Cylinder Lap? Hone?

[cfellows]

I've been working on an engine cylinder, 1" bore x almost 3" inside length. The head is integral to the cylinder so is not removeable. I thought I had the cylinder finished, but today measured a slight taper. The bore on the head end of the cylinder was about .002" less than the outboard end.

Not being very familiar with lapping and not having a hone of the right diameter, I came up with the following:

It's made from a length of brass pipe nipple and the last 1.25" is turned to a tad less than the cylinder diameter. There is a slight reduction in diameter on the back end so only the last 1.25" does any work. I then cut 4 slits in the tube and sprung the ends out so the end would bear against the cylinder. I mounted the pipe in my lathe, smeared clover lapping compound into the cylinder and set to lapping the cylinder. Since the pipe is sprung outward it exerts more pressure on the end than toward the middle. Within just a few minutes of lapping, I was able to the get rid of the taper and achieve a very nice finish on the cylinder wall.

Chuck

 

[gmac]

Chuck; Nice work, have I got it right that there is no internal expansion plug, you just use the natural spring of the hone to exert force on the bore wall?
Cheers
Garry

 

[blighty]

silly question time.......

but not having done any lapping my self, do you turn the bore 1-2tho under then lap to size or turn to size then lap it just to get rid of the machining marks, sort of thing?

 

[cfellows]

Chuck; Nice work, have I got it right that there is no internal expansion plug, you just use the natural spring of the hone to exert force on the bore wall?
Cheers
Garry

Yep, no expansion plug. Just sprung the ends out and that makes it bear on the cylinder wall.

silly question time.......

but not having done any lapping my self, do you turn the bore 1-2tho under then lap to size or turn to size then lap it just to get rid of the machining marks, sort of thing?

I'm no authority on lapping either but I think, ideally, you would try to get the bore to maybe a thou or so under, then lap it to bring it to size and remove the machining marks. In most of my experience, the finished bore isn't critical and a couple of thou under or over doesn't matter... you just machine the piston to fit.

Chuck

 

[Don1966]

Chuck I have read from the late Bob Shore that to lap a cylinder leave it 3 to 2 thousand smaller then the final bore. He also said that to lap a cylinder the lap must be at least 1/2 to 3/4 the length of the bore. In looking at you lap does it spring more at the end then the full length of its body. Would that create a problem if the end is not moved all the way across the length of the cylinder? And forgive me, but I want to know how to lap the correct way. So if any one knows about lapping please give us a reply?

Regards Don

 

[blighty]

you just machine the piston to fit.

i have made the pistons already.

i have had a play with a cylinder and the grinding/lapping paste i got was valve grinding past you get for redoing your valves in you big car. it comes in 2 pots, rough and smooth, but the smooth seems to rough....... i guess I'll have a hunt on ebay.

i did make the bore about .001 ish to big ..... well .02mm to big in my language.


plus i don't see how you brass lap works. i thought the bit that does the lapping should be parallel to the bore. in your lap and others i have seen, one end would be bigger than the other when you expand the lap. if this is this case, only a very small part of the lap is touching the bore. or is that how it is meant to work?

 

[cfellows]

Chuck I have read from the late Bob Shore that to lap a cylinder leave it 3 to 2 thousand smaller then the final bore. He also said that to lap a cylinder the lap must be at least 1/2 to 3/4 the length of the bore. In looking at you lap does it spring more at the end then the full length of its body. Would that create a problem if the end is not moved all the way across the length of the cylinder? And forgive me, but I want to know how to lap the correct way. So if any one knows about lapping please give us a reply?

Regards Don

Don, the method I described here probably isn't the best for general lapping. In my case, I wanted to remove a taper,so it made sense to me to have it bearing harder on the smallest diameter. Also, while lapping, I spent a higher percentage of time on the smaller diameter end and not so much in the middle and only a little bit on larger diameter end. I should think for general lapping you would want a longer lap that expands the same for the entire length, like a barrel lap. I think what I've done here is more like a hone than a lap.

Chuck

 

[cfellows]

i have made the pistons already.

i have had a play with a cylinder and the grinding/lapping paste i got was valve grinding past you get for redoing your valves in you big car. it comes in 2 pots, rough and smooth, but the smooth seems to rough....... i guess I'll have a hunt on ebay.

i did make the bore about .001 ish to big ..... well .02mm to big in my language.


plus i don't see how you brass lap works. i thought the bit that does the lapping should be parallel to the bore. in your lap and others i have seen, one end would be bigger than the other when you expand the lap. if this is this case, only a very small part of the lap is touching the bore. or is that how it is meant to work?

I actually used clover fine valve grinding compound on mine. I was more interested in removing metal than getting a super fine finish. However, the finish still turned out pretty smooth.

I turned the pipe to be about the same size as the bore of my cylinder, so even though I spring the ends out a bit, when I inserted them into the cylinder, the brass pipe was about parallel with the bore, or at least parallel with where I wanted to get it to. When I first started lapping, I'm guessing pretty much the whole lap was contacting the cylinder, but the lap was pushing harder on the smaller diameter so it removed material there faster. As material was removed, I'm guessing the outward pressure was more evenly distributed.

At any rate, lapping removes material pretty slowly if you don't spin it too fast. It's easy enough stop often and measure progress to see how you are doing.

Chuck

 

[Don1966]

Thanks Chuck for clearing t that up for me.

Don

 

[Stan]

In some respects, a model IC engine is no different than the full sized one. The fine valve grinding compound from the auto parts store will finish valves to hold much higher pressures than we obtain in models. After boring a full sized engine cylinder, you have to cross hatch the surface with a hone to provide some place for lubricating oil. You don't want a highly polished finish on the cylinder bore when you first run the engine.