Piston to bore clearance

[motogman]

I am sending a couple of cylinders (82.5mm) and pistons to Millennium to get the chrome striped and Nikasil plated.  When I talked with them they asked what the clearance spec is and suggested 0.003.

The question here to the experts is... 

1)  What was the clearance spec on a chrome bore set up
2)  What clearance did Guzzi use once they went to Nigusil bores
3)  What have others used when getting bores replated at Millennium?

Thanks

[Vasco DG]

If they've done a host of them, and it seems Charlie uses them regularly so my guess is they have, then they will of got it down pat one assumes. Do they cam grind the pistons prior to plating the bores?

Pete

[motogman]

Do they cam grind the pistons prior to plating the bores?

Pete

I don't know.  I wasn't smart enough to ask but I will when I call them this week.  From the previous discussions I have had with them, I don't think so because they did read me the "boiler plate" warnings about using used pistons etc. and did not offer a piston grind.

[motogman]

BTW...  I do hope and expect Charlie and a couple others to chime in to these questions and they have been extremely helpful so far.

Perhaps Flap Jack has a comment on this piston to bore clearance question?

[Antietam Classic Cycle]

1) piston-to-cylinder clearance on the original chrome bores is ~ .002"
2) from what I can tell by looking at the specs in shop manuals, Nigusil equipped Guzzis ran slightly tighter clearance, ~ .0013"
3) I don't tell Millennium any clearance to use, I trust that they know their job better than I ever will.

AFAIK, Millennium does nothing to the pistons I send them. They look exactly the same as they did before I sent them.

[lucian]

I am quite sure they don't cam grind the piston but will adjust the bore / hone to spec. If there is still chrome  one of the bores I am sure they will measure the clearance with your piston before  they proceed.

[motogman]

My understanding is that they strip the chrome, do any repairs if needed (my chrome is still in good shape so no repairs should be needed), electroplate on the nickle-silicon carbide and then hone to tolerance based on piston measurements.

[motogman]

I went into some factory manuals (thanks to Greg Bender’s fantastic web site) to do some research into piston to bore clearances and wanted to share what I found.  Note units:  um are microns (ie 1/1000 mm) and the " dimensional estimates are in thousandths.

V7 Sport – chrome bore
Cylinder   82.500-82.006   Min  36 um  ~ 1.5 “
Piston      82.458-82.464   Max 48 um ~ 2.0 “

850 T – chrome bore  (also LeMans II)
Cylinder   83.000-83.006   Min  26 um  ~ 1.0 “
Piston      82.968-82.974   Max 38 um ~ 1.5 “

850 LeMans – CI sleeve
Cylinder   83.000-83.009   Min  55 um  ~ 2.0 “
Piston      82.936-82.945   Max 73 um ~ 3.0 “

G5  – CI sleeve
Cylinder   88.000-88.009   Min  71 um  ~ 3.0 “
Piston      87.920-87.929   Max 89 um ~ 3.5 “

LeMan IV -  NiSiC  Cali 1000 the same
Cylinder   88.000-88.006   Min  26 um  ~ 1.0 “
Piston      87.968-87.974   Max 38 um ~ 1.5 “

So it would seem that the chrome bores and the Nikasil bores run tighter tolerances than the CI bores. 

I will be asking some more questions to other Nikasil designs to see what I find.

[n3303j]

Aluminum piston and cylinder can run a closer initial clearance because of a similar coefficient of expansion of the two parts.
Aluminum expands faster and more than cast iron so the combination requires more initial clearance.
At full operating temperature the fits will be similar.

[mtiberio]

Aluminum piston and cylinder can run a closer initial clearance because of a similar coefficient of expansion of the two parts.
Aluminum expands faster and more than cast iron so the combination requires more initial clearance.
At full operating temperature the fits will be similar.

I'm sure you meant to say "because aluminum expands more than cast iron, the combination of aluminum pistons and plated aluminum cylinders requires less clearance than with cast iron cylinders".

Can't believe anyone (not the author I responded to above) suggested grinding aluminum pistons. Never grind aluminum. As far as having to change the "cam and barrel" shape of the pistons. Going from chrome bore to nikasil does not require a shape change. Trying to use Iron bore pistons in a plated cylinder technically is sub-optimal.