, on the ground.

I would get as large diameter prop as your nerves will allowe you to install, and adjust it with the center adjuster, so your Onan will stall at 3000 RPM. That is much cheaper than for an unknown engine go trough a number of fixed wooden prop.

Also if you happen to be in a high altitude, and dont have all the power avaliable, and cant get the prop up on the power band, and have a potential dangerous take off, you can just adjust the prop to get up on the 'rpm and get going.

If you want to develop on this, you can also have the pilot cockpit adjustable electric gizmo installed, but it's not necessary for now.

The bolt on pattern in the back of the 60 Cu Inch Onan is the same as the 48 Cu Inch Onan and should be installed per plans.

Some sag have been known to happen on that engie installation, I would still do it per the Onan original installation plan, except do some extra layer of fiberglass where the drill hole into the firewall is to be.

There was a plate sold from the kit manufacturer, that was to be bolted onto the engine, the plate itself was to be bolted onto the firewall fasteners that goes through the firewall.

That plate was a kevlar plate, very strong, and light weight, Dont know today how to get hold of one of those, but they were manufactured once, so I guess they can still be made, if you know how to do them. Possibly you can go on the aftermarket and try to get one of those.

Weight difference between a 60 and a 48 Onan is not much, size difference is about 1.3/4 wider.shoud not be too hard to overcome, very close to the original plans.

One weight saving that can be done, if you have a friend that have a machine shop, ask him to do the Onan front plate, in aluminun rather than the cast iron front plate. You find those cast iron front plates on both aluminun and cast iron engines.

Get an electronic ignition kit, if you get an engine with points. They are sold as after market at Onan dealers.

Onan have a very weird way of firing the ignition, it has through the back of the engine, a little rod pushing up and down, actuating either the points, or the electronic sensor. I dont like it myself, but they have a very long track record as an industriaol engine.

Good luck

I don't have a picture but I assume it is slightly larger than the 48 cube
Onan. There should be identifying numbers on the engines. If I remember
correctly "B48M" identifies the smaller engine.

Dennis of Georgia
once upon a time: Dennis of Homestead Florida

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