Oversize piston question

Terry/Iowa

Terry/Iowa

Junior Member
I am working on rebuilding an engine that I picked up at an estate auction. The block had been rebuilt with new valves, babbitt, and lifters before the fellow rebuilding the car had died. I am ready to order a set of pistons for it and measured the bore. The cylinders are .0555 over. Does anyone know of a source for .05 over pistons? I have a new set of .06 over that they might have been thinking of using, but I would think they will be too tight. I am trying to avoid taking down the block again and having the cylinders bored again. Any suggestions would be appreciated.
 
Have you measured all four holes checking for round and taper? There is a problem somewhere and it's best to know where it is now. If the cylinders are true (as if freshly bored) and your measurements are correct then the pistons won't even slip in the holes.
 
As marco said don't think that their is uniformity in the wear. I started with 20 over pistons and by the time the machine shop got done we had to go to .80 over because of number 3 cylinder.
 
Thanks all for the advice. The cylinders were re-bored by the previous owner and now measure 3.9305 each. Since I can only find new pistons in even sizes ie .02, .04 etc I guess I will try honing the .0005 to make the ones I have fit.
 
Terry:
take it to the machine shop and have them hone it out to the final size for the clearance of the pistons. Usually the pistons will measure 3.932/933 so they have to final size to 3.935/936"
 
Terry/Iowa,
Have you ever honed cylinders to fit pistons before? If not, there is a LOT you need to know.
 
I'm assuming your measurements and paper work is correct. When the cylinder(s) are
bored...they are left undersize. Then they are honed too the correct finished size
with the clearance for each piston. Is this where your at?

"I guess I will try honing the .0005 to make the ones I have fit."

Is that what you meant ".0005" " or this .005"?
 
Terry/Iowa said:
Thanks all for the advice. The cylinders were re-bored by the previous owner and now measure 3.9305 each. Since I can only find new pistons in even sizes ie .02, .04 etc I guess I will try honing the .0005 to make the ones I have fit.
Click to expand...

Have you measured the the piston skirts? Granted, specs indicate they should be too large for the bores based on your measurements but the cylinders should be sized to the actual pistons (each) and not to what they are stamped. Just for ducks see if any of the pistons will slip in the holes, and if so check the clearance of the thrust side if the piston skirt with feelers.
 
I agree with J&M, have a machine shop do the final hone to fit. It can take awhile to hone 5 thousands.
 
I'm a retired Machinist/tool and diemaker also a shade tree mechanic. First off, Don't include half thousandths in your measurements (.0005) That level of accuracy just isn't needed and it's very confusing. .005 is ten time larger than .0005, So what did he really mean? Another often overlooked measurement is at the bottom of the bore/cylinder. I have a block that is .010-.012 over at the bottom and .036 at the top. Cylinders don't wear uniformly. Honing can take a while. I took me two days to hone a Kohler 1 cyl. to .010 over. Very tedious work. Also, when fitting pistons, there is the ring gap. This is the gap between the ends of the ring when it's compressed inside of the cylinder, Very important! Make sure you get this right. Too small and the rings expand in the cylinder when they get hot and break. Another thing is the "Cross hatch". The pattern of the stone marks as you raise and lower the hone. It sort of comes with practice, but the lines going down should intersect with the lines coming up at somewhere around to 60 deg. It's a case of how fast do I rotate the hone versus how fast do I lift or lower the hone. This helps the rings and cylinder walls break in better. Hope this helps!
Terry
 
So that everyone knows what "Cross hatch" is; here's a Model A cylinder done properly.
Cross Hatch aids in the lubrication of the rings and piston as it travels up and down the cylinder bore.
Typically the criss cross should be 25-35 degrees.

Furthermore there are different stones for different pistons rings.
I often hear the wives tale of Moly rings taking longer to break in.
Which isn't the case if the correct grit stone is used.
 

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Be sure to allow enough piston-to-wall clearance. Ford said a max of .002" but today's pistons need more, a minimum of .003" in my opinion. I bore to +.003 then finish with a light hone, about another half thou. Honing is a pain unless you have a power hone like most engine shops have.
 
When I was 17,a local garage owner let me use his Sunnen cyl hone to"bore" the badly worn cyls in a B block to fit a set of used .060 OS pistons.The cyls were .040 & most of the honing was in the lower 1/2 of cyls.The job turned out very well.
 
You should be able to tell if the bore is rough bored, or honed by looking at the surface -in the picture the center cylinders have been honed some, the front and rear are rough bored.

If you want to try it yourself you need a rigid hone, not a spring loaded glaze breaking hone ---in the picture are Aamco rigid hones, not the best but can give good results--perhaps not as quick, or as easy as a 10,000$ machine but can usually be bought on e-bay for under 100$, many times the better sunnen hones are double that
 

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