Turning piston undersize above top ring

[Brian Rupnow]

Last week someone mentioned that it was wise to turn a piston slightly undersize above the top ring to help getting the top compression ring on without breaking it. I've been playing Mr. Mechanic for almost 75 years, and I've never heard of that. Has anybody else heard of that, and if so, how much undersize do you turn it. I am dealing with a 1" diameter piston (actually 0.998") dia.) and it's up on my lathe right now.---Brian

 

[dnalot]

I don't think you need to do that Brian. When I built the Holt 75 the plans called for three rings and I feared getting a ring on the center grove would be a problem. I installed the bottom ring from the bottom of the piston and then worked the center ring into place from the top, skipping over the top rings grove. Proved to not really be a problem. I used STP as a lubricate, was so slippery I could barely keep my grip on it. Been watching your experimentation with rings with great interest.

Mark T

 

[rklopp]

I've never done it, and have had no problems with rings sealing. That is 14 pistons and counting.

 

[oldengineguy]

Most of the car engines I have rebuilt over the years do have a smaller dia. above the top ring. The smaller the bore the less the difference. " A " series BMC, about 2.5 in bore usually - .010 smaller. I make my little pistons (1 inch) about 005 smaller. Also makes rings somewhat easier to install. FWIW Colin

 

[rpf]

I think Eric Whittle did this on his V8, but the piston ring are quite small on this engine.

 

[Charles Lamont]

The purpose is not to make ring fitting easier but to make greater allowance for thermal expansion of the crown, which gets hotter than the skirt. This is a consideration with aluminium pistons.

 

[lohring]

We do that all the time with our 26cc race engines. Those engines were designed to be air cooled and have no problems in RC cars. However, when the cylinders are water cooled, the cylinder may not expand as much. It's insurance against siezing.

Lohring Miller

 

[MikeG]

Hi All
I don't know what all the intense discussion is about. I'm a nobody, but I've built two Panther Pups (1" bore) and installed 4 rings per piston..1 compression, 1 scraper, 2 oil rings.
All three have a different configuration on the circumference. Have I broken a ring upon installing? Yes, but very fewer than you may think ( 1 in ten) if you need X number of rings make X + ++! Be patient. After all this is a hobby.....right? That said, I get VERY frustrated with the stuff I'm doing too.

MikeG

 

[abby]

I am currently experimenting with cast iron rings in my 1/32 scale steam locomotive cylinders.
These have a nominal bore of half an inch.
I have found that the relationship between ring width and ring thickness is very important when it come to fitting without breakage.
I think my cast iron rings perform well against "O"rings , with non of the stick-slip phenomena.
Dan.

 

[Steamchick]

Hi Guys, For what it is worth.. Experience with Production car engines led me to work with Hepworth and Grandage (Piston makers) in the late 1980s. Of course there is a world of difference between the super-high precision (in design and development as well as manufacturing) of car engine pistons - that have to work "right first time, every time, for the expected lifetime or longer, under a wide variety of engine loads from long periods of idle to prolonged "flat-out" (Autobahn) - in environments from Finland (-30deg,C starting) to Dubai (+55deg,C tops).
That said: Models do not operate in such extremes of environment nor working life.
Pistons had a "nominal" diameter: To allow for the body temperature not to seize during a "rapid warm-up" - where the pistons gets hot quickly while the cylinder is still cooler than working temperature. But as there is a temperature profile from crown to skirt, that determined the worst case with extensive computer modelling...
At the ring pack - on a "3 inch piston" - there would be 0.002" to 0.004" reduction of diameter. The skirt would be "out-of-round" so that the pressure of the skirt resisting piston-slap was within certain limits, and as the skirt did not expand perfectly uniformly, (especially at the gudgeon pin bearings)so it was machined to be round at the "Working temperature". The tooling to machine this was a diamond tool, on a hydraulic servo carrier that moved the tool in and out at the speed of rotation of the work-piece (2000 rpm. or whatever?). They even shaped pistons to have their patented 3-pad shape on the skirt so the surface that the skirt "contacted" to bore was a small area to reduce friction. (Actually, no contact, just minimal oil film in shear - for the tribologists.).
SO - on your models, it may be worth having a 0.001" reduction of diameter at the ring pack, - to avoid seizure "pick-up" in this zone - but many engines will not be so precisely fitted to need that. Of course on aero-engines, - or full sized racing engines, where they are built very precisely, this will be necessary.
Oh, and as history shows, skirts have become shorter with the decades, so now they hardly exist at all....
Hope this is of interest,
K2.

 

[deverett]

Oh, and as history shows, skirts have become shorter with the decades, so now they hardly exist at all....
Hope this is of interest,
K2.

And the legs get longer as a result!

Dave
The Emerald Isle

 

[ajoeiam]

Hi Guys, For what it is worth.. Experience with Production car engines led me to work with Hepworth and Grandage (Piston makers) in the late 1980s. Of course there is a world of difference between the super-high precision (in design and development as well as manufacturing) of car engine pistons - that have to work "right first time, every time, for the expected lifetime or longer, under a wide variety of engine loads from long periods of idle to prolonged "flat-out" (Autobahn) - in environments from Finland (-30deg,C starting) to Dubai (+55deg,C tops).
snip
Oh, and as history shows, skirts have become shorter with the decades, so now they hardly exist at all....
Hope this is of interest,
K2.

I would hope that the piston clearance was not planned to 'only' -30C.
Have had far too many starts at far colder than that.
Get far enough north and you'll find that serious equipment just doesn't get shut off.
Sometimes the oil change also doesn't mean an engine shut off but that's another tale . . .

