Poppin Help

[student_Machinist]

Hi guys
I started a poppin engine quite few months a go but got discouraged when thing didnt quite go my way (quite a common thing for me to do unfortunately, now i have plenty of half finished projects :)
But i figured it was about time that i got started on it again and tried to fix the problem.
This is where im at at the moment

I need some suggestions with the piston and cylinder as no matter what i do it alway gets stuck.
When i clean it out with rubbing alcohol it has a really good almost frictionless airtight fit, but over night the piston gets stuck. i figure this was probably the piston/cylinder rusting so i then put some light oil in the cylinder and turned it over which freed it up. consequently after turning it over for a while the piston gets really stiff.
I dont seem to be able to win either way ???

any suggestions?

 

[mklotz]

What materials did you use for the piston and cylinder?

Is it possible that you have so little clearance that a slight expansion of the piston or, more likely, a slight contraction of the cylinder is causing binding?

Try heating the cylinder quickly (so the piston doesn't get too warm) with an alcohol lamp or butane barbecue grill lighter. If, immediately after heating, the piston moves freely, you may have found your problem.

Alternatively, cool the cylinder with some ice and see if the piston moves less freely.

Neither test is destructive and can be done quickly.

 

[student_Machinist]

they are cast iron as recommended. thanks for the reply I just tried your suggestion but found no discernible change.
Im wondering if it could be that around the middle of the piston is an area that just missed honing leaving a band around the middle that is ever so slightly lower that the edges

 

[Maryak]

SM,

You may be right, it's very easy to bell the ends when honing, especially if the hone/lap is less than 3, preferably 4, times the cylinder length.

Hope this helps

Best Regards
Bob

 

[student_Machinist]

Yea Im still not quite sure why it gets stuck though. could it be due to the oil thickening and geting stuck in the slight band around the cylinder? If this was the case would sticking it back in the lathe and turning a small amount out of the middle help?

 

[rleete]

Are you sure it's the piston and not the crank binding?


Edit: Looks like you're mostly done, and a fine job of it. Ought to finish it up and get a runner, which will go a long way towards keeping you focused.

 

[NickG]

Good point rleete,

you might want to try disconnecting it from the crank and just pushing it up and down to see if it has the same effect which will at least eliminate another cause of the binding.

I had problems when trying to lap / hone too with my Jan Ridders engine, in the end I reamed the bore with an adjustable reamer which ensured it was parallel and round and I turned the piston with a very fine feed and left it pretty much as it was from the lathe so as not to change the shape. A few seconds with some fine grit paper on a stick was all I did.

For my poppin, I went straight for a graphite piston and glad I did - it ran on the very first attempt and needs no lubrication. If i get around to it, I'll make a graphite piston for my other flame licker too.

Nice work, quite a few bits and pieces to do but sort the binding then get on with the rest and it will run. Poppin seems a lot more forgiving than the other I did.

Nick

 

[fcheslop]

Just a question are you getting any scuff marks on the piston either in the middle or to oneside
best wishes Frazer

 

[student_Machinist]

Its definatly not the crank binding coz it spins for ages with out the piston and cylinder attached

Just a question are you getting any scuff marks on the piston either in the middle or to one side
best wishes Frazer

Yea there is a band around the middle of the piston that wasn't touched while lapping. Ill try to see if i can get a pic tomorrow if i can find time after uni

 

[tel]

Its definatly not the crank binding coz it spins for ages with out the piston and cylinder attachedYea there is a band around the middle of the piston that wasn't touched while lapping. Ill try to see if i can get a pic tomorrow if i can find time after uni

That might not be any indication - if the alignment between crank and piston is a bit off it will give you problems!

 

[fcheslop]

Just noticed that the barrel is only held on with two screws and wondering is it pulling up square and true to the frame when they are tightened?
best wishes Frazer

 

[IronHorse]

On my Poppin, I used a aluminum cylinder and piston. I rub a bit of powdered graphite on the piston and it runs for months before I have to add some more graphite. At first I used oil, but it just stuck the piston in the cylinder. I have a chunk of graphite and one day will try a piston made from it.


IronHorse

 

[student_Machinist]

Just noticed that the barrel is only held on with two screws and wondering is it pulling up square and true to the frame when they are tightened?
best wishes Frazer

I dont think thats a problem because once its cleaned out it turns perfectly with little to no binding
This is a pic of the piston (im not very happy with it atm)

I would realy like to try some graphite at some point but i have no idea where to get it from
I'll probably have to make another piston, what do you reckon the best way would be to bring it down to size before lapping?

