Question - maximum piston to bore size

[Jack Tarlinton]

Hi all,

I have gone through a number of threads from previous years, and all seem inconclusive to say the least.

Simple question perhaps - what is the greatest tolerance between piston to bore with modern aluminium standard compression pistons?

I see some manufacturers recommend .002 to .003 and one thread I read someone posted up to .015, which seems a bit big to me!

Anyway, your advice is always appreciated. Thank you all in advance.

Best regards

Jack

[Kerry]

When I bore a T engine, .003/004", any tighter and you run the chance of the piston grabbing.

[Wayne Sheldon]

A common confusion comes from a difference of opinion in HOW that "clearance" should be measured. The diameter of the cylinder less the diameter of the piston? Or the radius of the cylinder minus the radius of the piston. In one, the clearance is the total amount, or in effect, piston pressed against one side of the cylinder wall and what the feeler gauge says the difference is 180 degrees across the cylinder and piston. THAT is the way nearly everybody USED to measure it and count it!
A lot of people today want and use a 360 degree measure (radius), basically half the total measure however measured all the way around the piston AND cylinder. This method can be more correct when figuring in cam ground pistons, which may or may not apply to model T rebuilds. But it is only more correct when the differences in the cam grind are taken into account. Some allowance should be made anytime cam ground pistons are used. (And don't ask me about cam ground clearances, I rarely use them.)

Common round pistons and model Ts are fairly simple. However, there is a lot of confusion created by the fact that model Ts originally used cast iron pistons. The coefficient of expansion for aluminum is much higher than it is for cast iron. And a cast iron piston expands very close to the same amount as the cast iron cylinder does when hot. So the clearance is very close, between .001 and .002 inch. A worn motor can actually run quite well all the way up to .010 inch, although it will rattle a little bit from time to time. Taper can also be run up to .010 inch without serious problems. It is actually best if the taper is nearly correct at the bottom of the cylinder, and the piston is a bit loose at the top of the cylinder.
I did a long time ago run an old cast iron wonder six with .019 Taper! Ran great, and for many years.
However, I digress.

Aluminum pistons expand much more than cast iron pistons do. Therefore, they require much more clearance! And for the full length of the cylinder. Modern machining practices for modern engines throw around numbers like .004 inch or .0035 inch clearance (diameter, NOT radius!). HOWEVER! Those figures WILL NOT work in a model T! Modern engines have full pressure oiling. And modern engines have full length water jackets, which cool the cylinders more evenly. The engines with pressurized cooling also run a bit hotter than non pressurized cooling engines generally do. Because of the cooling inefficiencies of a model T, the aluminum piston runs relatively hotter than does the cylinder, and therefore expands a bit more. They therefore require a bit more clearance (by either measure) than the same piston would require in a modern engine. A lot of people recommend .005 clearance (diameter!) for model Ts, however, I prefer just a bit more myself. I have seen engines set up at .006 inch have tightness issues until the engines got a couple thousand miles on them. I suspect more than a few rod bearing early failures I have seen were due to the cylinders being that bit too tight. For a model T, I consider .006 to be the absolute minimum clearance, and .007 inch a bit better (again, diameter, not radius). And again, that with round pistons. Cam grounds can be a bit tighter on the tight sides but must be a bit looser on the loose sides.

Again, wear and taper issues can be worse than a lot of people think. Many years ago, I rebuilt a 1970ish V8 for my work truck. It was NOT bored. Some wear and taper, a few thousandths. Except for one cylinder that had had a slight head gasket leak. That one had about ten thousandths wear and taper. Long story, I was in a rush for it, so I ran with it.
Believe it or not, that engine, without any significant work done after my rebuild, went well over 350,000 miles! I can't tell you how many miles over 350,000 it went? Because at 325,000 miles the speedometer/odometer drive gear in the transmission wore out and quit! I drove that truck for seventeen years! And I averaged about 30,000 miles per year. Do the math.

[Jack Tarlinton]

Thanks Wayne, that is helpful.

I have a set of brand new standard size 3.75 round aluminium pistons that came with my engine, and was told they should be useful.

