Piston/cylinder-lapped fit--no rings

[petertha]

Thanks for taking the time to explain your process in great detail, Gail. Very useful information.

 

[petertha]

Here is an example of two of my shop built external laps. One for 7/16 and one for 1/2 inch diameter.

Right up my alley! I was interested in these CI external laps
http://www.americanlap.com/External%20Laps.htm
...until I converted the US$ into rapidly declining Canadian pesos. I figured there must be a good reason for the outboard stress relief holes & inward radiating slits, at least for small shafts. A bit of bugger to make though. I see your examples you have inward slits on one & outward on the other. Can you comment on any performance difference. Maybe clamping pressure requirements or how the lapping compound gets imbedded?

I'll have to dig up my related post for a visual, but rather than have a tightening screw right adjacent in the hub, I have an extending arm & the screw is outboard. a) to give more mechanical advantage like a pair of pliers b) to give more clamp control due to extended lever for same TPI thread c) doubles as a handle. Are you clamping like the American lap method?

 

[petertha]

Found it. This is kind of the concept. A generic tool holder, insert a replaceable, relatively easy to machine lap cylinder cartridge into the center. Clamping screw puts mostly radial pressure around the lap. At least that's how the commercial ones look to me. Trouble is, I need a water jet to cut that radial slit profile

 

[bb218]

Sunnen also make an external hone very similar to the photo Blogwitch shows in his post. The times I used them I use the hone with the honing oil to flood the part.

In the shop I worked at we had a roll lapper, it had 2 cast iron rolls running the same direction, 1 was a larger diameter so the surface speed was faster. You applied diamond lapping compound and used a wood stick to put pressure on the part getting lapped. Really could hold very tight tolerances, I was lapping fuel injector plungers. The lap was really simple design, similar to an old wringer washer only the rollers were turning both the same direction so they would not try to pull the part in. If 1 end was larger use the stick and put pressure on that end.

Mike

 

[Brian Rupnow]

I had the same experience as Petertha---I called the good folks at American Lap, and the holder, handle, and 25 mm insert came to over $400.00--that inspires me to build a home-made external lap, similar to one I have seen on another post, which is simply a piece of brass with a "close to the right size" hole drilled thru it and a pinch bolt to close it up a bit, as required.

 

[Billitmotors]

Gday Brian
For internal lapping nothing can be easier.
Laps are very easy things to make. Just get a piece of brass that is bigger than the final size required and machine it down till it is .001” to .002”smaller than the bore of your cylinder, your cylinder is approximately 1.6” long so make the machined length of the brass 2” long plus your chucking length.
Once you have machined the parallel 2” length centre drill using a No 2 or No3 centre drill. then drill with a 5.1 drill or 13/64” drill around 1.5” deep. Then using a ¼” BSW or 1/4” UNC TAPER tap, only let the tap enter for 16mm or 5/8” of an inch. Next get your hacksaw and cut down the axis of the lap for about 1". Run your tap in again to the same depth, remove it and using a fine file remove the burrs from the outside. Then screw in a 1/4" allen headed cap screw until it just starts nipping up.
You can either use diamond past or aluminium oxide lapping paste, I prefer the diamond paste which you can buy cheaply on Ebay.
To load the diamond paste onto the lap put a small amount (it goes a long way) on a flat piece of steel preferably high speed steel and while pressing the lap hard against the flat steel roll the lap over the paste.
Cover your lathe bed with plastic and put the lap back in the chuck leaving a small amount of the chucking piece protruding from the chuck jaws or put a large plastic washer on the lap adjacent to the jaws so that the cylinder can't be damaged by the spinning chuck.
Set your lathe to run slowly and slide the cylinder, head first onto the lap, use kerosene as your lubricant and turn the lathe on. It is always worth making a shroud that the cylinder is attached to so it protects your hands if it picks up. Grasp the shrouded cylinder and slide it back and forth along the lap. Your aim is to get a fine even finish cross hatched surface finish over the entire length of the bore. You will need to keep incrementally tightening the screw as the lap wears and the bore size increases and becomes round and straight.
When you first start use coarser grit paste and once the bore gets an even matt finish over the entire length go to a finer grit. It is worth having a second lap prepared to use with the finer grit. Once you are happy with the finish of the bore wash and scrub it clean of any grit before you try and fit your piston and contrapiston. I purchased a small cheap ultrasonic cleaner which does a great job of final cleaning all the parts.
Rob.

 

[Brian Rupnow]

Thank you Billetmotors---Lots of good information there.---Brian

 

[gbritnell]

I had written something and then decided that it wasn't necessary.
Thanks,
gbritnell

 

[petertha]

My liner trials & tribulations FWIW
http://www.homemodelenginemachinist.com/showthread.php?t=25163

I used an Acro lap. Reasonably priced & replaceable brass barrels. The difference is the enlargement occurs in the middle of the barrel, not at an end. Depending on the cylinder & setup, this may or may not be an issue or advantageous.

What this exercise taught me is: if you really don't care if your "~1 inch bore engine" comes out 0.998" or 1.002", as long as it ends up circular & appropriate finish & parallel walls, lapping is a bit messy but not that difficult or time consuming. In other words if the resultant bore is 'whatever' & this becomes the starting point dimension to subsequently make a matching piston and/or ring to suit, that's a straightforward A to B procedure. But if you have to hit the specific dimension + final finish combo deal, that's a bit more interesting (especially multi-cylinders).

Now that I understand Gail's piston lapping as being fitted to (but not modifying) the bore, that makes more sense. You could muck up a few piston attempts getting the fit just right but confining the boo-boos to just one part.

Has anyone had adverse issues of lapping compound (particularly diamond) imbedding in aluminum? I keep hearing its a bugger to remove & why you should choose other compounds.