In need of a little cylinder lapping advice.

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Steamrguy

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So...
I am trying to lap a few cylinders. They are 1.5" diameter 303 stainless steel. I have read up on this some so I gave it a try. I made a lap out of aluminum about .002 smaller that the bore with a couple of grooves in it to help hold the compound. I used Time saver lapping compound. These cylinders are headless so the lap can not go all the way thru the cylinder. After lapping the first two there was a very noticeable taper in the lap. tapering smaller in the front. The lap was held true and there is no bel mouthing at the back of the cylinder but now I can not be positive the bore is true end to end!
What did I do wrong?
 
Steamrguy said:
So...
I am trying to lap a few cylinders. They are 1.5" diameter 303 stainless steel. I have read up on this some so I gave it a try. I made a lap out of aluminum about .002 smaller that the bore with a couple of grooves in it to help hold the compound. I used Time saver lapping compound. These cylinders are headless so the lap can not go all the way thru the cylinder. After lapping the first two there was a very noticeable taper in the lap. tapering smaller in the front. The lap was held true and there is no bel mouthing at the back of the cylinder but now I can not be positive the bore is true end to end!
What did I do wrong?
Click to expand...


it is not realistically possible to lap a blind hole and not get any taper (in spite of the fact that you can buy laps that are single ended for blind holes!)

in order to get a good lap you must have the lap move both circularly to even out the roundness of the bore and axially to even out any taper, barrel, or pincushion, of the bore, with a blind hole you can't do the later, that's also why you can't lap a taper (eg you can't "lap" a morse-taper) because you can't move the lap axially. you're screwed, your only real practical alternative, if it is so critical that it needs lapping, is to re-design the part and its surroundings.

Sorry to be the bearer of bad news, but I've lapped countless cylinders and liners, and countless main crankcase bearings, and countless cam shaft bearings, and failed at various forms of tapers, and this is how I know what I know, might not be true for everyone, but its true for me.

some personal (works for me) recommendations, if you just need to clean up the surface finish then an AccuLap (brass and pretty short for their diameter, diameter adjustable for wear, on a 1/2" shaft) will work, if you need perfection then get a HelicalLap (steel, long, adjustable) and their 1200 grit alumina as perfection requires a lap that is much longer than the part you're lapping, there's really no getting around this fact.

Also, its very difficult to lap a cylinder to an exact specified ID, you'll have to practice at it, don't expect to get it right the first try, make extra blanks. you have to first machine the bore to darn near where you want it, because with a fine finishing grit lapping compound you won't be able to do much more than remove the machining marks (you'll be taking maybe 0.0001" at a time, washing the part clean, measuring it, repeat until you're there, and if you have more that 0.001 to go from the start you'll die of boredom before you get there, and if you try to machine too close to the final dimension some of the machining marks will be too deep to lap out, its all trial-an-error with lots of error !

(also SS 303 might not lap well, its very soft, and if its thin walled it will all too easily crush to become out-of-round, but I'm wishing you lots of good luck and good fortune !!!)
 
Last edited:
peterl95124 said:
it is not realistically possible to lap a blind hole and not get any taper (in spite of the fact that you can buy laps that are single ended for blind holes!)

in order to get a good lap you must have the lap move both circularly to even out the roundness of the bore and axially to even out any taper, barrel, or pincushion, of the bore, with a blind hole you can't do the later, that's also why you can't lap a taper (eg you can't "lap" a morse-taper) because you can't move the lap axially. you're screwed, your only real practical alternative, if it is so critical that it needs lapping, is to re-design the part and its surroundings.

Sorry to be the bearer of bad news, but I've lapped countless cylinders and liners, and countless main crankcase bearings, and countless cam shaft bearings, and failed at various forms of tapers, and this is how I know what I know, might not be true for everyone, but its true for me.

some personal (works for me) recommendations, if you just need to clean up the surface finish then an AccuLap (brass and pretty short for their diameter, diameter adjustable for wear, on a 1/2" shaft) will work, if you need perfection then get a HelicalLap (steel, long, adjustable) and their 1200 grit alumina as perfection requires a lap that is much longer than the part you're lapping, there's really no getting around this fact.

Also, its very difficult to lap a cylinder to an exact specified ID, you'll have to practice at it, don't expect to get it right the first try, make extra blanks. you have to first machine the bore to darn near where you want it, because with a fine finishing grit lapping compound you won't be able to do much more than remove the machining marks (you'll be taking maybe 0.0001" at a time, washing the part clean, measuring it, repeat until you're there, and if you have more that 0.001 to go from the start you'll die of boredom before you get there, and if you try to machine too close to the final dimension some of the machining marks will be too deep to lap out, its all trial-an-error with lots of error !

