piston size help needed

mach0415

mach0415

Senior Member
Good afternoon! I am in the process of freshening up a 29 T.S. Compression test results showed low, so the head was pulled to inspect condition. A compression ring was stuck on one of the cylinders and 2 others had stuck valves. The one cylinder that had higher compression than the rest did not appear to have any stuck rings or valves. I pulled the pan to inspect bearings and found lower end stuff okay. I mic'ed the pistons to see which oversize they are, as it is clear someone along the way worked on the engine before.

The piston measured out at 3.920" at the top and 3.940" at the bottom of the skirt. I noted also that these are split-skirt pistons. My intent is to re-ring after a light hone. I have not measured ring gap, yet. So, if my specs are correct, that would put everything at a .045" oversize, but rings come in .020" oversize increments. I think I really need a .050" over set, but the next size up in .060" over.

What would you all do? I am not interested in a total rebuild at this time.
 
mach0415 said:
Good afternoon! I am in the process of freshening up a 29 T.S. Compression test results showed low, so the head was pulled to inspect condition. A compression ring was stuck on one of the cylinders and 2 others had stuck valves. The one cylinder that had higher compression than the rest did not appear to have any stuck rings or valves. I pulled the pan to inspect bearings and found lower end stuff okay. I mic'ed the pistons to see which oversize they are, as it is clear someone along the way worked on the engine before.

The piston measured out at 3.920" at the top and 3.940" at the bottom of the skirt. I noted also that these are split-skirt pistons. My intent is to re-ring after a light hone. I have not measured ring gap, yet. So, if my specs are correct, that would put everything at a .045" oversize, but rings come in .020" oversize increments. I think I really need a .050" over set, but the next size up in .060" over.

What would you all do? I am not interested in a total rebuild at this time.
Click to expand...

Std bore is 3.875 that is bore size not piston size. so 3.940 - 3.875 = .065
that would mean .070 over size bore. That is kind of a odd size. Are you sure you measured the piston right.

Why not check the bore size that is what counts.
 
Last edited:
they could be semi pistons they were cut to size in the 30`s. I have one engine aprt with 3 differant size pistons one is a 65 over ...Dave
 
Yes, I measured the piston diameter at the top, above the first compression ring with a digital micrometer (yes I calibrated it before I used it, hehe). I measured the bore size a month ago, as I was doing a general assessment and found the actual bore to be .054" larger than stock at 3.929" at the very top. I did not bring my snap gauges home to measure taper, out of roundness, etc. Even so, there would be a .009" difference from bore size to piston fitment size, right? Should I have measured the piston at the pin?

These pistons do not have a stamping of what oversize they are...that would have been too easy, I suppose.
 
mach0415 said:
Yes, I measured the piston diameter at the top, above the first compression ring with a digital micrometer (yes I calibrated it before I used it, hehe). I measured the bore size a month ago, as I was doing a general assessment and found the actual bore to be .054" larger than stock at 3.929" at the very top. I did not bring my snap gauges home to measure taper, out of roundness, etc. Even so, there would be a .009" difference from bore size to piston fitment size, right? Should I have measured the piston at the pin?

These pistons do not have a stamping of what oversize they are...that would have been too easy, I suppose.
Click to expand...

Now I'm confused you said the piston 3.940 at the bottom that is not going to go in a 3.925 bore.
Piston should be checked at the bottom perpendicular to the pin.
 
In choosing what rings to use, use the bore measurements not the pistons. Remember, if your at an unusual bore size, get the next size bigger ring and file fit the ring gap at the bottom of the bore.
 
Whew, I do not know what I did with measuring. So, I tried it again.

On the suggestion for piston ring installed and adjustment for end gap, will do. I have remeasured my pistons and found them to be 0.0755" +/- of stock piston diameter of 3.8745" So then, I will remeasure the bore to determine which rings to go with. Without measuring the bore more thoroughly, it looks like .080" over.
At which point do I need to consider oversized pistons, when there is more than .002" between it and the wall?
 
I think that I remember reading that Ford suggested that .006 taper was the time to bore to the next size. I think that he also said that many were probably running with .020 taper. Others can correct this if I'm wrong.
 
mach0415 said:
Whew, I do not know what I did with measuring. So, I tried it again..............it looks like .080" over.
At which point do I need to consider oversized pistons, when there is more than .002" between it and the wall?
Click to expand...

Go oversize pistons when the taper or out of round dictates boring, not before. You'd be amazed at how loose you can be with pistons and it will run a long time.
 
