Piston-to-bore clearance

[Littlestown Mike]

Looking for a real world answer, for cars that aren't driven much.  Bore is 3.68.  Factory clearnce was in the 0.002-.003 range with a limit of 0.005 inches.  

Of course, i don't want to hear rattling pistons, but the difference in cost is significant ( but not prohibitive).

 

On a related topic--what are you guys paying for rebore/ hone and chamfer on a per cylinder basis?
 

[joe_padavano]

And why do you think anyone would know more than the factory engineers who specified that clearance? In any case, clearance depends on the block material, the piston material (iron, cast aluminum, or forged aluminum), and the intended operating conditions.

[Littlestown Mike]

I am not sure what the Factory Engineers knew and wanted versus what the accountants demanded.  Maybe theirs was the best compromise.  Maybe real-world experience would confirm that; maybe it would require tighter clearances.  I am just asking.

But, you are right about materials--Cast Iron Block, Cast Aluminum Pistons, water-cooled,  "standard"-type engine IOW not a race engine, also Normally aspirated--low performance---1965 Falcon 200 Six  Probably will see less than 2000 miles a year.  I was surprised to see chrome rings.

[Bloo]

The clearance should be what the PISTON engineer specified. If not using factory supplied original pistons made in period, you can forget what the shop manual says. This is a common mistake. It has been a common mistake for decades. There is no rule of thumb.

 

Whether you need enough clearance for the pistons to make noise cold depends on what metal the pistons are made of, and also the design of the piston. Cast iron tends to use lower clearances. Aluminum varies a lot depending on whether cast or forged, whether there is an expansion strut, cam grinding (out of round to compensate for expansion), and how much silicon content. Skirt design also has plays a large role.

 

Ask the piston vendor, and believe what they tell you. If they specify a range, do not go to the tightest thousandth.

 

 

[Oldtech]

Are you getting it re-bored? If not, and bores are worn a bit tapered, the clearance applies to the bottom of the bore. Top will be much looser.  And I wouldn't bore it for .005 wear.   .010 yes.  for all the miles it will get. 

In my poorer youth I have re-ringed engines with maybe .015 taper and had good results for maybe 30,000 miles.  These were engines from 50's and 60's  I don't remember excessive noise.  

Don't think you could get away with that with a new one!

[Ben Bruce aka First Born]

I agree with oldtech.   I have a little noise in mine.   We used oversize rings back in the '50s.  Filed to fit.  Today I would not do so if I was building for someone else and on their dime.  For what you will probably use yours for, I would do it for myself.   Roll the windows down, turn the radio up.

 

  Ben

[Frank DuVal]

10 hours ago, Bloo said:

If they specify a range, do not go to the tightest thousandth.

Isn't that the truth! I've used a Sunnen hone to open up two Corvair engines that were too tight with new pistons and a fresh bore job. It took two the three thousandths to get rid of the scoring, to a total of 4 thousandths, and they ran great after that. That's 4 thousandths total, not per side of piston. That's another gotcha, some piston's specs for clearance are per side of piston, i.e. a feeler gauge measurement, not total bore. So "they" see 1/2 to 1 thousandths and bore to 0.0005 total larger. Very tight!😮

 

🦃🦃🦃🦃🍲🍯🍠🥔🐖🍾

[Rusty_OToole]

Pistons can be knurled if a little loose and oversize rings installed. This was a common part of an overhaul back in the day. Should not knock or make noise and will last a reasonable time, like 30,000 to 50,000 miles.

[Littlestown Mike]

Thanks all.  I am having that battle in my head---"Do it right"  versus " your funds are not limitless".   Good to know its possible to "get away" with up to .015" taper.  I don't think I will go that far, but its good to hear that it has been done and lasted for a whle. I think my self-imposed limit would be much less than .015", but probably a little more than the factory .005"

 

 

I have a snap gauge and a 6" caliper---but I don't think that combination is very trustworthy--or maybe its my technique that is at fault.
I like the method of using a piston ring and measuring the end gap with a feeler gauge at various point in the cylinder.

There is also a method in the old Ford FSM that is intriguing----turn the piston upside down and place a specific size feeler gauge between the piston and the cylinder wall and use a spring scale to measure the force required to pull the feeler gauge.  Then go to a chart and that combination of feeler gauge size and pull force gives a clearance measurment that to can add to the piston doameter and get a reading on the bore dimension.  Have any of you used that?  How does it compare to actual "precision measurements"?

[Oldtech]

I always used the ring end gap system, Use a ringless piston to square it in the bore.  I have the same problem with snap gages

[hook]

12 hours ago, Frank DuVal said:

Isn't that the truth! I've used a Sunnen hone to open up two Corvair engines that were too tight with new pistons and a fresh bore job. It took two the three thousandths to get rid of the scoring, to a total of 4 thousandths, and they ran great after that. That's 4 thousandths total, not per side of piston. That's another gotcha, some piston's specs for clearance are per side of piston, i.e. a feeler gauge measurement, not total bore. So "they" see 1/2 to 1 thousandths and bore to 0.0005 total larger. Very tight!😮

 

🦃🦃🦃🦃🍲🍯🍠🥔🐖🍾

Same with Franklins or other air cooled. Another thing is the construction of the piston.

