Lap modification

[Gordon]

Is it possible to resize a barrel lap? I want to lap a 1 1/16 bore. 1" and 1 1/8" laps are available. Could the 1 1/8" be turned down to 1 1/16"?

I have tried making my own "D" type lap without a whole lot of success. Getting copper with the proper ID is the problem. Also commercial laps are available in the $20 range and new sleeves are only about $15. You cannot spend a lot of time for those prices. I will also have to look into making my own sleeve for other sizes.

 

[WOB]

It is true that commercial laps are cheap. However, the problem of exact size is easily overcome by custom making your own. This is covered well here : http://modelenginenews.org/techniques/cylhone.html Brass is better than copper because it is harder, easier to machine and readily available. Diamond grit embeds readily and works so much better than AlO or SC.

One disadvantage of commercial barrel laps is that they are "barrels" . Having tapered ends makes a uniform bore size at the ends a little tricky unless you can control the lap stroke and position very well. The shop made lap only has one tapered end so uniform lapping to the end of the cylinder bore is easier.

WOB

 

[mayhugh1]

Is it possible to resize a barrel lap? I want to lap a 1 1/16 bore. 1" and 1 1/8" laps are available. Could the 1 1/8" be turned down to 1 1/16"?

I have tried making my own "D" type lap without a whole lot of success. Getting copper with the proper ID is the problem. Also commercial laps are available in the $20 range and new sleeves are only about $15. You cannot spend a lot of time for those prices. I will also have to look into making my own sleeve for other sizes.

Gordon,
I did exactly that for the cylinders on my Merlin. I modified the stock Acro 1-1/4" brass barrels by turning them down to 1.195".
Terry

 

[deverett]

Gordon
If you have an AcroLap, it is easy to make a new barrel to your size. I've made several custom sized barrels and also new stems as required.
https://www.homemodelenginemachinist.com/threads/hubbard-marine-engine.26387/page-2 #30.
It all depends on whether you have the 1-1/16" brass in stock. If not, it is probably cheaper to buy a new barrel and machine down as MayHugh suggested.

Dave
The Emerald Isle

 

[Gordon]

Thanks for the input. I will probably buy an Acro lap and an additional sleeve and give it a try. It looks like there are many variations on how to make your own. Some look pretty crude and some look like they require some pretty precise machining.

 

[deverett]

If you have a real Acro Lap, then it is not hard to see how to machine one for yourself. Have a look inside the barrel and see how they have made it.

Basically, drill through the brass so that the hole is about 1/4 the diameter of the large end of the adjuster. The first one I made I tried to copy the adjuster taper, but this isn't necessary - leave it parallel.
Drill a tapping size hole for the adjuster and stem threads.
Relieve the bore between the mid point hole and stem thread area, so that the wall thickness of the barrel is not more than about 3/32". The mid point hole where the adjuster bears needs to be about 1/2" long.
Thread the ends for the stem and adjuster and finally put 5 longitudinal slits in the barrel. If you find that it is too hard to adjust, try thinning the wall thickness a bit.

Dave
The Emerald Isle

 

[Gordon]

Actually I have never seen an Acro lap. When I get to the project where I need the lap in the next few weeks I will purchase a complete lap and an additional sleeve to modify. I have been trying to find the range of laps covered by each body. I anticipate needing laps for about 5/8 through 1 1/4 and I am not sure how many bodies that would require.

 

[StephenB]

another option would be a brake cylinder hone

 

[deverett]

Have a look at the sizes available and the relevant stem sizes.
http://acrolaps.com/index_005.htm

Dave
The Emerald Isle

 

[editor123]

It is true that commercial laps are cheap. However, the problem of exact size is easily overcome by custom making your own. This is covered well here : http://modelenginenews.org/techniques/cylhone.html Brass is better than copper because it is harder, easier to machine and readily available. Diamond grit embeds readily and works so much better than AlO or SC.

One disadvantage of commercial barrel laps is that they are "barrels" . Having tapered ends makes a uniform bore size at the ends a little tricky unless you can control the lap stroke and position very well. The shop made lap only has one tapered end so uniform lapping to the end of the cylinder bore is easier.

WOB

I agree that this style of a lap, which expands and contracts evenly throughout its length is the best possible lap. Acro laps are not a good solution for the novice because they are not uniform over their length and it is easy to non-uniformly lap a bore. The best engine builders I know, build and use the lap style described in Ron's ModelEngineNews.

 

[Gordon]

Dave: Thanks. I thought that I had read that each arbor handled a range of sizes. According to that each size needs it's own arbor.

 

[Gordon]

I guess that I am just dumb but I am not getting now the lap is made as shown in the model engine news. I assume that the arbor and the sleeve both have slits to expand but I do not understand why the arbor is tapered. If it is tapered it would seem like it should be tapered in the other direction.

It may not matter since I do not have a taper attachment and the article implies that a taper attachment is necessary for accurate taper. What taper are we aiming for and why is accuracy important?

 

[mayhugh1]

Gordon,
The larger diameter (one inch plus snd maybe slightly smaller, if I remember correctly) Acro barrel laps use the same arbor and tapered screw. The smaller ones have a much narrower overlap range. - Terry