# What's the procedure in lapping a piston/bore to get an air-tight seal??



## Philipintexas (Apr 28, 2013)

The title line pretty well says it all. I need to get a 3/8 piston (bronze) to fit a 3/8 bore (bronze) with an air-tight fit while still allowing rotation of the piston. I've reamed the bore to a smooth finish but don't know how to proceed. O-rings are out of the question as the movement of the piston is rotary covering/exposing a length-wise slit port. (Corliss valves).


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## Tin Falcon (Apr 28, 2013)

first of all you need a .3755 bore +.0005 -0  and a .3745 piston plus nothin minus .0005. in a perfect world. 
lapping is easy if you follow the basics. 
for an inside lap use a piece of aluminum a couple thousands smaller than the bore coat with a fine abrasive. real lapping compound is the best but tooth paste or auto finishing compound will work. 
To lap an OD drill a hole in in a pice of wood and use the same lapping compound . two things to remember smooth finish and accurate measuring. 

The piston should fall in the open cylinder and not fall if one end is closed. 
Tin


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## Philipintexas (Apr 28, 2013)

So, do you don't actually lap the piston in its bore?


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## rebush (Apr 28, 2013)

Philipintexas: You're lap should be a softer material than that being lapped, so it can absorb the lapping compound. I'd turn my piston then bore the cylinder undersize as Tin said. Then lap the cylinder to fit. I use wooden dowel rods about four inches longer than the cylinder. Turn it a little oversized, use rough grit sandpaper to size that I can push the cylinder on to it with a snug fit. Then I sand the first inch a little smaller so that I minimize the chance of bellmouthing the cylinder. Hope this helps. Roger


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## Tin Falcon (Apr 28, 2013)

Lapping is not always needed .  when we did the  first team build parts were made in shops miles apart. 
I assemble my engine and it ran . I used plenty of light oil. there were some very light rubs at first but no fitting or lapping required .
People just measured carefully and debuured there parts. 

Sometimes an engine will require running in under external power again plenty of oil.  too rough or tight things can seize and break. so be careful. 
tin


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## cfellows (Apr 28, 2013)

I'm not an expert at lapping and others may disagree with my methods, but I've had pretty good luck using a piece of thin walled aluminum or brass tubing for a lapping tool.  I simply cut a lengthwise slit longer than the piece to be lapped, then cut a crosswise slit half way through the tube at the end of the other slit.  Then I spring the slit tube apart a little so it's a spring fit in the cylinder and use that for the lapping tool.

Chuck


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## Ramon (Apr 29, 2013)

Hi Phillip - don't know if you've seen this thread on the Super Tigre build -http://www.homemodelenginemachinist...g32-1cc-diesel-5cc-version-17674/index16.html 

Lapping for very close fit is covered in detail. This may be of use to you but as you are lapping bronze you will need to keep the lapping medium to a minimum and very fine too - if the surface finish is good to begin with on both parts then lap the piston to fit the bore letting the lapping process size the bore to the piston. If the finish needs improving first then lap the bore first before lapping the piston to fit.

A word of caution - should you decide to use the readilly available diamond lapping compounds. Lapping any soft material -ie unhardened - the compound will embed itself in all surfaces not just the lap. The usual cleaning solvent based cleaning methods will not remove minute particles from the material and as such the parts can continue to lap themselves in after assembly. Only cleaning in an ultra sonic cleaner will prevent this situation from happening. 

Hope this helps

Ramon


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## Mainer (Apr 29, 2013)

No, you don't lap the bore with the piston or the piston with the bore, depending on how you want to look at it. If you try doing it that way, you have no control over what gets taken off where. You need an internal lap for the bore, and an external lap for the piston, if you want to lap both of them. 

You can buy bore laps (see "barrel laps" in tool catalogs).  Use a fairly small amount of abrasive so it doesn't pile up at the bore openings, as that can lead to a bell-mouth bore. Plan on lapping no more than 0.001" and 0.0005" is probably better if you can manage it. #320 grit does pretty well. If you do have to take off quite a bit (i.e. around 0.001") you might want to start with #240. #600 to finish up, if you want.

I make external laps out of split aluminum disks that I put in a holder that has setscrews to close up the lap to the desired tightness. See photo.


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## Ramon (Apr 29, 2013)

Mainer said:


> No, you don't lap the bore with the piston or the piston with the bore, depending on how you want to look at it. If you try doing it that way, you have no control over what gets taken off where. You need an internal lap for the bore, and an external lap for the piston, if you want to lap both of them.
> 
> QUOTE]
> 
> ...


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## Philipintexas (Apr 30, 2013)

Thanks for the input guys, I hate to admit my ignorance but I was trying to use 5/8" bearing bronze, turned to .500 OD and drilling & reaming to .375. After 6 failed attempts to do this I gave up on bronze (I also ran out of a 22" stick I purchased for these 4 valve bodies)! I could not get a straight/concentric hole in 2.4"!  I switched to 1/2" drill-rod and got 2 out of 2. I know bronze requires different drill lips but It just wouldn't work. Anyway, I used a little of each answer to my lapping question and after many hours finally got 4 Corliss valves/chambers that will rotate and seem to hold air. Thanks for all the help.


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## kilocharlie (Apr 30, 2013)

Phillip - just a note, perhaps getting too far into details, but fwiw, the goal for the finish inside the cylinder is usually a 55 degree crosshatch pattern, although this is usually applied to piston ring seating, not a rotating piston, so you may consider highly polished with 0 degree "crosshatch" (or lack of it...). My experience says finer grit is better (I'd lean toward 1,000 grit or finer), and I would back what others say about .0005" clearance is better that .001" clearance, especially for an air seal in that small of a bore. Best of luck to you.


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