(Re: skirts - - - you may wish to look into something called the 'hem line index' - - - used by some in the financial industry.)

 

[Steamchick]

Hi A Joe, I am. Most manufacturers do different things to limit cost, where the part of the market "outside limits" can be managed separately. But an engine for a common European production car is likely to be designed to wider limits, but only sold to function within the limits in the Sales Brochure. Maybe I would be in breach of Company confidentiality if I told you all the truth in print? So best if I just tell you what the Sales Brochure has published. Being fair, a commuter in Liverpool or Lisbon isn't going to see extreme temperatures, so the range I quoted is pretty much the Norm. (Ask Daimler Benz! I heard they make quite good cars?).
I do recall a tale of some engineers waiting a week in Finland for the weather to warm-up enough before they could do the correct cold-start tests on the cars, which were the cars they were driving in temperatures well below that.
Also, Most people used full-strength Glycol mix to keep from freezing, and ran the cars all day when cold, or had sump heaters to keep the engine above "glycol freezing" limits.
K2

 

[ajoeiam]

Hi A Joe, I am. Most manufacturers do different things to limit cost, where the part of the market "outside limits" can be managed separately. But an engine for a common European production car is likely to be designed to wider limits, but only sold to function within the limits in the Sales Brochure. Maybe I would be in breach of Company confidentiality if I told you all the truth in print? So best if I just tell you what the Sales Brochure has published. Being fair, a commuter in Liverpool or Lisbon isn't going to see extreme temperatures, so the range I quoted is pretty much the Norm. (Ask Daimler Benz! I heard they make quite good cars?).
I do recall a tale of some engineers waiting a week in Finland for the weather to warm-up enough before they could do the correct cold-start tests on the cars, which were the cars they were driving in temperatures well below that.
Also, Most people used full-strength Glycol mix to keep from freezing, and ran the cars all day when cold, or had sump heaters to keep the engine above "glycol freezing" limits.
K2

Hmmmmmmmmmmm - - - - dunno if I should but I'm a turkey so here goes!
If one uses full strength glycol your 'liquid' will gel at some -20 odd C.
(Industrial heat exchangers were run at 20% glycol but they were NEVER shut off in winter. (They ran 24/7 most of the year in fact.))
If you use a 60% glycol 40% water mix you are still have a liquid at some -45C.
At colder than -45C your cooling liquid is only very lightly gelled so you won't have any issues.
(I haven't experience too much colder than -45C so would refer you to someone who runs equipment in the Arctic for colder temperatures.)
A 50-50% mix is rated to -38C.
A straight glycol will at 'only' -35C be stiff enough so that you will have issues - - - - even with a new car!!!!!!!!!!
If you don't respond in time you will need a new engine - - - that's worst case. Its easy to need new heads.
We were using 'block heaters' and such from the early 70s.
I do remember the battery being in the house and straw being burned under the air-cooled car engine in the mid-late 60s.

I do understand the auto executives thinking that they covered the norm - - - - one of the reasons their cars just weren't as attractive to the user when those
temperatures aren't the norm. And yes, I have had a Mercedes 220D diesel and starting it was less than enjoyable at even -30C never mind at colder - - - great car though even though the starting system sucked.

I have had equipment with block heater, engine oil heater, transmission oil heater, battery warmers (2 - 1 for each battery) and a battery charger. The block heater and oil heater were on when the tractor wasn't running and the rest was on over night so about 8 to 10 hours per day when I needed operation every day. This is the kind of stuff that those that haven't lived it find sorta weird. But I have gotten much better engine life than most of my peers and I prefer the lower costs and less of the large scale headaches.

Also was told by an owner that he had an employee that went to start 2 cats both with large Cummins engines (KTA1900 is the series or used to be) that used a lot of ether. He got them going all right but both engines were rebuilt before the winter was out. Don't think the employee saw the end of winter either. More commonly a parachute would be used and then a kerosene or propane heater run for likely 24 hours to warm up the equipment - - - - before starting! That's arctic starting guidelines.

 

[Steamchick]

Factory fill was 50% glycol. But I had an idea that was enriched to 65% by Finnish dealers at PDI. Not sure.
When it is cold here it is maybe -2 ~ -4°C. But when it is hot we get an exciting 22 °C! Lovely and stable in N.E. England. Why live anywhere else?
K2

 

[Vietti]

I believe water plus glycol is called a eutectic mixture, the mixture freezes at a lower temp than either component. I don't remember but I think the boiling point is also higher than the components.

John

 

[Steamchick]

Ta. I'm not technical on cooling of mixtures. Just had to be sure the statistical distribution of the mixtures in cars being shipped was as narrow as possible, with 99.997% of product leaving the factory within spec. I.E. a minimum 50% mix meant fill average mix was around 54% Glycol. (Zero cars with sub-50% mix). A large variety of engines and vehicles were filled by the same machines which affected the total process capability. Factory experience is just that. Limited to the Factory. "Meet the specs and you can ship product. Outside spec - fix it quick! - and fix the process!" Design experience is a bit different, "Meet the customer's spec and prove the design at the spec limits." - Outside that it is not on the bill. (I.E. if the customer wants more he'll have to pay more). That's life where you have to make money or give-up! Very few Politicians, civil servants, teachers, etc. experience that. Just corporate and small private business folk - who make the profits to re-pay their investors, pay their taxes, to pay for all the politicians, civil servants, etc. Stop making profits and the investors and taxmen will take everything away - and you'll have nothing. I'm not bitter, the bitters go in my Pink Gin!
K2.