Hi,
I haven't been on here for quite some time due to other distractions
but as a result of our weekly chat's my friend Chris (Model steam) has just brought this to my attention.
I have two sets of Delapena external hones bought expressly for pistons and crankshafts. The round bars are indeed setting bars to bring the stones parallel but - as far as I am aware - the bar is held in the lathe and rotated and the stones brought onto the shaft (well lubricated of course) as if in use properly - this will set the stones (particularly new ones) and take out any potential slight taper.
The nub is that having bought them (and a bench mounted internal hone) I still prefer to lap pistons down with a brass lap made for the relevant piston. Having used them (the Delapena) I feel that the external hone definitely has a tendency to barrel the piston blank as it moves over either end whereas the brass lap doesn't. They do work extremely efficiently however and produce a nice surface finish. I have used them on crankshafts but again there can be a problem getting up to the shoulder near the web.
I can't see a stop would be any great advantage - particularly, as Jason remarks, if making the piston (or a shaft) to fit a bore. Even on a multi cylinder I would still want to do that as opposed to 'interchangeability' though that said a stop could help prevent over doing things 'per pass'
If I were setting out again would I buy them? Probably not as the results achieved so far using the laps and methods described elsewhere have proved more than adequate for the task in hand - others of course may feel differently.
Hope that helps some - hope everyone is well too - I'm afraid I'm a hopeless cause being well and truly distracted at present but 'Forncett' isn't too far off so maybe that'll be the catalyst for revival
Regards - Ramon