I don't think we can give you much advise with the info given.. I'm concerned about the measurements you've posted.. Your first post kinda stated you just wanted to perform a 'quicky' re-ring to get you by a few years.. Without knowing the bore measurements advise is 'iffy' at best.. Ford says .002" fit, general fit is usually .001" per inch of bore.. But it takes quite a bit more than that for the pistons to start to 'rattle'.. But from what you said the top bore measurement was, I suggest you be careful with the hone. It kinda sounds as if you may have quite a ridge to deal with..
 
You can get an idea of what your cylinder taper is with a piston ring, measure the end gap at the top, push it to the bottom and measure again ---the bore taper is approxamatly 1/3 of the eng gap difference ----when fitting new rings you have to fit the end gap at the bottom where the cylinder is the smallest.

If you look some you can find piston expanders---it is a spring that fits inside the split skirt pistons to make them a little larger to make up for wear --not as good as a proper job

If you hone the cyls in the chassis what are you going to do to clean out the grit from honing ???---any leftover grit will make it's way into the babbitt and work on grinding the crankshaft.

The picture is an example of how not to "fit" rings by grinding the ends, this engine had a fresh overhaul on a standard bore --+.015 at the top ---the new oil rings were broken (if you look close you can see the missing part of the ring in the picture-the lower ridge that touches the bore), it smoked bad
 

Attachments

  • IMG_0166.JPG
    IMG_0166.JPG
    117.9 KB · Views: 52
Looking at this again, something still isn't making sense.. If your pistons measure .0755 over 3.8745 as you stated in [#8] then that appears to be a piston diameter of 3.950.. Post #4 indicates a bore[top] of 3.929..Post #1 says the piston is 3.940.. I think I'd grab a set of snap gauges and a real micrometer and get dirty.. There was a time when pistons could be 'got' in .010" sizes and it wasn't uncommon to find several different sizes in the same engine.. A larger ring can be fitted to the smaller piston if required..
 
Yes, I have a set of snap gauges, but lack one big enough for this. Looks like I will be borrowing one from a friend of mine. Thanks. I will repost more accurate results. The piston diameter is 3.950" to be sure, though.
 
mach0415 said:
Yes, I have a set of snap gauges, but lack one big enough for this. Looks like I will be borrowing one from a friend of mine. Thanks. I will repost more accurate results. The piston diameter is 3.950" to be sure, though.
Click to expand...

When you say snap gauge are you talking about a telescoping gauge. To me a snap gauge would be a go no gauge.
 
A little note about telescoping gauges.

I bought a set of low end gauges. They did not work so well. Not very good at repeating the measurements. Found the pin that locks the plungers was denting the metal and causing it to snap to that position. Turns out they used brass with poor machining. So it did not work very good at first and got worse as you used it.

I medium quality set that is steel. I get repeatable measurements, but they do not feel quite right as you rock them to get the size. The milled flat area is not as smooth as I think it should be, but they work good enough.

My brother brought over his Starrett gauges and they work nice. They feel like they are getting the right measurement every time. Very repeatable. I will get a set one day.
 
Kevin in NJ said:
A little note about telescoping gauges.

I bought a set of low end gauges. They did not work so well. Not very good at repeating the measurements. Found the pin that locks the plungers was denting the metal and causing it to snap to that position. Turns out they used brass with poor machining. So it did not work very good at first and got worse as you used it.

I medium quality set that is steel. I get repeatable measurements, but they do not feel quite right as you rock them to get the size. The milled flat area is not as smooth as I think it should be, but they work good enough.

My brother brought over his Starrett gauges and they work nice. They feel like they are getting the right measurement every time. Very repeatable. I will get a set one day.
Click to expand...

Starrett is what I have, had them about 50 years. Used them many times.
With a telescoping gauge you should check the same place more than once, and make sure you get the same reading. Even with inside mikes it is a good idea.
If you make your living with precision tools, only the best will do. The cheep junk should not be used for close measuring. Same with mikes I have a couple pair that my Dad had when he worked at Gm back in the 40ts they are still right on.
 
Kevin in NJ said:
A little note about telescoping gauges.

I bought a set of low end gauges. They did not work so well. Not very good at repeating the measurements. Found the pin that locks the plungers was denting the metal and causing it to snap to that position. Turns out they used brass with poor machining. So it did not work very good at first and got worse as you used it.

I medium quality set that is steel. I get repeatable measurements, but they do not feel quite right as you rock them to get the size. The milled flat area is not as smooth as I think it should be, but they work good enough.

My brother brought over his Starrett gauges and they work nice. They feel like they are getting the right measurement every time. Very repeatable. I will get a set one day.
Click to expand...

The gauges I am referring to are 25 year old Craftsmen telescoping snap gauges. Are you all using this type or the dial bore gauge?
 
You must log in or register to reply here.
Back
Top