One thing of note is that these pistons are undersize:

The widest measurement I'm getting is 3.734 just below the level of the pin hole - 90 degrees around from it. I would image that stock pistons would be around .004 under to allow rings to do their work - not .016 under

Now if I place these in the bore with the top of the piston at the top of the block and place feeler gauges in there I get a pretty consistent .020 to .024 gap across all 4 bores.

I think that illustrates some of my confusion in this matter - lets say:

you start with stock 3.75 and bore .030 over

you order pistons advertised as .030 over but the pistons are actually .016 under

This means that you're left with a lot of room for the rings to compensate.

Am I missing something or is this standard practice?

Thanks again

Jack

[Wayne Sheldon]

I wonder if the pistons were maybe cut/ground undersize for some reason? Do you know any history on them? They could have been ground undersize to fit a re-sleeve back to a standard minus? I have heard of people doing that as a way of not boring the sleeve too thin. I do not think it is a common practice, but just a thought?

I have never had much money to spare, so I do almost all my work myself, and usually refit used and/or original pistons back into engines that I rework for my own use. I have never run into standard pistons that were that undersized before myself. But like I say, I have heard of it a few times.

Did you check the cylinders for taper? They normally wear much more at the top than they do at the middle or bottom. When I fit pistons, I use a simple feeler gauge and the skirt of the piston. However, I check all four compass points (front, back, right side, and left side) at the very top, about the middle, and right near the very bottom of the cylinder. That is twelve points, and it gives me a good picture of bore, fit, oval, taper, and even egg shaped.
If cylinders look decent, I often use a flexible hone. They break the glaze and smoot minor imperfections. However, if I have any doubts, I prefer a ridged hone, which can with careful use actually clean up a cylinder to the next larger size if need be.
One model T block I had was thought to be .020 inch over, because they only measured at the top. I ordered .020 over pistons for it and then discovered it was standard at the bottom, and more than .015 over at the top! More than .015 inch taper! I used my ridged hone, and straightened all four cylinders to a beautiful fit with .007 inch clearance. It was a great running engine and served me well for several years. (Until I had to sell it.)

I notice that you are in NSW. However, you were using "inch" measurements. That really helps some of us up here. However, I have also worked with metric measurements for many years, and can translate if need be. Although I usually don't go down to the tiny numbers in metric. I use metric on larger numbers (millimeter plus?) quite often.

[Henry K. Lee]

Wayne,

Perfect explanation to others, Thanks from everyone! I do set mine at or around .002 per inch of bore.

Hank

[Kerry]

To measure modern aluminium pistons at the wrist pin is the cam grind, you need to measure on the thrust face of the piston/bore for clearance. Sticking the piston in the bore and measuring the top at the ring lands is mute as well as the top is under cut as well.

[Jack Tarlinton]

Wayne,

I may be in Australia, but these are imperial engines and it makes sense to all to use imperial measurements in this case. Thanks for your compliment regarding this - I personally would rather use metric, like NASA, but this isn't the place for it. Ha ha.

I am not a mechanic by trade, so I am compelled to ask questions in order to learn.

To clarify, the pistons were purchased from Snyders this year, brand new, in the box - pins are marked 2019.04.08. I am less interested in my current bore size because I can have it bored oversize and use different pistons.

What troubles me is that how do I know what pistons to order when the advertised size does not reflect actual size?

I am using a brand new pair of digital vernier caliber purchased 3 days ago.

The pistons measure at the widest point across the round piston:

Lip above top ring - 3.725

Between rings 1 and 2 - 3.726

Between rings 2 and 3 - 3.726

Between ring 3 and pin hole - 3.730

Immediately below pin hole - 3.736

Absolute bottom of piston - 3.734

All 4 pistons reflect these sizes. The box states that they are "sorted in matched sets by exact size and weight".

To further clarify, when checking the gap between the piston and the cylinder wall, I am using feeler gauges that are 5" long - much longer than the pistons - and when checking the bore gap I feed both the gauges and the piston up and down and around and around the bore looking for the biggest gap:

Thus my maximums of between .020 and .024 gap across all four cylinders.

