Senft "Poppin" engine

[Brian Rupnow]

Some very delicate work indeed going on here. This is the arm that operates the valve on the front of the cylinder. The arm is 1/16" thick material. The roller that you can see is 5/16" diameter. The axle inside that you can't see is .175" diameter with a .062 spigot that goes thru the arm.--And the roller has to turn freely after it is all assembled. The plans call for "peening" the head of the 1/16" part which extends thru the arm. I have never had a whole lot of luck with peening one part of an assembly while still leaving the other part free to rotate. I wimped out and silver soldered the projecting 1/16" spigot.---And my roller does spin freely!!

[Brian Rupnow]

Nice little crankshaft. That is interesting observation Brian- what is the source of measured runout @ 0,016"? Assuming the rod is straight there should not be much more runout than spindle runout multiplied by lenght of rod at measurement point

I like your cylinder btw 

Perry--I never know where the reason lies for a bit of misalignment on little shafts this small. They tweak so easily that even when pushing them thru a pair of reamed holes that are not press fits they will move around a little. The only good thing about it is that it takes very little effort to realign them after the Loctite has dried and the center portion cut away.

[mklotz]

Some very delicate work indeed going on here. This is the arm that operates the valve on the front of the cylinder. The arm is 1/16" thick material. The roller that you can see is 5/16" diameter. The axle inside that you can't see is .175" diameter with a .062 spigot that goes thru the arm.--And the roller has to turn freely after it is all assembled. The plans call for "peening" the head of the 1/16" part which extends thru the arm. I have never had a whole lot of luck with peening one part of an assembly while still leaving the other part free to rotate.

One way to contollably peen thin shafts is to drill a shallow hole in the end of the shaft; then use a punch to slightly spread the shaft material and trap the roller, etc..  I've used this method successfully on several sub-miniature models I've built.

[NickG]

Yes, I was a bit apprehensive about that roller Brian, I can’t remember how I did it, but it worked. Think the pin was just left long enough to avoid nipping the roller. Anyway, you’ve got there another way so all is good. I was looking back at some videos of mine, when I first built it, thinks kept moving about - that arm is kept in place by the friction of something like a 6 or 8ba nut! There’s only so much you dare tighten those, could have done with a square filing or something to lock it. Then there was the 1/16” rod that goes into the rocker shaft - mine kept coming loose so needed dabs of loctite in the end!


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[Brian Rupnow]

Thank you Marv. Nick--I've been wondering about exactly the same thing in regards to holding the arm in place on the rocker shaft. At the other end of the valve rod where it screws into the cross shaft I would have used some loctite anyways, because there isn't room to get in there with a lock nut.

[Brian Rupnow]

I'm having to retool my shop just to build this engine. So far I've had to order a #2-56 die, a 0-80 die and a tap, and I've found that nobody in Barrie carries 0-80 hex nuts. I'm working below my comfort level, size-wise, but so far it's coming together well.

[Brian Rupnow]

Okay, I'd be the first one to agree, at first glance this looks uglier than original sin. Both brass pieces of the two piece big end have been carved out, one being silver soldered to a 3/32" diameter cold rolled steel rod (which I had to turn from 1/8" steel rod because I don't stock anything that small.) They are currently crazy-glued together and later today I will tap them while still crazy glued together, insert a full thread #2-56 bolt thru all, then drill and ream the hole for the con rod journal. Seems like a very strange way to make a con rod big end, and yet it doesn't require anymore pieces than a  "normal" con rod big end does. It does cut down quite a lot on the "swing radius" of the con rod assembled on the crankshaft.--It is setting on a bigger aluminum block covered with saran wrap so that I don't glue it to the big aluminum block with a couple of 0.015" thick shims under them to maintain alignment .

[Brian Rupnow]

Oh wow--Christmas just came at my house!!! The laps that I ordered the other day came in just now. No more screwing around with home made laps. This is very timely, because now I can finish lapping the Poppin cylinder before I make the graphite piston.

[Perry]

Hmm what a fine set 

[bent]

Still following along, Brian, nice work so far! 

Art, thanks for the reply - from the picture it looks like your bushing has a single radial slit?  Is it a hacksaw cut, and do you do it before or after machining the taper?  And apologies, Brian, for the continuing hijack. 

[mklotz]

That unusual big-end design saves drilling and tapping one hole.  That might matter in a production environment but hardly seems relevant in a miniature engine that barely has enough power to operate its own valve.

[b.lindsey]

Those laps look sweet Brian. Please let us know how they work.

Bill

[Brian Rupnow]

The con rod big end does look okay now that it is finished and fitted. I have never made one this way before, so it was a new experience. If you really, really need radial clearance around the crankshaft, then this is probably a good idea. I don't particularly care for this method, as your options are very limited for any kind of adjustment. At any rate, it seems to work, and I've managed to do something "just a little different".

[Brian Rupnow]

I used one of my new laps to lap the bore of the cylinder. I only used a single stage of lapping using 600 grit paste. This isn't perfect, but it's good enough for what it will be doing. To be perfect, I would have used grit in three successive stages, using #300 first, then stepped up to 400, then up to 600. There's a little story that goes with this. When I first started machining eight years ago, I bought a number of new, expensive reamers. They would ream a bore and leave it almost flawless inside. Over the years, the reamers have grown more and more dull, not leaving a perfect finish anymore, but that was okay--My 3 stone brake cylinder hone would clean up any imperfections prior to lapping. But--On a cylinder with a 5/8" bore, the hone will not fit into it. The finish after reaming did have scratches and flaws in it, and I had no good way to clean the bore up. I have been contemplating buying a set of expanding laps for a few years now, and this seemed to be the time. I will still probably shop for a smaller diameter hone, but for now the lap has cleaned things up sufficiently. Another factor in all of this is that on all of my i.c. engines, I have been using Viton rings, which are very forgiving of surface finish. If the surface finish isn't perfect, it will be after running for an hour.--The Viton rings will polish the inside of a cast iron cylinder to a mirror finish in about an hours running.-You might have to replace the ring after that first hours running, but Viton rings are cheap.

[Brian Rupnow]

Those laps look sweet Brian. Please let us know how they work.

Bill

They work great to take out minor flaws in the bore. Really, the bore should be honed before the laps are used. If your reamers or boring tool is very sharp , you might possibly skip the honing and go straight to lapping, but I recommend the honing before the lapping process.