Twin aero CI/diesel - some advice needed

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pat_pending

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Hi, after completing 5 single-cylinder compression ignition engines that run nicely. I thought its high time to get a twin under my belt. I had a look around and they seem pretty rare! I was looking at the FigTree Pocket twin (plans by Ron Chernich from the Motor Boys plan book) but haven'y made my mind up yet.

I was wondering if any of you folk could give me some pointers / alternatives to consider. Also, there seem to be very few build logs out there so I'm struggling to get my head around machining the crankshaft and making the 2-piece conrods. I understand eccentric turning and have made a bunch of crankshafts for singles but how to make two polished journals out of a single piece stumps me due the the difficulty of getting a finishing tool deep into a cut/polishing with emery paper etc. Any build logs that you know of would be greatly appreciated.

Thanks,
Patrick
 

Peter Twissell

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Hello Patrick,
I am building a glow ignition twin, to my own design.
I have avoided the issues of crankpin finishing and two piece rods, by adopting a built up crankshaft and using hard dowel pins for the big end journals.
The assembly will be pressed together.
 

pat_pending

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Hello Patrick,
I am building a glow ignition twin, to my own design.
I have avoided the issues of crankpin finishing and two piece rods, by adopting a built up crankshaft and using hard dowel pins for the big end journals.
The assembly will be pressed together.
Hey Pete thanks for the reply. That sounds like the way forward. Do you have any builds that show an example of doing this? I am worried about getting the press fit right and making sure everything remains true. Also id be nervous that the assembly falls apart at 13K RPM.

But I'm here to learn new skills!

PS: would love to follow your build if you are documenting it.

-Patrick
 

Peter Twissell

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I have based my pressed crank design on the numerous two stroke motorcycle engines which used pressed up cranks and run ridiculous amounts of power at silly rpm. The Yamaha RD250 comes to mind.
My build is being documented in the engines section, a 15cc opposed twin.
 

pat_pending

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Thanks Pete. I will be watching your build with keen interest especially when it comes to assembling the crankshafts.

PS: I love RD250/350s... scary to ride though. If you want to be impressed what someone can do with machines of that era using only modest model engineering equipment you HAVE to check out Allen Millyard if you haven't already :)

 

Jasonb

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I'd probably hold the roughed out crank in a square block oreven a collet block held in a 4-Jaw or angle plate fixe dto th efaceplate Turn the rear pin first then just rotate the block 180deg to do the front pin, this will keep throws the same and make sure the pins are 180deg apart
 

josodl1953

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Hi Patrick,
You might consider silver soldering a built-up crankshaft rather than pressing it together. I use round HSS toolbits for crankpins, they remain fairly hard after being heated up. Be sure to finish machine your crankshaft journals after soldering to make sure they are perfectly in line.

Jos
 

mole42

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I'd be wary of silver soldering - I don't think it's strong enough for a crank. I brazed my Gnome crankshaft following advice - I had proposed silver solder because I've been using that material for 50 years but engineer friends said it may not survive so use braze instead. The only problem with that is that the metal need to be a lot hotter!
 

Peter Twissell

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Part of the idea of the pressed up crank is to avoid split rods.
The pressed up cranks in 2 stroke motorcycle engines typically use roller big end bearings. The crank is pressed up with the rods in place.
 

pat_pending

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Thanks for all the advice. Im wary about silver soldering because of heat distortion and the need for cleanup/machining at the end. I think braising might be the same. The engine i'm thinking about is 5cc so the crank parts will be very small. Id' be worried braising as I might also melt something.

All the commercial glow crankshafts ive taken a look at seem pressed and also use split conrod ends (see below). I've never pressed before. I guess I need to do some reading up of fits in the engineering tables etc to get it right (and also making some sort of assembly jig). Any examples would be really really useful!


1637322973789.png
 

Peter Twissell

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Hi Pat,
The crank in your picture does not look like it is pressed up. If it was, I would expect to see the ends of the crankpins in the 'outside' faces of the cheeks.
Regarding tolerances, there is a general rule of thumb of 1 thou per inch for a press fit. Clearly, if your crankpin diameter is about 0.2 inch (5mm), you would be trying to achieve 2 tenths of a thou interference. When I make interference fits at this scale, I will do so by making the first part to nominal size (or perhaps using an off the shelf hard dowel pin etc.), then make an identical dummy part, but polished down to a suitable undersize and using that as a gauge, machining the second part to a light push fit on the gauge.
Others will be better capable of machining to tighter tolerances than I!
 

abby

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Silver soldering is very strong and certainly good enough for a model crank , I make my single throw cranks by this method and you would have to destroy one to get it apart.

Dan
 

Richard Hed

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Silver soldering is very strong and certainly good enough for a model crank , I make my single throw cranks by this method and you would have to destroy one to get it apart.

