# Lapping cylinders



## SignalFailure (Apr 2, 2008)

I've seen lots of people mention lapping cylinders for a very good fit/finish but have yet to find a practical explanation of the process other than in a book I have which simply suggests twirling a slotted rod carrying a piece of abrasive cloth inside the cylinder.

So come on, how's it done and when? 

Say I've made a nice bore of the 'right size' in a piece of brass,aluminium or cast iron by drilling and boring or reaming and I'm ready to turn the piston to suit. What's the sensible order of operations?

Cheers

Paul


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## tattoomike68 (Apr 2, 2008)

Bog goes through it on this thread, its a good read.

http://www.homemodelenginemachinist.com/index.php?topic=1614.0


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## Mcgyver (Apr 2, 2008)

Paul, 

I've been trying to write an article about them, did one years ago but it needs an update. Basically, a lap is softer than what is being lapped so the abrasive can get embedded in it and it is adjustable so it can be expanded. I make mine out of steel and soft solder on a thin piece copper - who wants to buy 2 1/4" dia copper? In use, the lap removes very very little material. You use progressively finer grits of compound. I use clover compound because it's easy (already mixed) and will finish up with 600 or 1000 grit. Here's a pic of ones I've made over the years. I build mine with smooth handles so i can hold the cylinder (often an irregular casting) in the lathe and hold the lap by hand with the lathe running at say 50 or 100 rpm. This lets the lap float and if it sticks, it slips in the hand (don't knurl!) - the other way around and the casting takes your hand off. It is important though that one of the two float.







Be carefully lapping brass or aluminum. there is special lapping compound that you need the breakdown quickly else you'll turn the cylinder wall into the lap.

I wouldn't bother if we're talking a little brass or aluminum wobbler, just bore or even ream and turn the piston to the clearance you want. I don't want to be mistaken for saying you have to lap a cylinder, only that this is what lapping is and how its done. It would be over kill in a lot of instances. Also, these engines tend to have smaller bores. It would be difficult to make the style of lap i've shown for very small bores; laps for small bores tend to be barrel laps - the 2nd & 3rd from the left. This style of lap more easily leads to a bell mouthed bore unless you careful, another reason why i don't bother on simpler and smaller engines

Here's a pic of the cylinder of a flame licker i did recently, you can see the finish it put on the cast iron cylinder. 






with a lap, you can correct errors in the bore - out of roundness, taper etc as well as the great finish. This can be really important for low power atmospheric engines. In another thread Kevin points out that the cross hatch pattern of scratches left by a hone assists in keep oil in place. I'd always thought the same but recently read some credible account claiming this to be bunk, who knows, but i've heard both from people who should know. I suppose one could lap it to get the bore straight and round and then run a hone in it to get the hatch pattern

and thats what i know about internal laps ;D


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## Hilmar (Apr 2, 2008)

Mcgyver

  Be carefully lapping brass or aluminum. there is special lapping compound that you need the breakdown quickly else you'll turn the cylinder wall into the lap.


Aluminum? The Lap or the Cylinder?
Hilmar


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## Mcgyver (Apr 2, 2008)

> Aluminum? The Lap or the Cylinder?



the cylinder, as in trying to lap an aluminum cylinder. because the AL is soft, there's' a danger the compound becomes embedded into it, it will then act as a lap and wear away the piston/rings. I forget what its called, but there is a lapping compound intended for lapping softer materials. Never bothered myself, I've made engines with bronze and AL cylinders but didn't lap them. the ones I've lapped are cast iron, or one case steel


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## snowman (Apr 2, 2008)

What soft solder do you use?


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## Mcgyver (Apr 2, 2008)

whatever's handy, plumbers etc. I tin each side, wrap it up with binding wire and sweat it together.


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## snowman (Apr 2, 2008)

Ok, a realistic question then.

HOW!?!?!

I have never had ANY luck getting plumbers solder to stick to steel. Tried lots of different fluxes, everything.


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## compound driver 2 (Apr 3, 2008)

Hi
Clean the steel with wire wool so its a nice bright surface. Coat it with Bakers fluid. Slowly warm the steel re fluxing from time to time. Once the solder melts at a touch 
it should flow onto the steel.

Think clean steel

Cheers kevin


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## snowman (Apr 3, 2008)

Well I'll be danged...

I couldn't sleep, so I decided to go into the basement and play.

