wrote:
On Fri, 04 Jul 2008 14:15:47 -0700, Tim Wescott
wrote:

I'm a wannabe miniature internal-combustion engine builder. To further
the task of taking the 'wannabe' out of the description, I'm considering
taking an old Cox Medallion .15 and machining a new cylinder and piston
set for it (the original was allowed to run too lean, overheated, and
now cylinder and piston are slightly oval; since the piston is free to
rotate on the rod you can run it for just a little bit before it binds up).

This cylinder has an extra-thick wall up to the exhaust ports, and
rather than transfer ports going through to the crankshaft, it has
transfer slots that come up from the crankshaft to between the exhaust port.

So from the bottom of the cylinder it looks kind of like this (slots
exaggerated, and I hope I can make it more round than the drawing!):

-------
/ ----- \
/_/ \_\
|(_ _)|
\ \ / /
\ ----- /
-------

The normal operations that must be performed on a part of this type by
amateur builders is to bore the inside of the cylinder to size, then lap
it to final finished size, then fit the piston.

My problem is that I am trying to decide when I should cut the transfer
slots. Should I build the whole cylinder, get it lapped and fit to the
piston all nice and pretty, and then take a mill and hack out the slots?
Or should I make the slots, bore the cylinder, finish it off and lap
it? Or should I finish machine it, make the slots and then lap?

Each one of these carries interesting possibilities for making a
cylinder that isn't quite round at the bottom.

Making the slots first of all would let me use a plain old drill bit,
but would make for a lot of interrupted cutting while I'm making the
bore, and possibly machining in some not-quite-roundness, and would
force the lap to work in an oddball cylinder, possibly softening the
transition from cylinder wall to slot.

Making the slots after boring but before lapping would save me from
machining in the not-quite-roundness, but would create the possibility
of the cylinder warping from the stress of milling or from relaxation of
internal stresses. It still leaves me with the odd lapping problem.
Finally, it means that I have to use my 3-in-1 Smithy as a milling
machine, and it is neither convenient to use nor terrifically rigid in
this role.

Making the slots after lapping leaves me with the stress-relief and
milling issues, and it may well raise honking big burrs. It just
doesn't strike as the clean way to go -- ripping into a nice, lapped
surface with a milling cutter just seems _wrong_.

So how would you do this? And why?

Thanks.

Greetings Tim,
I would lap last. Because a properly made and used lap will keep the
bore round despite the port cutouts. This means that you can mill the
ports and then file if need be to final shape. Care must be taken when
deburring the cylinder side of the ports because the boring and
lapping will only be removing a small amount of metal. The interrupted
cut when boring may seem to be a bit of a pain but when filing the
ports to get them to shape and smooth inside it will be easier if you
don't need to worry so much about the edges being rounded over where
the port intersects the bore. The lapping operation of course needs to
leave the ports with sharp edges.
Eric

Yea, I think lapping last makes the most sense. I think drilling the
slots first, then boring the cylinder, then lapping is probably the way
to go -- but I've never done it before (or I wouldn't be a wannabe!).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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