Piston Clearance Question
 

About 60 years ago, my brother purchased 2+ complete Model A motors all in pieces. His idea was to have "spare parts". Till now, that's all they ever amounted to while being stored under our dad's house and, later, at my brother's storage shed.
Recently, I learned my brother was looking to "make space" so I asked him to "donate" those two motors so I could attempt to assemble one, on a display stand, from all the best parts (some of us are a bit "strange" and actually "enjoy" work). I'm just getting started with this process. Have sorted parts and am working on some cleanup and parts sourcing (little Dearborn).

Finally, my question: Can someone tell me what the piston to cylinder clearance be on a Model A engine?? The motor I'm going to use is a 31 and appears to never have been rebuilt. It still has "Ford" pistons that appear to be standard bore. Babbitt looks thin (mains) but, otherwise good. Rods look really good!!

Thank you.

Re: Piston Clearance Question
 

If you are to purchase new pistons for this build, purchase Silvo-lite pistons from United Engine Machine. They are superior to the ones offered by the major suppliers. Set the clearance to .0035".

This is based on personal experience. I purchased the pistons offered by the major suppliers and my engine machinist set the piston to cylinder wall clearance to .0025". I was getting a lot of metal in the oil; opened the motor to find scored cylinder walls and pistons.

Re: Piston Clearance Question
 

Search "Model A Babbitt Bearings" on this site. There's a lot of info there!

Re: Piston Clearance Question
 

As a rule of thumb, a thou per inch of bore is good.

Re: Piston Clearance Question
 

The original pistons were set very tight, around .002-.0025".

As Synchro mentioned, the rule of thumb is .001" for each inch of bore size.

I'd rather set them a bit loose rather than too tight.

Re: Piston Clearance Question
 

I think the rule of thumb is for overhaul of a used engine, whereas 0.002" is for newly machined iron cylinder bores.

Re: Piston Clearance Question
 

For a stock 'A' engine, .003" minimum, .004" for a performance engine, more if using forged pistons.

Re: Piston Clearance Question
 

There are millions of cars running around with bearings and bores a bit too lose. There are NO cars running around with them too tight.

Re: Piston Clearance Question
 

LOL. Not for long anyway.

Re: Piston Clearance Question
 

Solid skirt pistons need more clearance than the split skirt style originally used by Ford. Using the clearance originally specified by Ford will cause potential failure if solid skirt pistons are used. The Ford specs are for split skirt pistons. I too would recommend Silvolite pistons. The Chinese pistons need more clearance than specified on the packaging IMO.

Re: Piston Clearance Question
 

Thanks for all the input everyone. I'm anxious to measure everything on this 1931 engine with what appears to be the original (Ford Script, split skirt) pistons. Only measured one so far and the block is currently soaking in a electrolysis bath.

Re: Piston Clearance Question
 

Your main concern should be the condition of the mains! The cylinders can be easily and cheaply (relatively speaking) rebored. But repouring the mains, not so much! Make sure your solution is not etching the Babbitt. Mike and then drop in the crankshaft and check the main bearing clearances before spending any money on the block. If you can't get the bearing clearance within spec by removing shims it's time for new Babbitt or inserts.