Now, I have no issue having the block bored oversize, but how do I select a piston to fit?

These brand new pistons are .014 under the bore they are meant to fit (3.75), and if that undersize measurement is reflected across the entire range of other overbore sizes, what am I to do?

Buy a certain piston size and then have the bore cut .006 over the random piston size?

Thanks again everyone for your input.

[TXGOAT2]

I wonder if that set of pistons has been re-boxed or mis-marked.

[Mark Gregush]

If the bore is worn, any measurements of a STD. piston is a moot point. Starting with a base bore diameter of 3.75 your machinist would need to first see what oversized it could clean up to, .020, .030 etc. Which may involve doing a test bore to clean up any out of round or taper. Then pistons would need to be ordered so they would be in hand to be fitted with the correct clearance for each hole. That means they would need the 4 pistons in hand to do the job correctly.
The block would be bored just under the oversize used, then honed to fit the piston with required clearance. If all pistons are same diameter, that make the job easier.
The aluminum pistons do not have one base diameter top to bottom, the top part where the rings are should be smaller.

[Kerry]

All your measurements you have listed are of no use really to work out what size piston you have. New pistons are not round but oval, called cam ground, and measuring at the pin side is of no help, you must take your piston to bore clearance at 90 degrees to the pin hole on the thrust face of the pistons and the thrust side of the bore on a worn block. It can take as much as 4 test re-bores to see at what size it's going to clean up at before ordering pistons. On a STD bore I have had to go as much as .060" to do so, the max over size is .080 for what pistons can be bought of the self, over that and then it's re-sleeving the block.

[Allan]

Jack, presumably you do not have the gear to bore the block yourself. Before I ever order pistons, I have the machine shop test bore the block to be used. If it doesn't clean up at .020", they know to go to .040", and you know which pistons to order. They are only a week or so away by Fedex. leave the measuring etc. to those who are doing it all the time.

With alloy pistons I have them set the clearance at .004", {+.002, -.000] Too close and they will be prone to sieze.

Allan from down under.

[Dan McEachern]

Always measure modern aluminum pistons 90 degrees to the pin, and as a general rule, about ½ way between the pin bore and the bottom of the skirt.
Most pistons have very little skirt taper ground into them- .001-.002" at most. IF, your piston skirts are really round and measure 3.734 , throw them in the trash and get a new set. Disregard the diameter at the top in the ring land area. Its the skirt dimension that determines the proper bore size and subsequent clearance. I'm not going to address the range of needed clearances stated in the above postings, but I do find them "interesting"- and I do agree with Allan regarding a set of cast pistons.

[Adam]

I'm not going to address the range of needed clearances stated in the above postings, but I do find them "interesting"

I concur!

[Jack Tarlinton]

Thank you Dan, everything I needed to know you've summarised it all beautifully.

As stated, my measurements are all from the widest point, which is around 90 degrees to the pin and not in line with the pin as some suggest, so these pistons are of no use.

Always measure modern aluminum pistons 90 degrees to the pin, and as a general rule, about ½ way between the pin bore and the bottom of the skirt.
Most pistons have very little skirt taper ground into them- .001-.002" at most. IF, your piston skirts are really round and measure 3.734 , throw them in the trash and get a new set. Disregard the diameter at the top in the ring land area. Its the skirt dimension that determines the proper bore size and subsequent clearance. I'm not going to address the range of needed clearances stated in the above postings, but I do find them "interesting"- and I do agree with Allan regarding a set of cast pistons.

[Jack Tarlinton]

Thank you Allan, I appreciate the advice.

Jack, presumably you do not have the gear to bore the block yourself. Before I ever order pistons, I have the machine shop test bore the block to be used. If it doesn't clean up at .020", they know to go to .040", and you know which pistons to order. They are only a week or so away by Fedex. leave the measuring etc. to those who are doing it all the time.

With alloy pistons I have them set the clearance at .004", {+.002, -.000] Too close and they will be prone to sieze.

Allan from down under.