[Rusty_OToole]

A quick web search turned up Silvolite pistons with rings for $155.75 a set of six. Should not break the bank if you really need them. I don't know what it costs to get a block bored where you are.

[Littlestown Mike]

11 minutes ago, Rusty_OToole said:

A quick web search turned up Silvolite pistons with rings for $155.75 a set of six. Should not break the bank if you really need them. I don't know what it costs to get a block bored where you are.

So far, one shop quoted me $40 per cylinder for bore, hone and chamfer.  No, that wouldn't "break the bank", but it may have impact on other details in getting the car back on the road.  It would be nice to KNOW that the clearances are all within oem specs.

 

[Bloo]

This is getting really confusing. Are we talking about skirt clearance or ring gap or taper? All matter.

 

On 11/24/2023 at 5:50 AM, Littlestown Mike said:

I have a snap gauge and a 6" caliper---but I don't think that combination is very trustworthy--or maybe its my technique that is at fault.

It is not trustworthy. It can work in a pinch, but you may chase your tail. Also, a caliper is not good enough, a micrometer, even a cheap one is more like it. I don't recommend any of this, unless you have mechanically inclined friends you can get to also do the measurement and then compare to yours. There are lots of ways to screw up here, but if 2 or 3 mechanically inclined people get the same results, I would be inclined to accept it. This is for taper. There is a triangular tool that attaches to a dial indicator that gives way more consistent results. There are cheap ones now on Amazon last time I looked, and one of those might be an option. I've not used one of those cheapies, so can't necessarily recommend it. The triangular thing that attaches to the dial indicator is the right tool for the job though. A quality one like Starrett or SnapOn is going to be way too expensive for one engine. It may be cheaper and better to have a machinist measure all your parts.

 

On 11/24/2023 at 5:50 AM, Littlestown Mike said:

I like the method of using a piston ring and measuring the end gap with a feeler gauge at various point in the cylinder.

 You have to do this if you haven't rebored. Just check down low in the cylinder where there is no wear. Nothing else matters. Well, it matters but you can't do anything about it. The tightest measurement is what will bite you and what you must pay attention to. If you put standard rings in a worn bore, the ring gap will be huge. It will still be better than old rings but you may be able to do better. When overhauling engines with a bunch of taper back in the day you would either use "file fit" riings (aimed at racers) which were slightly oversize, or just use .010 oversize rings. File the ends. Keep them square. Shoot for the low end of spec but, just like when fitting pistons, not the absolute bottom of the specified range. Ideally I would want about .003 more ring gap than the absolute minimum, and this is measured on the un-worn lower part of the cylinder. If you are too tight, the ends might touch, and an acccidental overheating might make them touch when they wouldn't otherwise. If the rings touch, they will break. That destroys the piston and scores up the bore badly. If you fit the rings at the bottom, they will still be way too loose at the top, but there is nothing you can do about that, and it is "less bad" than standard rings in worn cylinders. Lots of engines with excessive taper were overhauled this way back in the day. With a new ring set using 3-piece oil rings, oil consumption will be acceptable and better than it has a right to be.

 

Full disclosure though, these are backyard methods from the 1930s-1980s period. Almost no one does this anymore.

 

On 11/24/2023 at 5:50 AM, Littlestown Mike said:

There is also a method in the old Ford FSM that is intriguing----turn the piston upside down and place a specific size feeler gauge between the piston and the cylinder wall and use a spring scale to measure the force required to pull the feeler gauge.  Then go to a chart and that combination of feeler gauge size and pull force gives a clearance measurment that to can add to the piston doameter and get a reading on the bore dimension.  Have any of you used that?  How does it compare to actual "precision measurements"?

Yes, I have heard of it. Meh. Again what matters for piston skirt clearance, just like ring gap, is the clearance in the unworn part. Piston skirts too tight will score, might seize, and even if they don't seize, might refuse to crank well with the engine hot. Nothing substitutes for precision measurements. The only time I have heard of that method being used was fitting NEW pistons to NEW bores. It might be OK. You can also do that with just a long feeler gauge.

 

On 11/24/2023 at 2:00 AM, Rusty_OToole said:

Pistons can be knurled if a little loose and oversize rings installed. This was a common part of an overhaul back in the day. Should not knock or make noise and will last a reasonable time, like 30,000 to 50,000 miles.

 

Yep, this is how we fixed skirt clearance way back when, and in a backyard overhaul like I have described above it is fine. The knurlings hold oil, and it lasts a lot better than it has a right to.

 

Skirt clearance is pretty critical though if you can't stand noise cold, know there is a VERY narrow window where the skirt is loose enough where it won't seize, but still won't make noise cold. Quite a few piston designs cannot do this at all because the rate of expansion is too much. Old fashioned forged pistons, for example, cannot do both. Sufficient clearance to prevent scoring hot will result in a little noise cold.