First of all, the shop is not in the basement. I couldn't find my plumbers torch, so I figured I'd use my iron. Couldn't find my big iron, so I figured I'd use the torch aim n flame. Couldn't find that, so I ended up using my coleman camp stove as heat.

Then I couldn't find a piece of steel. Pretty sad. Finally decided a utility knife blade would do. lol

No sandpaper, it's all in the shop. Had to use a file. Carefully filed a couple high spots on the blade, coated with flux, heated on the camp stove and wallah...a bead that stuck. 

Further then I had ever gotten before. I have always heated TOO hot I think. 

So, I found another couple pieces of steel...1/8 x 1/2 CRS...filed to fresh metal. Tinned both pieces of metal after I spent a quarter hour looking for a brush of some sort to flux and tin with. Then I even soldered them together. Tough enough joint that I couldn't break it apart, just bent the steel. Not too shabby.

Only took an hour, but I got it figured out. I had always thought you needed silver solder to solder to steel. 

Guess the membership fee to this site has already paid itself off. 

Thanks for the motivation to learn...as well as the help, albeit arriving during my adventure. 
*
NOW....*

Got me thinkin during all that mess.

Why bother with the copper sleeve? Why not just load up the shaft with a little solder, turn it to size, then load with the lapping compound? Just durability?

When you use a lap with coarse grit, is that lap always coarse grit, or do you expect that grit to break down through the stages?


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## compound driver 2 (Apr 3, 2008)

Hi
A few years ago I was restoring a cylinder block from a full size Fowler traction engine.
After re machining the bore it was honed using a cast in lead plug. The bore was blocke with a lump of oak at one end a mandrel was set in the middle of the bore and a lead slug was cast arouns the mandrel. Once the lead was solid the oak plug was removed and the lead slug tapped with a club hammer. Once removed from the bore a liberal coating of tallow was applied with some grinding grit.
half a day later the bore was a perfect cross hatched finish and to this day its still in steam.

As for solder im not sure id try to machine it and I doubt id use it as a hone.

Cheers kevin


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## Hilmar (Apr 3, 2008)

Mcgyver

It is OK then to use an aluminum Lap to lap steel, cast, brass or bronze cylinders?
Hilmar


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## Mcgyver (Apr 3, 2008)

> Why bother with the copper sleeve? Why not just load up the shaft with a little solder, turn it to size, then load with the lapping compound? Just durability?



yes, mostly, more complete answer below



> When you use a lap with coarse grit, is that lap always coarse grit, or do you expect that grit to break down through the stages?





The correct use of a lap is to charge it, press the abrasive grit into its surface by say rolling it over some steel, so that the lap is holding the abrasives and it becomes a cutting tool. If you don't charge the lap, the abrasives tend to roll between the lap and work cutting both. 

I have a slightly different approach. A scientific theory is one that support the observations; it can never be proven only disproved.  what follows is theory I've conjured up - it seems to fit the observations, but I've never read of it have no authoritative texts to reference. take it for what its worth....

I don't charge the lap, I just put some compound on it and lap away expecting this rolling action to occur. Why that is considered wrong is that I am going to incur far more wear on my lap than if i charged it properly (by traditional techniques) - I'm using it in such a manner that will knowingly and drastically shorted the tool life. Why such cavalier abuse in a hobby which is supposed to be par metalworking part frugality?

The answer is that I want the lap to wear away so that it forms a perfect cylinder. As the work and lap wear together, they will form a perfect cylinder. This is where its important that it floats and that its shorter than the bore length - you can feel any tights spots and through working the lap the length of the bore you'll abrade both into perfect cylinders. doesn't work as well with barrel laps and not at all holding the lap in the tailstock. 

This rolling action will embed particles, so having the lap softer is still important so they embedded in the lap rather than work but solder wouldn't be thick enough and wouldn't imo stand up. if you did charge a lap I suppose you need different laps for different grits, although they are supposed to break down. I've found with my semi-charged laps that i can use the same lap and after a thorough cleaning move to next higher grit without penalty. Perhaps this application creates less embedding and the rolling a faster breakdown? 

So the way i do it the surface of the lap is a bit sacrificial. In industry you wouldn't want to have to re-clad a lap after a couple of cylinders, but then again in industry you'd use Sunnen hone. However in the homeshop making onesies and twosies its a very doable technique to produce a quality of bore and finish not available any other way.

Hilmar, seems logical, but I don't really know or have any first hand experience. there are compounds made specifically for brass and bronze that are supposed break down quickly, but again, I've never lapped bores of those materials.


