piston clearance

Hi Can anyone tell me the piston clearance on a fresh 30 thou oversize rebore on a 401 Buick nailhead thanks

 

.005-.007 should do it, unless you are planning on a blower or turbo.

 

It depends on the piston type you are using, cast or forged? They each use different amounts of clearance. Skirt design and taper also dictates the clearance along with piston material type.

 

This is for Stock Cast pistons, any excess clearance on the cast piston will probably result in cracking. With a forged piston, use the piston's manufacturer specs.

 

WTF!! you obviously have never built or even looked at specs for piston clearance !! Cast pistons with that clearance would sound like hammers inside the engine.

 

This is a loaded question for sure.
Manufactures measure their pistons in different areas depending on material and design.
If you know the manufacturer contact them for a clearance and where to measure from.
Chances are they are cast,forged haven't been readily available for years and custom forged are very expensive.
Just had a set from Ross made and they suggested to start at .0050 for their forged piston.measured at the bottom widest point.
I have fitted pistons that I had no idea of manufacture or specs,I would put it on a hot plate to 250 degreese and measured it with a micrometer and welding gloves too see how much it would grow.
Depending on application would determine how much clearance I would add to the piston mesurement.

 

Std engine .0015 to .002, forged pistons .004 to .005, blower pistons or racing pistons .006 to .007 maybe .008.
It all depends on the application. Cast pistons don't expand that much where as forge do expand and racing/blower pistons have to dissipate a lot of heat and therefore expand the most. That's a brief breakdown

 

Its not ment to be a loaded question the machine shop just needs the piston clearance

 

What makes it loaded is the variables, for a general rule of thumb if you are using forged pistons .0010 per 1" of bore will get you close. Cast pistons will work at whatever Buick says unless otherwise stated by the piston manufacturer and hyeruetecs (sp?) are different from all of the above.

The reason that some pistons come with a recommendation from the manufacturer is that today there are some many different alloys used and thermal expansion can very from one manufacturer to the next.

More often than not the piston is measured just below the wrist pin on the skirt. To get an accurate measurement you want to measure the piston where is it least likely to flex.

 

The Old Country Mercedes Mechanic would simply say; "GUTTENTITE"

 

Oops. I should have said "..if you..." instead of "unless." Silly me.

As has been said, it all depends on application and type of materials. Heating up the piston to measure it is pointless unless you are planning on heating up the block as well during machining. For our purposes, iron blocks and aluminum pistons have approximately the same rate of expansion anyway.

 

Hey Snot: Yep I too have been bitten by the wrong turn of words. Piston clearance really depends on more than cast or forged. Material and size of piston also enters into it. Back in the 60s I set up a Mopar 383 with .007-.008 piston clearance and as I recall forged pistons. It was quiet and long lived street car that set a NHRA Winter nationals class record after several thousand steet miles. One bike I had with the pistonclearance set were it would not seeze sounded like the hammers of hell, tighten up the clearance a few thou and it would seeze. Piss poor design of a piston ,good old MC supply product ,'way to much skirt taper. When I build a engine for any sort of performance I always have the shop finish to the top of the recomended clearance.

 

There was a time when the starting point was to finish the bore to nominal - 0.000" + 0.001 inch and the oversized pistons would have the clearance "built in."
I would still want to locate the (piston) manufacturer's spec, and measure every piston.
Even Rules of thumb like where to measure the piston are kind of risky as that varies with manufacturer too.

 

Well if that's the case all pistons are set up to the same clearance,man the directions they give with piston sets must be meaningless.
Now I know.

 

Motors calls for .003 for stock pistons.

 

Hi thanks for all your reply's Russell from Centerville auto repair where I got my pistons from has given me a clearance size of .002