Edited November 25, 2023 by Bloo (see edit history)

[Frank DuVal]

8 hours ago, Bloo said:

There is a triangular tool that attaches to a dial indicator

aka Three Point Bore Gauge....😉

 

I have one that will do 0.5" to 0.8", might be a bit small for engine cylinders....

[Littlestown Mike]

Bloo--Thank you.

I took advantage of a Black Friday Amazon special and have a 4 piece micrometer set on its way, 0" to 4".  should cover anything I need to measure.

In my professional life, I was ( semi-retired, now) a technician in the soils and foundation engineering field.  I established labs for several of my employers over the years and shepherded them through the arduous Certification process.  I have a fairly good appreciation for accurate measurements in length, mass and volume, and being able to trace those measurements back to recognized standards. My weakness is experience in making those measurements on engines, and i will address that somewhat by making multiple independent measurements at different times, on different days.  If I get the same measurement at three different times, i will have more confidence in it.  In the end, my measurements will be just to help me decide "go or no go" for an overbore.  If I decide to rebore, then the machinist is responsible for matching the new pistons to the bore--or rather bore and then hone the bores to fit the new pistons.

I understand that piston skirt clearance in the unworn portion of the cylinder is critical and that ring end gap must be sufficient to prevent binding and breaking--ouch!

I remember that knurling was a common procedure "back in the day", but I always thought it was frowned upon by most as a temproary fix.  As you say, the knurling creates pockets to trap oil and that helps lubricate the cylinder walls so it often lasts longer than anyone had a right to expect.

I appreciate your real-world experience--and Rusty's too.  

 

 

[Pfeil]

On 11/23/2023 at 6:53 AM, joe_padavano said:

And why do you think anyone would know more than the factory engineers who specified that clearance? In any case, clearance depends on the block material, the piston material (iron, cast aluminum, or forged aluminum), and the intended operating conditions.

Exactly! I did a re-rebuild on a friends 37 Ford woody, a V-8 60. The first rebuilder installed aluminum pistons and clearanced them for the factory spec. steel slugs. Every time the car warmed up it wouldn't restart and had to completely cool down again. Had to .060 it to bring it back. Very lucky. I did that engine in 1979, it's still running good after all those years.

[Rusty_OToole]

You should see my collection of 1930s MoToR Magazines. This is the American garage trade magazine not the English enthusiast magazine. They are full of ads for "10 UP" piston rings, "Cord" rings that resemble a Slinky, piston skirt expanders, all kinds of devices to avoid reboring worn engines . An overhaul was typically required every 20 to 30 thousand miles  with a complete rebuild after 50 thousand and a total life of under 100 thousand.

[arcticbuicks]

the "Cord rings " i remember those........like a stack of razor thin rings.....to replace one ring......guaranteed to stop oil burning on any engine no matter how worn............problem is i bet it quickly wore the engine out even faster after that ?

[Hemi Joel]

On 11/24/2023 at 7:50 AM, Littlestown Mike said:

Thanks all.  I am having that battle in my head---"Do it right"  versus " your funds are not limitless".   Good to know its possible to "get away" with up to .015" taper.  I don't think I will go that far, but its good to hear that it has been done and lasted for a whle. I think my self-imposed limit would be much less than .015", but probably a little more than the factory .005"

 

 

I have a snap gauge and a 6" caliper---but I don't think that combination is very trustworthy--or maybe its my technique that is at fault.
I like the method of using a piston ring and measuring the end gap with a feeler gauge at various point in the cylinder.

There is also a method in the old Ford FSM that is intriguing----turn the piston upside down and place a specific size feeler gauge between the piston and the cylinder wall and use a spring scale to measure the force required to pull the feeler gauge.  Then go to a chart and that combination of feeler gauge size and pull force gives a clearance measurment that to can add to the piston doameter and get a reading on the bore dimension.  Have any of you used that?  How does it compare to actual "precision measurements"?

 

 

If you are checking with a feeler gauge, remember that most pistons are cam ground, not round. You need to check at the largest diameter of the piston, usually the skirt 90 degrees from the pin bore. You new micrometer will find it for you. 

 

You can get a dial bore gauge pretty cheap on ebay nowdays. Combined with your new mike set, you can get very accurate measurements. I prefer .0001" graduations, but .0005" will work and are cheaper.

[Ronnie]

14 hours ago, Littlestown Mike said:

My weakness is experience in making those measurements on engines, and i will address that somewhat by making multiple independent measurements at different times, on different days.  If I get the same measurement at three different times, i will have more confidence in it.

Do some reading on the internet and learn the proper way to hold your new micrometers when taking measurements. How you support it with your hand makes a difference. Then take some time to get the "feel" of using micrometers. That is something that is acquired with practice. Without taking the time to do that you might find it will be hard to get an accurate micrometer reading three times in a row within the close tolerances you will be working with. You might get frustrated and give up thinking something is wrong with your new micrometers.

 

A dial bore gauge is best for checking the cylinders, but you can do it accurately with a telescoping gauge once you get the "feel" for using it. Unfortunately getting the feel for using a telescoping gauge is probably harder to learn than getting the feel for using micrometers.