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## kf2qd (Apr 8, 2008)

Taught Machine tool practice Lab when I was in college and we built some .60 diplacement 2 cycle engines. Cast Iron Cu\ylinder. To lap the cylinder I took a piece of cold rolled steel, centered both ends, turned it to just under my finished bore size (seems like it was 0.002" small) and then tapered both end slightly leaving the middle 2" at the diameter. I then coated it with some fine valve lapping compound and worked the cylinder back and forth with my hand until it moved freely back and forth. I think the bore was 0.937" x 2" long

2 things I accomplished - 
#1 I was not lapping in the chuck as I have seen so many setups do. No having to tear a chuck down and clean the lapping compound out.
#2 got nice finished straight bore very quickly.

Did over 30 engines with minimal wear on the lap.

We then made the piston to fit the bore -


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## d-m (Apr 8, 2008)

Here is a co that makes laps and they are priced reasonable you can also find some small ones on ebay if you search NEEDLE EYE LAPS but these are just the barrels ( I think haven't resarched yet)
Dave

http://www.acrolaps.com/


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## SignalFailure (Apr 11, 2008)

Many thanks all for the advice, especially Mcgyver your explanations and photo are a great help - thank you for taking the time and trouble :bow:


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## snowman (Apr 11, 2008)

McGyver...

can you show a little more detail on one of the laps? Or describe?

It looks as if you have cut a slot and relieved the end, with a set screw to adjust diameter of lap...is this correct? How much movement do you actually apply to the lap? Just enough to make it fit the bore "snuggly"?


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## Mcgyver (Apr 11, 2008)

SF, you're welcome 

snowman, the collection was done over many years as i needed them, so construction was a bit of an evolution. Probably the style i've decided i like the and is easiest are the ones on the far left and far right. The bar (handle) isn't slotted but disk is. I only braze 1/2 of the slotted disk onto the handle. the two halves of the disk are still joined but the smaller amount of steel that isn't slotted - this allows a little bit of flex, the two holes through the copper are for set screws that push to two halves away from each other. In use i get so there is some resistance, snug would be hard to hold on to. as material is removed plus moving to finer grits, it's gradually expanded.


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## Cedge (Apr 12, 2008)

I have been studying this thread closely since I knew I'd be needing to try something to finish out a cylinder I'm working on. Sure enough, my small iron left me with a bit of taper in the bore.... mostly due to the spring in the cheap Chinese boring bars I've been stuck with using. Some of the taper was from bad technique but I soon discovered the problem and corrected what I was doing wrong. I wound up with .0015 variance which was not going to work for the .0001 clearance I had planned for. 

I had a piece of copper bar that was growing dust, so I turned it down to just under the desired bore size and cut two relief grooves down the sides. I then loaded the thing with a very fine metal polish and began lapping the bore on the lathe while letting the "lap tool" float in my hand. I'll admit to having some doubts, but when all was done, according to my mic I was only .0006 out end to end. 

Tonight, I made the piston "to fit", almost using it as a go no go gauge. When it slipped into the bore I called it a fit, After drilling it for the piston rod and doing a cut off, I coated the piston with fine grit metal polish and began to hand lap it in the cylinder, strictly by feel. Once it felt "right" I measured the bore using an expanding bore gauge and my digital micrometer. The variance had dropped to .0000*2*!! That, my friends, is the tightest tolerance this relative newbie has ever achieved and far beyond all my expectations. 

Thanks to those who shared their ideas and techniques. Even though I was a bit skeptical of the accuracy claims I was reading, I have to admit the process worked rather damned well. That copper slug is now a new tool sitting in my box for another day.

Some decent boring bars are definitely my next tool building project. 

Steve


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## SignalFailure (Apr 12, 2008)

Nice work Steve and more useful tips.


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## snowman (Apr 12, 2008)

Steve,

How deep are you boring, and what diameter?


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## Cedge (Apr 12, 2008)

Snowman
The bore dia. is 13/16" and 1 5/8 in length. The problem I was having was due to boring bar flex and a shallow attack angle while retracting the tool. The cutting edge was climbing over the cut and creating a taper. I didn't spot this as being the problem until after I'd bored the cylinder to diameter. The lap corrected the problem much more quickly than I would have imagined and the piston fits beautifully. This morning I used a similar process to "line bore" the gland end to a friction free fit for the piston rod. 

Steve


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