Lapping cylinders - compounds

[Mike Ginn]

I am about to lap a cast iron cylinder using an expanding lap as per shown in Blondihacks video and in one of Joe Pieczynski videos. This type of floating lap seems to make sense but I am confused re the compound. I only want to use one compound otherwise I will need to make more than one lap. Blondihacks used valve grinding paste of 400 grit (which is about 20 micron) on the cast iron. Others use 5 micron diamond paste. In the past I have used the traditional brake cylinder 3 stone laps with moderate success.

Please can anyone advise on the best/workable approach to the lapping compound?

Thanks

Mike

 

[Charles Lamont]

I would not use diamond myself for fear of getting it embedded. I have 600 grit silicon carbide and would use that. For softer metals I also have 600 grit garnet paste.

 

[wazrus]

A set of 1-1/2" bore, cast iron cylinders I had made were inspected by a very experienced fitter, whose specialty was reconditioning hydraulic cylinders. He concluded that 'my' bore was straight and, very importantly, parallel. I was intending to lap them and he answered with a very emphatic "no, no, no!!" He went on to say that the surface finish was quite OK and the rings would do the lapping AND keep the bore parallel. He went on to say that, with the lapping, I'd probably bell-mouth the bores. Fair enough: so I didn't lap 'em and the engine chuffles along 40 years later. That all assumes that your cylinders are cast iron, of course!

 

[Jasonb]

I use silicon carbide powders mixed with a little light oil down to 1000grit often sold for the stone polishing hobby.

I work through the grits 240, 320, 600 and 1000. If you just use the last grit you will never remove the larger ridges and end up just polishing them, not found embedding into the lap to be a problem just clean out well with kero

Your brake hone will be fine if using iron rings I only tend to lap for viton rings or if not running any rings

 

[Mike Ginn]

Jason
That is very good advice and what I was looking for. I had hoped that I could re-use the expanding lap and you clearly find that they can be cleaned.

I guess I now need to decide if lapping is necessary!

Many thanks

Mike

 

[grahamgollar]

As an alternative to, or in association with some of the above it is worth considering the use of 'running-in' or 'breaking-in' oils.

Graham

 

[Mike Ginn]

Has anyone ever considered using wet/dry paper suitable held in an expanding lap. I have often wondered if it would work provided it could be held without tearing.

Mike