The Ford Barn

SofaKing · 11-02-2014, 02:23 PM

Hi All,
I have a 21 stud V8 that I am attempting to mic and I cannot find dimensions for the piston to bore clearance except the 6-10 Lbs of pull using a fishscale. I have purchased several books and done some web surfing to no avail. Does anyone have a source for an accurate number using a micrometer instead of a fishscale?
Thanks.

Ralph Moore · 11-02-2014, 03:32 PM

Not sure if this will help, but my book covers back to 1941 and piston skirt clearance is listed as .0025 with a limit of .005

supereal · 11-02-2014, 03:45 PM

The original "pull" is 6 to 12 pounds, with a wear limit of 6 to 10. This is measured with 1/2 inch wide feeler gauge between the cylinder wall and the piston. Use .003 for original, .005 for a used block. At our shop we use a special Sunnen gauge to measure the cylinders for wear, and a wide throat micrometer for the pistons.

rotorwrench · 11-02-2014, 05:41 PM

Piston clearance is always determined by the manufacturer of the pistons. Forged aluminum have to be set up more loose than stock so all the drag pull testing is moot for them. Only Stock Ford or Mercury pistons can be clearanced by the FoMoCo specs. Stock cast aluminum 4-ring pistons would be sized as mentioned in the previous posts. If it still has steel pistons, I don't have experience with them but I would consider changing to a cast aluminum replacement.

SofaKing · 11-06-2014, 06:28 AM

Thanks for the help guys. They are within limits so they will be going back in, they are aluminum 3 ring pistons with Ford script. I appreciate the assist.

barnfind · 11-06-2014, 10:10 PM

Sponsored Links (Register now to hide all advertisements)

I used a fish scale to measure ring tension in stock class race engines. Some brands were HP robbers.

11-02-2014, 02:23 PM	#1
SofaKing Senior Member Join Date: Jun 2014 Location: Salt Lake City, Utah Posts: 756	Fishscale Lbs to Inches Conversion for Pistons Hi All, I have a 21 stud V8 that I am attempting to mic and I cannot find dimensions for the piston to bore clearance except the 6-10 Lbs of pull using a fishscale. I have purchased several books and done some web surfing to no avail. Does anyone have a source for an accurate number using a micrometer instead of a fishscale? Thanks.

11-02-2014, 03:32 PM	#2
Ralph Moore Senior Member Join Date: Oct 2010 Location: North Pole, Alaska Posts: 1,470	Re: Fishscale Lbs to Inches Conversion for Pistons Not sure if this will help, but my book covers back to 1941 and piston skirt clearance is listed as .0025 with a limit of .005

11-02-2014, 03:45 PM	#3
supereal Senior Member Join Date: Jul 2014 Location: Iowa Posts: 1,634	Re: Fishscale Lbs to Inches Conversion for Pistons The original "pull" is 6 to 12 pounds, with a wear limit of 6 to 10. This is measured with 1/2 inch wide feeler gauge between the cylinder wall and the piston. Use .003 for original, .005 for a used block. At our shop we use a special Sunnen gauge to measure the cylinders for wear, and a wide throat micrometer for the pistons.

11-02-2014, 05:41 PM	#4
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,509	Re: Fishscale Lbs to Inches Conversion for Pistons Piston clearance is always determined by the manufacturer of the pistons. Forged aluminum have to be set up more loose than stock so all the drag pull testing is moot for them. Only Stock Ford or Mercury pistons can be clearanced by the FoMoCo specs. Stock cast aluminum 4-ring pistons would be sized as mentioned in the previous posts. If it still has steel pistons, I don't have experience with them but I would consider changing to a cast aluminum replacement.

11-06-2014, 06:28 AM	#5
SofaKing Senior Member Join Date: Jun 2014 Location: Salt Lake City, Utah Posts: 756	Re: Fishscale Lbs to Inches Conversion for Pistons Thanks for the help guys. They are within limits so they will be going back in, they are aluminum 3 ring pistons with Ford script. I appreciate the assist.

11-06-2014, 10:10 PM	#6
barnfind Senior Member Join Date: Jan 2011 Posts: 427	Re: Fishscale Lbs to Inches Conversion for Pistons Sponsored Links (Register now to hide all advertisements) I used a fish scale to measure ring tension in stock class race engines. Some brands were HP robbers.