Wrist pin clearence

K

kimlinh

Senior Member
I want to put new pistons in a stock model A engine. The wrist pin diameters of the new pistons are smaller than the wrist pin diameters of the old pistons I will be replacing. ( It's a long story) The new pistons wrist pins will have a .002 inch Clearance between the rod bushing and the pistons wrist pin. This seems like a lot to me. My question is can I live with a .002" wrist pin clearance? Does anybody have any experience with loose wrist pins in a Model A?
 
Don't even think about running those pins at .002 inch. You will hear them all the way down the street. :)
 
New bushing time. Find a shop that knows what they are doing. The bushings need to be expanded/burnished in the rod eye prior to machining them. Either they need to be bored using the big end bore as a reference or honed and then the connecting rod pins aligned to the big end. Clearance should be on the order of .0002/.0004". If the shop wants to just press the new bushings in and then hone them, find another shop.
 
The pins should be a tight push fit in the rod ends and pistons. Some engines require the pistons to be heated to get the pins in.

While you have the rods at the shop, have them check the rods for alignment between big and small ends. Plus, have the weights checked and adjusted if necessary to make them all the same within reason.
 
The Ford Service Bulletin of September-October 1932, Page 54, notes that the piston pin fit in the connecting rods should be 0.0002" to 0.0004" Clearance. The piston pin fit into the pistons at a temperature of 70F is a slight interference fit so that the pin floats in the piston when the engine is at operating temperature. The procedure for fitting piston pins is elaborately covered in the service bulletin.

However, I suggest that you buy pistons with the pins already fitted, and ream the connecting pin bushings to achieve a clearance of 0.0002" to 0.0004."
 
Dan McEachern said:
New bushing time. Find a shop that knows what they are doing. The bushings need to be expanded/burnished in the rod eye prior to machining them. Either they need to be bored using the big end bore as a reference or honed and then the connecting rod pins aligned to the big end. Clearance should be on the order of .0002/.0004". If the shop wants to just press the new bushings in and then hone them, find another shop.
Click to expand...

Dan is right! .0002/.0004 will give you a light palm push into the rod, with oil.
 
Flathead said:
Don't even think about running those pins at .002 inch. You will hear them all the way down the street. :)
Click to expand...

You got that right! I recently tinkered on a doodlebug that had that wrist pin noise, loud as shaking a can of pennies into a microphone! I hate that ******
noise
 
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