Scott flame licker build

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madjackghengis:
I'd definitely agree with your final finish desire, it does make a substantial difference in useful power, and longevity. The issue with rings was the lack of full understanding of the loading, and the need for the rings to be substantial harder than the bore, as the contact surface of the rings is a factor of far more than a hundred to one, and cast iron has those nice little nodules of carbide in it. Ring wear today is mostly due to exotic additives in the ring material, and partially due to better control of surface angles and contact surfaces designed to follow the twists and flopping caused by friction and direction changes, with angles designed to work with the natural forces on the ring, rather than trying to counter them, by making it more rigid. I use several different kinds of rings in rebuilding Harley engines, each brand of which claim to have solved all the world's problems with new technology, but I never know what the rings will look like when I order them until I open the package. There are three or four differing engineering perspectives on rings, all of which are at least somewhat contradictory to each other, with the major difference being in minute differences in cranking pressure and final horsepower, all of which tends to be accademic to the mechanic and the owner, except for racers, where it does really matter.
Lots of machinists don't know that good quality cast iron can be brought to a mirror finish under the right conditions, and that, coupled with accuracy in roundness and taper can allow an engine to run at full temp with less than half a thousandth clearance if the piston and cylinder are iron. I've found delrin does an excellent job in carrying lapping compound for a final finish in iron and is cheap and easy to get, and easy to throw away after its dead and gone. :beer: cheers, jack

Bogstandard:
Nick,

I will most probably spend more time on getting a good finish between the bore and piston than I have spent making the two parts.

To me, they are the most important part of an engine, and getting it right first time will eliminate any queries about them when it comes to get the engine running, if there are any problems.

Because there was so little spare length to hold the billet in the chuck (3/16") to complete the piston by normal methods, I am using a feature that is normally put in last to actually help me make the part. When you see the finished article, you will see why I went down this route.
First off, I did a fine face off on the end, then drilled and tapped a 3/16" x 40 tpi thread in it (normally the last bit to do). This is going to be the main datum for the whole machining exercise.

I now went over the whole billet, rough skinning it down to within about 0.050" of finished size all over.

This is removing metal from the other end of the billet.

The O.D. was taken down in two stages.

The final part was to remove the bulk from the inside.

So this is what I ended up with. I now need to be able to hold this fairly rigid while I bring everything down to size.
Time to make a mandrel.

Using a piece of bar end, I turned a spigot sticking out on the end and then faced the end off smooth. The spigot was then threaded 3/16" x 40 tpi.

The part was then screwed on tight onto the spigot screw and faced end.

I need to get the heaviest of the machining done first, as the screw will be gradually shortened to a couple of threads by the time the centre is bored out.

First off, the piston was brought down to exact length.

Followed very gently by removing all the inside of the piston by boring. You can just see at the bottom of the hole where the screw has been shortened by the boring exercise.

The OD of the piston was then brought down until it just fitted inside the bore, just a nice push fit.

You can now see how much metal was removed, I suppose because the piston needs to be very light just so that friction and reciprocating forces don't get too high.

If I had tried to hold this with a normal chuck, I would imagine it would have collapsed.

My mic said I had 0.0006" clearance (about 0.015mm). That will be spot on for when I start to lap the piston to bore. Hopefully I will end up with about 1 thou clearance (0.025mm).

You can also see that the bar end will act as a perfect piston holding handle for doing the lapping with.

Bogs

Dean W:
I like the idea of the spigot with the fat end for supporting the piston. A good multi-task jig, John.

Bogstandard:
Jack,

Sorry I didn't answer you before. I use Delrin sometimes as well, it is very easy to embed the cutting agent into it's surface with a piece of hardwood. But I don't thro it away afterwards, I just do an outside skim on it and it can be used for something else.
These pair though are going to be lapped together. I find cast against cast gives a lovely finish when final lap is with something like jewellers rouge or toothpaste.

Dean,

I suppose no matter which way it was done, some sort of mandrel would have been required. this way was the first to come to mind, so it got used.

Now the dirty work begins with the lapping.

Almost everything after that is straightforwards machining and sticking or silver soldering together.

John

NickG:
John,

Great work on the piston, I remember when I first did my jan ridders piston without hollowing it out and I believe it did make a difference. The engine wouldn't run with the extra mass. I got away with just putting the slot in the graphite as it was so much lighter anyway.

I don't disagree that correct lapping must improve the performance of such an engine. However, what I am trying to say is, I have proved although it's obviously a critical fit, it's not as critical as people think to get a running engine and that goes for flame gulper, stirling or whatever. I'm not saying my surfaces had a poor finish, they were as you said - done on the finest feed and very smooth, could not see (by eye) or feel any ridges. It'd be interesting to try and quantify any differences in the two methods.

I guess the difference is, I'm aiming for a much lower standard! I'm happy with an engine that will run well for the moment. The easiest and quickest way I've found of doing that is by reaming the bore and leaving the piston as turned from the lathe with a fine feed.

Nick

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