Not my area of expertise – I've only done 5 small bores, max size 25⌀ x 25mm.

The lapping rod should be softer than the cylinder. I used wooden and Aluminium rod turned to an approximate loose fit with a succession of fine emery papers wrapped round them. Started with 1000 grit, and moved up via 1500 and 2000 to 3000 or 4000. 4000 was probably overkill.

The cylinder was spun in the lathe and the rod/paper combo was rotated slowly and slid to and fro slowly in the bore by hand. Fine papers remove tiny amounts of metal, basically polishing only, so I felt there's not much danger of spoiling a bores parallelism or circularity.

I experimented with valve grinding paste and toothpaste once: in my hands seemed a lot more messy work than emery paper. Three grades of valve paste came in a kit. The coarse paste was too coarse, and the gap between fine valve paste and toothpaste is probably too big.

Paper worked well enough for me, but I wouldn't bet the farm on it being the best answer: see what others say.

Dave

Steve

If the liners are brass are you hard chrome plating them to run with a high silicon content ringless aluminium piston.?

Viewing your Album it seems you only need one other gear to connect the 2 idlers for a complete train, are the gears fitted not up to the job ?

Emgee

Edited By Emgee on 02/01/2022 19:15:13
typo again 

Edited By Emgee on 02/01/2022 19:17:42

Same could be said for twin overhead camshafts on an air engine I suppose, however that is the design of the engine so best to stick with it.

If indeed it is an air powered engine best guess is it will be a pistons with 1 "O" ring fitted, no doubt we will get some info on this from the OP.

Emgee

Whether an OHC, or DOHC design is 'appropriate' or 'over the top' depends on whether valve bounce is a limit to the output without this added cleverness. But it also depends on what the engine is for. If 'merely' to produce power, that is one question, but how about if it is intended to demonstrate the abilities of the designer, or maker, or both?

Cheers, Tim

Hadn't seen the other posts.

Emgee

Flap rod polishing does work very well. So does making a spring lap out of something softer than the material of the liners. To polish liners but not alter the shape, I have used hard balsa wood to polish and it works amazingly well. I know people who have also used other materials like tufnol to make polishing laps. The Op wants to to keep the roundness, as well as the the bore being parallel . I have had some great success of making laps that resemble the geometry of a hone. It does not take much out, but will polish very well, especially if the lap is a charged lap, so no extra compound sitting in the bore.

There is lots of ways to polish things and seldom is there a wrong way when the method used achieves the end results that is required.

Hi Steve – yes most definitely. Nearly all the laps I have made (quite a few now) have used copper as the wrapper but the principle is the same. I've not tried cyano as such but would think a gel type cyano best as only the leading edge of the wrapper should be fixed. Make sure the leading edge to the motion is the fixed edge – otherwise it tries to unwrap it i.e. the motion wants to be away from the fixed edge. The wrapper slowly looses its concentricity in it's thickness but the outside remains perfectly circular. I'd just use a 2mm grubscrew as the expander.

This type of lap expansion works extremely well and I have used them for many projects – not my idea but one that came from Len Mason in his building the Mastiff series in ME if I recall correctly

With regard to the wet and dry method make the paper width wider than the length of the bore but no more than twice the length of the liner as the more that's outside on axial movement the more it can loosen and expand and begin to lead to bell mouthing as it compresses going back in – yes axial movement should be used otherwise you will create rings but not excessively so compared to the radial movement – WD 40 will be a fine alternative to paraffin – I always have the latter on hand because of the diesels.

As you have a couple of spare practice first as you suggest – I'm sure you'll soon see how little action is required to achieve the surface smoothness you are after.

Regards – Tug

Edited By Ramon Wilson on 03/01/2022 21:01:09

The success achievable with abrasive finishing, and the amount of resultant diametrical enlargement, will depend on the present state of the bore. How much material removal can be tolerated (how deep are the present scratches/imperfections)?

A possible problem with abrasive finishing is the risk of embedding abrasive into the soft brass. Consider a non-embedding abrasive for lapping?

You could consider 'Ballizing' (I think that was the name of the commercial process), in which a highly finished ball, very slightly larger dia than the existing bore, is pushed along the bore. In brass, it should work-harden the surface usefully. It will retain, or improve, circularity, but will not straighten a 'bent' bore. Parker Hale offered this process for gun barrels. Compacting the working surface of the barrel was said to prolong barrel life. Probably impossible to get a ball of the desired size – for a reasonable price.