piston and cylinder lapping?

[billyboy]

hi fellas, what is the best way of lapping a bronze piston to its cylinder, i have bought a fine lapping compound but before i try to do any lapping i need your advice please, i have left the piston about 2 thou oversize. i dont intend on fitting piston rings.

bill

[SGW]

There have been some pretty good notes on lapping in the past, so a search of the archives will likely turn up some good information.

You may get other ideas on this, but here is what I'd do:

First, do you also need to lap the cylinder bore? There is little point to lapping the piston if the bore is rough.

To lap the bore, get yourself an expanding barrel lap of the correct size. Places like www.mscdirect.com sell them. You could make one, but they're cheap enough so it's hardly worth the effort. Set up the cylinder in the lathe, chuck the lap in the tailstock, apply a SMALL amount of lapping compound, turn on the lathe about 200 rpm, and by sliding the tailstock back and forth move the lap in and out of the cylinder. Tighten the lap slightly as it wears, add a little light oil as the lapping compound dries out. If the lapping compound seems "worn out," clean off the old before applying much more, as too much lapping compound contributes to getting a bell-mouth hole.

When you have a decent finish in the bore, stop, wash out the cylider well, measure the i.d., and turn down the piston so it's about 1/2 thou oversize. Lapping is not for removing a lot of metal...2 thou is way too much allowance.

You'll need to make a ring lap out of a disk of aluminum to finish the piston o.d. A question comes up here...how big is this piston? I'll assume it's about 1" dia, just for discussion. Bore a 1" dia. hole in the center of a disk of aluminum maybe 1/2" thick and 2" dia. Slit the disk nearly in half, and drill/tap for a screw so you can close up the slit. If you can counterbore this screw so it doesn't stick out, that will be extremely Good.

Set up the piston in the lathe, again turning around 200 rpm. Apply a small amount of lapping compound, wear a heavy glove (!), and by hand, work the ring lap back and forth along the piston as it turns, adjusting as necessary. Pay attention to what you're doing here, and be really careful that the lap doesn't seize on the piston, which could lead to some injury.

[black powder]

I have allways been told not to use carbide lapping coumpound on brass ,copper or alum.
What do you use as coumpound.

bp

[billyboy]

i dont know what type of lapping compound i have got, i bought iy from a auto accessories shop, ill have a look tomorrow

bill

[rkepler]

I always embed the lapping compound in the lap , this to keep any loose lapping compound from embedding in the work. The only exception to that would be with non-embedding lapping compounds like TimeSavers, but those are really intended more for running in a bearing or the like.

I also like to hold the cylinder by hand and run the lap on the headstock as it gives me a better feel for how much cutting is being done. If you are careful to embed only the center of the lap it'll cut true, and you can feel where the lap is cutting and where it is not, and get a constant diameter with that (hoping that it's axial). Ring laps I hold while rotating the work.

[billyboy]

is there a proper lapping jig of pacific measurements ie 1" od for both internal/external diams, i get the jist of the expanding mandrel idea but was wondering if there was an actual jig to buy?

bill

[Mcgyver]

Ok, if you're having trouble sleeping tonight, tune into Mcgyver's special on homemade internal laps

As the engines I've made have rings, I've never had to lap an external surface, but I've lapped every engine bore.
here's how I lap a bore. I've made two styles of lap as you can see



The 2nd and 3rd from the left are straight forward; a bar split down the middle with a set screw to expand them. The 3rd from left followed the time honoured approach of drilling some holes and then splitting with a hack saw. I'd grown more brain cells by the 2nd from left one and used the band saw and tack welded a retaining ring on afterwards –as per following close up



This little fixture I’ve posted before can be used vertically or horizontally in the horizontal bandsaw makes it easy to cut the split.



The other style, which if done before I've never seen, I'll claim as my own. The larger cylinder split and only half of it is brazed or welded to the handle. A couple of set screws allow for expansion.



This second style is terrific to use. Imo they avoid bell-mouthing as the length of the cylinder is less than that of the bore. As you are working them in the bore, you can readily feel the tight spots giving you a good change of feeling your way to a very round, smooth. parallel bore.

In all cases I hold the casting in the lathe and manually hold the lap. There are no sharp edges on the handle and if it does bite it just slips in the hand. calluses are the worst I've received. Holding an irregular casting posses obvious dangers in the event it grabs and the idea of using a glove around a machine tool scares the crap out of me, I won't do it.

I run fairly slow for this work and have clover compounds going up to 1200 grit , although 600 is probably good enough. You will need a variety, say 240, 360, 600 or such was in a kit I bought – that is a really good way to start. Obviously work your way up, clean with kerosene in between grits, cover the lathe with paper towels, etc

You'll notice all the laps are clad in copper. This is the right material for the lap, the grit gets imbedded in the softer copper and that is what does the cutting. The copper also quickly wears into a perfect round shape – despite the slight egg shape of the lap. Copper is expensive so my frugal solution was to solder copper onto the steel lap, and then cut some groves in it. Don’t worry that the OD of lap is not perfectly concentric with the handle, its hand held and hence floats so its not a factor. The lap is refurbished after wear by soldering on new copper – vs a through away commercial lap.

The question was raised about how to lap softer metals like brass, AL etc. The short answer is don’t because the grit becomes embedded in the soft metal. I've heard that there is a compound (designed for these metals) that breaks down quickly so it does not leave the part to be lapped a lap itself...... I don't though, seems counter intuitive and how do you know whether its true or not until the part is worn out. I don’t know the name or source for it – anyone?

Hope this helps

[Luke55]

Nice post Mcgyver I print it

[SGW]

One (allegedly) non-embedding compound is aluminum oxide. I think Brownell's ( www.brownells.com ) sells it.

[This message has been edited by SGW (edited 01-31-2006).]

[Weston Bye]

Excellent post, McGyver! Well written and illustrated. Completely understandable. And the bandsaw bonus fixture. I'd recommend your post for the next Tip Book.

Wes

[This message has been edited by Wes1 (edited 01-31-2006).]