(also SS 303 might not lap well, its very soft, and if its thin walled it will all too easily crush to become out-of-round, but I'm wishing you lots of good luck and good fortune !!!)
Click to expand...

peterl95124, Thanks for the info. I understand the pitfalls of lapping into a blind hole,​

I have built engines before that run well and I did not lap them, This engine I thought to go an extra mile so to speak. I am finding a .002 to .003 taper in the cylinders, larger at the head (blind) end. perhaps I was not holding the lap as true as I thought I was. all I really wanted to do really was take out the machine marks. ( I did accomplish that) I do not need perfection.​

 
peterl95124 said:
it is not realistically possible to lap a blind hole and not get any taper (in spite of the fact that you can buy laps that are single ended for blind holes!)

in order to get a good lap you must have the lap move both circularly to even out the roundness of the bore and axially to even out any taper, barrel, or pincushion, of the bore, with a blind hole you can't do the later, that's also why you can't lap a taper (eg you can't "lap" a morse-taper) because you can't move the lap axially. you're screwed, your only real practical alternative, if it is so critical that it needs lapping, is to re-design the part and its surroundings.

Sorry to be the bearer of bad news, but I've lapped countless cylinders and liners, and countless main crankcase bearings, and countless cam shaft bearings, and failed at various forms of tapers, and this is how I know what I know, might not be true for everyone, but its true for me.

some personal (works for me) recommendations, if you just need to clean up the surface finish then an AccuLap (brass and pretty short for their diameter, diameter adjustable for wear, on a 1/2" shaft) will work, if you need perfection then get a HelicalLap (steel, long, adjustable) and their 1200 grit alumina as perfection requires a lap that is much longer than the part you're lapping, there's really no getting around this fact.

Also, its very difficult to lap a cylinder to an exact specified ID, you'll have to practice at it, don't expect to get it right the first try, make extra blanks. you have to first machine the bore to darn near where you want it, because with a fine finishing grit lapping compound you won't be able to do much more than remove the machining marks (you'll be taking maybe 0.0001" at a time, washing the part clean, measuring it, repeat until you're there, and if you have more that 0.001 to go from the start you'll die of boredom before you get there, and if you try to machine too close to the final dimension some of the machining marks will be too deep to lap out, its all trial-an-error with lots of error !

(also SS 303 might not lap well, its very soft, and if its thin walled it will all too easily crush to become out-of-round, but I'm wishing you lots of good luck and good fortune !!!)
Click to expand...
Good advice but it appears you wasted your time.
 
Steamrguy said:
So...
I am trying to lap a few cylinders. They are 1.5" diameter 303 stainless steel. I have read up on this some so I gave it a try. I made a lap out of aluminum about .002 smaller that the bore with a couple of grooves in it to help hold the compound. I used Time saver lapping compound. These cylinders are headless so the lap can not go all the way thru the cylinder. After lapping the first two there was a very noticeable taper in the lap. tapering smaller in the front. The lap was held true and there is no bel mouthing at the back of the cylinder but now I can not be positive the bore is true end to end!
What did I do wrong?
Click to expand...

I used to go with my father when he worked on engines in the field. He had a portable lapping and grinding tool which he often used. But most importantly before starting he would meticulously mike the cylinder bore. He said if a taper is there when you start it will be there when you finish. The tool will follow the contour. If you were working with one end closed most likely the tool will have an end point which I think will naturally create a taper since it can not reach all the way in the cylinder against the closed part of the cylinder. It might be possible to create a lapping tool that ensures it can reach the total depth of the cylinder. But you must ensure that the grinding or polishing elements have a constant pressure on them This of course is a matter of rpm and tool movement. Later I once asked him why he did not do that service anymore and his reply was it can be done so much better in the shop with the newer machines. I dont think you did anything wrong based on the tools you were using. Its part art, part science. You could probably modify the grinding tool but it gets down to will it improve the operation of the machine you are building. However there may be people on this site who have built or designed the tools you need to do the job you want to do.
 
I think you will have to make your lap adjustable, so that it can be expanded at the front end.. And I am assuming that bores are small at the blind end.

Bore a hole at the front (leading) end of the lap about 3/8" deep and leaving a wall thickness of about 3/16" on the outside of the lap. Saw the bored end of the lap into about 8 pie wedges, similar to the slits in a collet. Make a tapered plug for the front end of the lap, held in place by a socket head screw so that you can pull the tapered plug into the bore of the lap to expand it. From that point, it is just a bunch of tedious work lapping and cleaning and measuring. Over and over again. The lap will have to be shortened periodically to remove the portion that wears down. Again, experience, magic, and luck.
 
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