Jason's Firefly .46 Build

Ian, there are no ball races in this engine.

You can ream the crank pin hole using a hand reamer and take advantage of the reamers taper so that the pin will start to enter the hole but is then pressed into an ever tightening tapered hole which helps keep it lined up

Hi Jason

A couple of people have mentioned fitting ball races as an improvement, wondered if it is worth while?

Would you have the glow plug type handy, never had any experience with model aero engines, but it seems that the glow plugs react similarly to spark plugs in a tuned car engine.

Ian

Ian

An OS # 8 would be a good starting point. I use them in most of my 2-stroke aero engines.

Peter

Yes OS #8 was what I ran mine with.

As to bearings, I suppose it depends on what you want to do with the engine. For me just the odd bench run is all it will get so the bronze ones are fine. A bit of gentle flying will also be OK with teh bronze it is only if you want to push it harder that ballraces may be worth thinking about.

Thanks Peter and Jason

Solid bearing it is then, always good to have advice, just been checking my taps, gues which one I don’t have for the plug lol.

Make sure you get 1/4x32 UNEF not ME

Thanks Jason, just noticed the two different threads whilst looking for the best supplier.

I am now making a second Firefly learning from the mistakes on the first and adding cooling fins to the head and beam mounting.

On the first one I used a commercial prop driver but this time I want to make my own.

Two questions.

How do I drill a hole with a flat. Jasons excellent thread does not mention this part.

Would I be better making a tapered split collet to secure the driver on the prop shaft?

Secondly how do I mill the radiused grooves on the driver face?

Peter

Peter, this came up the other day in this thread if you follow my link in that thread you will also see one simple way to do the serrated face.

Peter,

If you don't want to make a stepped broaching tool you could make a keyed driver instead.

I made one for my firefly. you would of course have to mill a keyway slot in the crankshaft.

If you go back to page 5 in this thread you will see the driver and crankshaft with the key fitted in my parts photograph.

Geoff

Jason

Thanks. I missed that thread. The use of a vertical knurling tool is a great idea. I have previously made prop drivers for larger engines for friends by tilting the mill head at 45 deg and just making grooves across the face but I felt this needed proper radial knurling.

However not having an arbor press I am not keen on using my mill or Myford in that role.

Geoff.

Milling the keyway in the crankshaft is not a problem. Thinking about it I have made internal keyways as required on the prop driver by drilling the correct size hole on the joint line of the crankshaft hole and a temporary plug. Then it is just a matter of squaring the hole with careful filing.

Meanwhile I am still contemplating a split tapered collet as was made in the RCME build article.

Peter

Jason

Just tried the knurling technique with an old rusty tool I had in my scrap box./Worked like a charm. Thanks

Peter

Peter, I'm not sure about your arbor press comment, I just did mine in the bench vice with an odd 1/2" drive socket as a spacer

Aaaaah. Misunderstood the picture. I thought it was vertical not horizontal.

After my breakthrough with the old knurling tool i will give your broaching method a try. Thanks again.

Peter

A question about the port dimensions, the cylinder block exhaust port is shown as 10.5mm x 5.5mm, whilst the cylinder liner port is shown as 15.285mm x 4.96mm.

How critical are these dimension as the are shown to three places of decimals?

I would have thought that the liner would have been the important dimension for the exhaust, is it to do with back pressure as in a car engine?

As I mentioned before, I have not had any experience with 2 stroke engines.

The three decimal places of the exhaust port are more a product of the CAD package when it rolls the liner out into a flat plane, the 14.35mm is what you will actually machine.

Width wise you have a bit more leway but the heights of the ports are what really set the timing of the engine so they are what you should aim to get as close as possible.

The crank case hole is a little larger than the port in both directions so it does not offer any resistance to gas flow and is less critical than the liner.

Hi Jason

Thanks for your prompt reply, appreciated, so the holes on the liner are the critical ones, and should not be restricted in diameter by the holes in the block when it is fitted?

The 10.5mm exhaust hole was the one that was confusing me as it seems to restrict the port in the liner.

Just measured the plan, and I can now see that the 14.35mm is due to the curvature of the cylinder, didn’t want to bu**er up the block on the last cut.

Ian

That's it liner is the one to get right particularly height.

The 10.5 and 14.35 are basically one and the same dimension, if you were to project two vertical lines upwards on the drawing that shows the 14.35 the distance apart would be about 10.2 as the 45degree cut is the long edge of a 10.5 x 10.5 triangle with a bit to spare.

Edited By JasonB on 06/01/2019 07:08:57

Another question, do you use any gaskets or sealants, for the cylinder head, engine backplate or block to cylinder?

I didn't but if you feel the need then a smear of a liquid gasket would be the best bet to keep backplate clearance and compression ratio the same.