A Titan

[Jasonb]

I suppose you could consider a remote needle which would mean you just need the spraybar in the venturi, this would allow you to keep the diameter down and the remote needle valve will keep your fingers away from the prop Works well on the firefly

I also wonder if the spraybar needs to go all the way through, many glow and 4-stroke engine carbs have the bar stop in the middle of the venturi so the boss on the other side could be filed flush. Your venturi is no different to a carb when the barrel is fully open. Something like this, the dia between the hex and small end is theaded to suit your boss.

You may also want to look at the spraybar on those 3cc glow engine drawings I sent you the other day, that would also work

[Jo]

Thanks Jason 

The Piston rod was recycled from one of my bits of scrap satellite  Having been taken to size I used a Wobbler to find the centre, then drilled and reamed the two holes on the mill.

I then got side tracked by the phone and mistakenly tapered the sides before I had taken the two sides down so the rod had to be clamped to the mill bed and the offending metal removed  The ends were then partially rounded on the mill before mounting the rod up on a mandrel on the lathe and turning the four faces.

It was when I tried fitting the rod on the crank I found that one of those little dimension changes between the drawings and the article caused the piston to hit the crank  So this needed the difference between the two sets of drawings removed from the bottom of the piston, I hope it doesn't impact the timing 

The gudgeon pin was then bored out and two aluminium rivets turned up and fitted, one into each end of the gudgeon pin so that the pin cannot rub the liner.

So putting it all together: with the connecting rod at BDC the piston no longer hits the crank but  am I meant to be able to see fresh air across the exhaust and inlet ports 

Jo

[Roger B]

 am I meant to be able to see fresh air across the exhaust and inlet ports 

Jo

If you mean the transfer ports yes. Around BDC the transfer ports and the exhaust ports are both open. The magic of gasflow means that the new mixture from the crankcase flows up into the cylinder whilst the exhaust flows down and out through the exhaust ports.

[Allen Smithee]

It was when I tried fitting the rod on the crank I found that one of those little dimension changes between the drawings and the article caused the piston to hit the crank  So this needed the difference between the two sets of drawings removed from the bottom of the piston, I hope it doesn't impact the timing 

It's not unusual in this type of engine for the front lower edge of the piston skirt to have a crescent-shaped portion cut away to clear the crank web, but shortening the whole piston is another approach. It shouldn't affect the timing as that's all done by way of the top edge of the piston on the ports.

Actually it could do - that's just triggered a memory from the 70s! If you have so much removed that the bottom of the skirt comes above the exhaust port as the piston gets to the top you open the crankcase to the air at a time when it has low pressure inside, and fresh air is drawn in. This can augment the power and was sometimes designed-in (it was called "sub-piston induction"), but it only works where the engine is run without any form of silencer - if there's a silencer then it can only drawn exhaust gas in, and that does nothing for you. When I was a teenager flying C/L combat (in the year after Richie Wilkins and Kevin Linsey turned an all-diesel event into an all-glow one) I boosted the power of my Super Tigre and Rossi 15s by taking the pistons into school and using their lathes to shorten the bottom of the pistons to give about 20 thou of sub-piston induction, which was worth over a thousand revs on a Taipan 7x4. What amazes me now is that I did this by clamping the alloy piston in a 3-jaw chuck (no one had told us kids anything about collets) yet I never seem to have distorted one because they all ran a treat afterwards!

AS

[Allen Smithee]

The design of that crankshaft surprises me. On all the engines I've been involved with the back face of the crank disk is flat and the crankpin is centred in the bore so that the con rod runs centrally with the big and little-end bushes positioned centrally over the rod. That "offset" rod looks like it would be prone to bending at high revs.

If you want a needle/jet assembly to get you going while you're deciding what to do you're welcome to a stock MVVS or Irvine one (I have loads in my spares boxes). These would both be one-sided units with a "banjo" fuel inlet and an open-ended jet that simply needs to protrude into the venturi by about 25% (truth be told they work fine even if they are flush with the side of the venturi). If you want one let me know and I'll drop one over to you. They're sized for racing 40s (6.5cc) so they should be fine for a low-powered 60.

PDR

[Jo]

Actually it could do - that's just triggered a memory from the 70s! If you have so much removed that the bottom of the skirt comes above the exhaust port as the piston gets to the top you open the crankcase to the air at a time when it has low pressure inside, and fresh air is drawn in. This can augment the power and was sometimes designed-in (it was called "sub-piston induction"), but it only works where the engine is run without any form of silencer - if there's a silencer then it can only drawn exhaust gas in, and that does nothing for you.

