Hi, i've bought new pistons, they're speed pro's .030 over with coated skirts (h631cp030). Since I'd like to have engine blueprinted, piston's diameter should be measured to calculate exact bore size. The point is, manufacturer's instructions say:

"do not measure the skirt and compensate for the increase in diameter due to the coating as this could lead to excessive clearances and piston failure"

ok, the goal is to reach .0010-.0015 piston-to-bore-clearance. how can i find bare, uncoated piston's size?

Of course I can bore cylinders to 4.030000000 (as far as machinist's tolerances let), but this wouldn't be based on any real measurements, just theory...

Your help will be appreciated

Mike

 

.001" = .0015" is too tight.

The pistons expand more than the block when they heat up. You need enough clearance when they're cold so that they are still no larger than the bore when fully heated up. If the pistons expand more than the clearance, they will seize to the bore and destroy themselves. A little too much clearance will not hurt anything, and is far better than not enough.

.003" - .004" is a better figure to aim for.

Don't bother trying to measure them. Instead, have the block bored and then honed such that a feeler gauge will just barely fit into the bore along with the piston.

 

Thank you for quick reply What should be thickness of this feeler gauge?

 

 

ooops

 

That first clearance spec you posted is right. That's what the clearance should be WITH the coating.

I have a box of those and they say
If you look on the box it will also say
What they meant by don't measure then compensate is exactly what RB told you to do (no offense RB ).

I know it sounds out of the normal relm, but here's a quote from their website... http://www.federalmogul.com/cda/cont...5_6800,00.html
If you want to double check, just to make sure, call them

World Headquarters
Federal-Mogul Corporation
26555 Northwestern Highway
Southfield, Michigan 48034
U.S.A.

Main Telephone: 248-354-7700

 

I think I'll call 'em, just to make sure... there's no hurry, right now i'm polishing piston decks, after 2 hours first one appears to be in exact middle of processing :/

 

Well, if the mfr says .001", then .001" it is.... maybe they designed the coating to be smashed hard against the cyl wall.

But the technique would be the same; except use a .001" feeler gauge. Hone until you can just barely slide the piston in, then hone some more until the piston and feeler will just barely go in.

 

I do have a question for you RB.

When someone says "piston-to-wall" clearance, is that one side, or all the way around? As in, would the "side clearance" of the piston be .002" then?

Thanks.

By the way z_power,

It appears you lost a bearing.

 

Usually they mean on "one side", so to speak.... the difference in diameter between the slug and the hole. Not that a round hole has a "side", but....

 

Did you just call me a butt? :nono:

OK. That makes sense... I think.

That means .0005" all the way around. That's tight as a nun's ... well, you know.

 

my English is poor, what's "nun"? And what does it have, that is so tight? j/k