 

[Longboy]

I would realy like to try some graphite at some point but i have no idea where to get it from
I'll probably have to make another piston, what do you reckon the best way would be to bring it down to size before lapping?

On my Stirlings I turn graphite rod pistons to size of bore and hand fit . This Aluminum piston in your photo.......I have made a friction fit arbor outta Delrin to recieve the open end of piston and once chucked up in the lathe, held some carbide paper, #400-500 grit to it and refit to bore. With graphite, #800 paper and polish with ordinary news or printer paper. Remember check often cuzz once undersize......... Dave http://www.graphitestore.com/items_list.asp/action/prod/prd_id/25/cat_id/22

 

[tel]

Any Auto Parts place should sell you a little puffer bottle of graphite, or failing that pull the guts out of an ordinary HB pencil

 

[NickG]

Is that an aluminium piston in a cast iron cylinder? If so, you'll have real trouble getting it to work. At least use 2 metals the same for cylinder and piston or use a graphite piston. Just turn the piston in the lathe and it will be parallel and round, just make sure you get a good finish on it. I didn't do any lapping on either of my flame lickers or my stirling.

Cheers

Nick

 

[student_Machinist]

Is that an aluminium piston in a cast iron cylinder? If so, you'll have real trouble getting it to work. At least use 2 metals the same for cylinder and piston or use a graphite piston. Just turn the piston in the lathe and it will be parallel and round, just make sure you get a good finish on it. I didn't do any lapping on either of my flame lickers or my stirling.

Cylinder and piston are both cast iron. I had quite a bit of trouble getting the piston to the right size and with a good finish (went through two attempts before this one) What would you recommend in terms of the lathe tool shape and RPM for cast iron?

 

[fcheslop]

As Nick mentioned I don't lap the piston either just machine to a good finish using a standard knife tool but sharpen it after grinding on a fine oil stone and put a small rad on the tip and set it exactly to centre height and turn it at a reasonably fast spindle speed with the finest auto traverse speed selected a bit vague I'm afraid but i don't know the rpm of my lathe.When the piston is fitting correctly it should snap up the bore if you blank off the hole in the head and pull the piston then release it.Sorry but I'm not very good at explaining
best wishes Frazer

 

[90LX_Notch]

Here's a few ideas-

I would start trying to isolate the moving parts. Ie: Disconnect the piston from the rod. Ensure that it moves freely, then let it sit over night. Then if it is frozen you know it's an issue with the cylinder/piston. While it's disconnected, turn the crank over and watch the rod as it moves up and down. If the crank pin is is not parallel to to crankshaft the rod will move in a V pattern. This would cause binding. Check that the bores on the connecting rod are parallel to one another. When the connecting rod and piston are assembled make sure that connecting rod is parallel to the piston outside wall. If they are not the Yoke face may not be parallel to 3/64 bore or the ID face of the piston may not perpendicular to the piston wall.

Another thing you could do is place a degree wheel on the crank with a pointer and and see if there is any relationship to where the freezing occurs.

I hope this makes sense.

Bob

 

[NickG]

I did as Frazer said for the tool, I always use HSS tools, I find they give me a better finish. Cast iron should be turned pretty slowly actually, it's quite a small diameter we're talking about though. I'm lucky as I can get right down to 34 rpm in my lathe - that's for really big stuff though, I think for the pistons I used the slowest non-back gear speed which was about 200rpm I think. As Frazer says, slowest feed - again I'm quite lucky as my lathe has a screw cutting gearbox and at the finest feed you can hardly see the carriage moving.

Sorry if you've already said but how did you make your bore? I struggled the first couple of times then used a method that Jan Ridders suggested, using an adjustable reamer after boring to keep it round rather than trying to lap and bringing the unknown into it.

Bob's trouble shooting is logical, I'd do this before going any further if I were you. If any of the rod is out of alignment though, just enlarge the holes a touch - people get hung up on having close fits, which is all well and good but if you can't hold the geometric tolerances it will all just bind up. Give it a bit of clearance so it can move a bit and find it's own place. As long as there isn't a massive amount of play in it, it won't affect anything, it will just reduce friction and stop the binding, unless of course it's way out.

That is the trickiest thing about 'poppin', some of the clearances on the drawing are very small - I found myself looking at things thinking, "that's not critical" but then thinking more about them "oh yes it is because it will foul on this, that or the other" !

Keep plugging away, you'll get there.

Nick