  I was concerned about that: it is touch and go if the bottom of the piston is going to clear the exhaust port, until I fit the main bearing and put the crank into place it is a close thing. There is no silencer 

Fitting that main bearing might be fun it is another of those heat the casting to 300 degrees, freeze the bearing and start praying 

Jo

[Jasonb]

I see that the article shows a twin tapered venturi insert, if you are fitting this make sure you allow for it on any spraybar you make.

Looks like there was not much of a conflict between the parts, I wonder if when the piston is in place the crank web would actually have gone up inside the hollowed out piston bottom. Think I may have just skimmed the dia of the web down if it needed clearance, maybe a 45deg chamfer would have been enough.

[Ramon Wilson]

Jo - a few things

The first of Jason's images show what used to be termed as a fuel post inlet. Of all the spray bar arrangements this is probably the most efficient. I have converted a couple of aerobatic engines to this system in the past and the improvement in consistency of run is very noticeable. However for your engine a straight forward conventional needle valve assembly will be just fine.

AS is right in that you may have introduced some sub piston induction. It depends on how much, as too much and you may lose crankcase pressure.
I have just checked the drawings and, taking the middle of all the tolerances, there is no SPI on the original. How much have you removed from the piston skirt? If you can tell me the height of the piston (not including the baffle) I can see if you have any SPI

Venturi inserts are usually used to 'tame' the engine a tad. More air - more rev/power to a point. As drawn with a .375 inlet and using a conventional bar you should not need to make an insert - IMHO.

Something I note on the drawings is that the inlet hole in the crankshaft is not dimensioned width-wise. (Nor a diameter come to that) I have used a hole of .375 diameter set at 45 degress to the crank for the timing calc. If you mill a flat across to break through to create the inlet it is the edges of the flat that define the timing any wider than .375 is going to give you too much inlet period.

If you do have too much SPI and have to make a new piston I would compromise a little by reducing the crank web diameter a bit and scallop the piston as AS suggests.

Finally, personally, I would Loctite the crankshaft bearing in - if you don't get that bearing in exactly when heat shrinking - specially with ally - it will mean having to bore it out again - once again yer pays yer money

Regards - T

[Ramon Wilson]

I have just checked the drawings and, taking the middle of all the tolerances, there is no SPI on the original. How much have you removed from the piston skirt? If you can tell me the height of the piston (not including the baffle) I can see if you have any SPI

'Avast there shipmate - belay that last order'

Jo - I was trying to do that for you just before I had to run Sue to work. In my hurry I forgot to put in the exhaust height in the calc. As drawn - using the middle of the tolerances there is 34 degrees of Sub Piston Induction - depending on how much you took off I'm hating to say it but I think you are going to have to make a new one

Sorry 

T

[Jasonb]

What conrod length are you using Ramon and Jo what length did you use?

[Ramon Wilson]

Hi Jason - the amended one on the drawing you sent me. I did wonder who amended it and as to why.

To be truthful I feel the tolerances of the port positions on the liner to be far too big. Couple them with tolerances on the build up and the timing could be way off one end of the dimensions to the other. That's why I went for the middle.

I have to dash out now but will be back this afternoon - don't despair Jo the very least you'l have to make a new piston   

Regards - Ramon

[Jo]

41.91mm between centres on the rod. I will have to wait for this evening to be able to measure the piston to find out its length.   

I don't like the sound of having to make another piston, I had to do that with the Stentor The bottom of the piston hardly comes above the bottom of the exhaust port if at all  .

Jo

[Jo]

  The distance between the centre of the gudgeon pin to the bottom of the piston = 12.4mm

I will be fitting the crank bearing shortly then I can tell you if there is any gap below the piston in the exhaust port and how big it is  .

Jo

[Ramon Wilson]

41.91 - 1.650 is the dimension used Jason

Jo - the total height of the piston is required - from the base to the top edge not including the baffle.

Drawing shows .875 - 22.22mm

As drawn there should be a gap under the piston at TDC - just depends on how big it is 

T

[Jo]

Ok the bearing is now in and I need to wait for the Loctite to go off... in the meantime this is the gap under the piston in the exhaust port:

It measures with the feeler gauges as 0.3mm or 12thou, which is about what I took off the bottom of the piston  I could put a 0.4mm gasket/shim under the fins that would raise the port and make the gap under the bottom fin nearer the width of the other fin gaps 

I'll go and fish the piston out and have another measure after dinner 

Jo