Dan
I have never silver soldered a crank but I silver soldered carbide bits to a holder then cut threads with immensely powerful counter pressures against the solder. It holds. If this will hold, certainly, I thimpfk, the cranks will hold as you say. On the tool, the solder is just sitting on the end. In the crank, the shaft is inside a hole. I don't see how this could possibly come loose. In fact, I would venture that the crank would break before it came loose.
 

Steamchick

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The silver solder would be in torsion, and at small diameters the shear forces are very low anyway, so should be no problem. The only issue (that I don't understand!) is how to get the correct alignment of shafts and have the require thou or 2 of CLEARANCE for the silver solder to use capilliary action to draw itself into the joint...?
I have only made press-fitted cranks (when I haven't made solid cranks!) and relied on the press-fit to hold the journal - as the steam models didn't develop a lot of torque...
But do use proprietary tables for interferrence fits, and carefully jig parts for pressing together. Preferrably, press to a shoulder, or other solid "stop" so it is "right first time - every time".
K2
 

abby

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Actually the idea that clearance is necessary is incorrect , the manufacturer of Easyflo (Johnson&Matthey) used to run an ad where a 1" dia bar was inserted into a 1" reamed hole in a steel plate then silver soldered. The assembly was then cut in half along the axis of the bar to show that the silver solder had penetrated the joint.
Cleanliness , good fluxing and correct heating will give good results.
Dan.
 

Makin chips

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The silver solder would be in torsion, and at small diameters the shear forces are very low anyway, so should be no problem. The only issue (that I don't understand!) is how to get the correct alignment of shafts and have the require thou or 2 of CLEARANCE for the silver solder to use capilliary action to draw itself into the joint...?
I have only made press-fitted cranks (when I haven't made solid cranks!) and relied on the press-fit to hold the journal - as the steam models didn't develop a lot of torque...
But do use proprietary tables for interferrence fits, and carefully jig parts for pressing together. Preferrably, press to a shoulder, or other solid "stop" so it is "right first time - every time".
K2
I would think wether you have clearance in the joints or not you would need a holding fixture to keep everything in line while heating and cooling.
just my $.02
 

Vietti

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Last built up crank I made I spot welded the assembly held true, with wire welder tacks, very small, then silver soldered. When cold I locktited pins thru the silver soldered joints. I've made quite a few silver soldered cranks and only had one come apart. It was in a Red Wing and I ran out of solder on the last joint and rationalized enough solder had penetrated. I like fairly tight joints for soldering as it makes holding the unsoldered assembly pretty true. I use a triangular file to file 6 or so notches around the inside of the webs to insure the solder penetrates.

I like the ease of making a built up crank and I can use drill rod for the shafts and pins, yields good steel journals, properly sized. My current project is a built up crank with a needle bearing on the rod journal. To avoid overheating the bearing I put deep chamfers in the webs and tapered the ends of the pin. I then wire welded them together, stopping when they started to get too hot. Going to be a fairly high compression engine so time will tell if it holds together. Pinned also.
 

methuselah1

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I've got a FTPT on the go, I've put it on a radial mount. I did meet up with a fellow builder who made one for marine use. He explained that he made his conrods to the drawing, but he had a problem with almost immediate failure, and that assembly was impossible without boring through the bottom of the case!

I forget his solution, but mine will include fitted steel bolts and loctite. If you can find a solution using one-piece rods, I'd do just that...

My own crank was turned from solid. The trick is to put three centre drilled holes in each end of stock and do the work between centres- the crankpin finishing tool is nothing special; just a little narrower, but with two or three little grooves cut into the nose. That way it can be wiggled from side to side, lowering the cutting forces. I always machine the crankpins before the conrods... It's easier that way if there's an error.
 

Richard Hed

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I've got a FTPT on the go, I've put it on a radial mount. I did meet up with a fellow builder who made one for marine use. He explained that he made his conrods to the drawing, but he had a problem with almost immediate failure, and that assembly was impossible without boring through the bottom of the case!

I forget his solution, but mine will include fitted steel bolts and loctite. If you can find a solution using one-piece rods, I'd do just that...

My own crank was turned from solid. The trick is to put three centre drilled holes in each end of stock and do the work between centres- the crankpin finishing tool is nothing special; just a little narrower, but with two or three little grooves cut into the nose. That way it can be wiggled from side to side, lowering the cutting forces. I always machine the crankpins before the conrods... It's easier that way if there's an error.
Do you have a photo?
 

Steamchick

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In discussion with a professor of engineering from the local university, he explained the "conventional" thinking on rod-ends was flawed. He had done some stress analysis - computer Finite Stress Analysis was then in its infancy in 1983 - and explained that most rod-end failures come because rods are designed to "look right" - which doesn't make them as strong as expected. Simply, you need twice as much end thickness as side thickness to resist distortion fatigue of the end (middle of an end cap) for durability. Similary the rod-side-leg failures are usually fatigue related as you need a lot more material than you expect. Without knowing the failure of your fellow builder's rod, this may not be relevant...?
K2
 

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