Old School Barstock 2 Stroke

[Jasonb]

Once you have cut your ports part it off and then lap it.

Would not lap into a blind hole that's why I said part off first. Don't really have a problem with boring into a blind hole myself, just put a stop on the lathe bed do you don't get a dig in

[Roger B]

Still following along with interest  A couple of points:

I think that the oil retention grooves in the piston are rather too big, they will potentially feed your crankcase compression directly to the exhaust port.

I'm not sure about lapping the piston into the bore  I have lapped the bore as true as possible and then lapped the piston to fit. As this will be a spark ignition engine you will be looking at a compression ratio of 5 or 6 - 1 rather than 15 to 20 - 1 for a compression ignition engine so the fit is not so critical. My 25 mm bore horizontal engine would start quite happily without rings, but the aluminium piston expanded too much so I had to increase the clearance and add piston rings.

[Brian Rupnow]

Jason--if I cut it off after I cut the ports, then hold the liner in the 3 jaw for lapping it, won't the 3 jaw distort the thin shell and give a strange internal shape.?

[Jasonb]

Hold the lap in the lathe and the liner in your hand

[NickG]

I agree with Roger re lapping. I've made flame lickers and hot air engines where minimising leakage and friction is as, if not more important than it is for an ic engine. I've made one with cast iron cylinder and piston, one with chromium steel cylinder and  cast iron piston and 2 with cast iron cylinders and graphite pistons. All of the cylinders were reamed with an adjustable reamer and the pistons just a turned finish on the slowest feed.  I found this kept the cylinder bore and piston as square as possible - previous attempts at lapping had not been successful. I've also seen graphite pistons used successfully on ic engines which was very easy to machine and get a good fit.

All this said, if you use the technique described by Ramon for lapping you will probably end up with a superior fit to mine. Am just wondering whether all the work lapping is necessary though.

[Ramon Wilson]

I would agree with Roger here - lapping the piston to the bore is a testing point of the operation. Though I have done it on all my diesel engines I wouldn't say it was exactly necessary for spark ignition. Yes it's a more certain way of getting a virtual fit without recourse to some pretty esoteric measuring kit but lapping the piston such that it enters the bore and moves up to the top of the liner with a slight push would be fine.

I also concur with Jason on the diamond paste. If you do use it you need to clean the part in an ultra sonic cleaner. The diamond will embed as said and quickly lap the piston in - and out again.
Silicon carbide powder is all that's needed but don't go any coarser than 320 grit to begin with - it can quickly grip and bind solid

There should be no need for the piston grooves - and could lead as Roger suggests to crankcase compression losses - far better to reduce the skirt of the piston by a thou on diameter for 75% of the piston length from the bottom

One last point and that's a personal preference - I always do the bore first, mount on a mandrel and turn the outside to size - that way any potential for distortion is eliminated. Cut the ports, de-burr without rounding the edges then lap.

Ramon

[Brian Rupnow]

Okay--Then the modification to my process is that I don't lap the piston into the bore. However, if I don't lap the piston to the bore, does this require that I make up some type of plug gauge to check the bore with?--Otherwise, I have no way of knowing when the bore has been lapped enough.

[cfellows]

Hey Brian, I'm a little late catching up.  That carburetor design is definitely unique and quite simple.  I don't see any reason why it wouldn't work on a 4 stroke as well.  What do you think?

I've been using TimeSaver lapping compound and am quite happy with it.  It comes in a powder which you mix with light grade oil.  It's friable and breaks down as you use it, ending up almost as a polish.  I've used it to lap valves and should think it would work as well with a piston and cylinder.  Here's a link to the product...

http://www.newmantools.com/lapping/time.htm

Chuck

[Jasonb]

Okay--Then the modification to my process is that I don't lap the piston into the bore. However, if I don't lap the piston to the bore, does this require that I make up some type of plug gauge to check the bore with?--Otherwise, I have no way of knowing when the bore has been lapped enough.

No need for a plug gauge on a one off,  bore the liner to a fraction below finished size and then work your way down through the grits looking at the surface finish each time, when you can't see the previous marks you have lapped enough with that grit. does not really matter if you end up with 24.04 or 23.96mm

See last post on this page

Then turn and finally lap the piston to suit the bore over the page

[CHP]

Jason
that link that you include is 
perfect love it

[Brian Rupnow]

Jason--Thank you. What a great link you provided for me. I really like how you secured the piston the the arbor for the final stages of honing it. That link answers a lot of my questions.---Brian

[Jasonb]

Its nothing more than Ramon's postings. Have a look back at his ED Racers and Tigers threads as they show the same methods.

That arbor is basically teh tee bar you mention elsewhere.

[Brian Rupnow]

And so it begins---I hadn't planned on beginning to cut metal so soon. But----For the moment I've ran out of intelligent questions to ask. It is plain that for me, the most difficult part of this build may very well be achieving the proper fit between the piston and liner, and the method used to do so. I have ordered a barrel lap, and some "light" diamond compound, and decided that I could build the piston from cast iron and leave it about .002" oversize from the final 24 MM (0.945") that I want to eventually achieve. I have received a lot of interesting and conflicting information so far. The plan calls for three .005" deep oil grooves, while well meaning advisers have told me not to put any oil grooves in.---I chose to put them in, because if I do achieve the fit I want in the liner, I don't want the engine to "seize up" from friction. The original plan has a drip oiler and oil port in the side of the cylinder, so the original designer must have had the same concerns. I have been advised not to use diamond compound by one highly respected and accomplished engine builder. I have been told it really isn't a problem by another equally respected and knowledgeable builder. I had to buy some compound anyways, because the finest compound I currently have is 600 grit, and I need something along the lines of 1000 grit, so I ordered the diamond. i will clean everything out with solvent and a toothbrush when I am finished fitting everything to my satisfaction. The piston shown here is cast iron, "as machined', with no other prep work yet.

[Brian Rupnow]

Hey Brian, I'm a little late catching up.  That carburetor design is definitely unique and quite simple.  I don't see any reason why it wouldn't work on a 4 stroke as well.  What do you think?

I've been using TimeSaver lapping compound and am quite happy with it.  It comes in a powder which you mix with light grade oil.  It's friable and breaks down as you use it, ending up almost as a polish.  I've used it to lap valves and should think it would work as well with a piston and cylinder.  Here's a link to the product...

http://www.newmantools.com/lapping/time.htm

Chuck

Sorry Chuck---I wasn't ignoring you. I have never seen a carburetor designed like this one before, and as soon as I seen how it worked, I thought of you. Would it work on a four stroke?---I'm not sure. The only thing I can think of, is that it might require the full crankcase vacuum from a two stroke to lift that ball and keep it lifted enough to run. Then again, a four stroke pulls a good vacuum on the intake stroke. I think perhaps on a two stroke with vacuum created on every rotation of the crankshaft, that the ball probably "oscillates" rapidly between the seat and the top throttle adjusting screw. On a four stroke, where it pulls vacuum on every other stroke, the ball might be too heavy to lift fully on every other stroke. at any rate, it certainly is a novel design.---Brian

[Jasonb]

Should work fine on a 4-stroke, as I said its very similar to a Lunkenheimer carb except that uses a valve rather than a ball to shut off the air and fuel

http://www.oldengine.org/members/diesel/OldCarbs/Carb52.htm

If you look at about 20secs in you can see the valve lifting at the bottom of the carb on the hit have it on full screen

<a href="https://www.youtube.com/watch?v=b3InhC_WiaI" target="_blank">http://www.youtube.com/watch?v=b3InhC_WiaI</a>