Piston Ring Gap

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rhaipol
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Piston Ring Gap

Post by rhaipol »

Hello sirs!

Could anybody please explain/elaborate - the importance of taking ring gap measurements. Also, ways on how to determine that piston rings are in good condition based on its ring gap. Is there a formula that can be use as a guide/reference?

I’ll appreciate your brilliant ideas sirs. Thank you!
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Merlyn
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Re: Piston Ring Gap

Post by Merlyn »

Years ago whe I started out we were shown how to manufacture piston rings, not only for internal combustion engines but reciprocating engines and Weir pumps and all general service steam driven pumps etc.As with older engines ( back in the early sixties on ) spares were not always available straight away so you had to turn them up.
"Blanks " were supplied of close grain cast iron, if the blanks were in a tubular form ( i.e. A hole in the center ) it was easier to machine, if not you had to work a big drill into the center of the blank to start the process and then bore it out to the correct ID.
You then turned the OD to spot on measurements and then set up for a parting off tool.
Cutting the ring to a smaller than required clearance gap the rings would then have to be gapped in the cylinder.
Here your filing flat skills came to the fore.
Both edges would have to be parallel and measure the manufactures clearances, and in thous. not metric.
Now as we well know liners and parent bores wear with a barrel effect, the maximum wear being halfway down ( or halfway up ) i.e. mid point of the travel.
When gapping new rings here is where the skills came in.
We had two persons out of a workshop of about 22 people who were skilled in this.
Turning up a mandrel the new ring would be pushed into various depths of the bore and the gaps measured.
The gap would be wider at the midpoint of the liner/ parent bore than it would be at the top or bottom.
Hereby a satisfactory compromise would have to be achieved which as I well know was very difficult to achieve.
Two small a gap at the top/ bottom could result in the ring expanding and kissing causing a possible breakup.
Too wide a gap would result in a blow by situation.
Furthermore if the top of the bore had a ridge present you might have to machine a small step to prevent ring contact and another smash up of the top rings.
Ridge dodgers they were know as.
Sometimes the parent bore would be machined out oversized and a new interference fit liner made up and pulled with a strong back into the block. Liner flame ring protrusion heights would obviously have to be correct.
Then came the chrome plated piston rings into a cast iron liner and several years later that position was reversed, chrome plated liners with close grain cast iron rings fitted.

Personally I found all the chroming never to be a long standing achievement .
Sometimes when the piston ring grooves were worn or stepped the pistons were put up into a three jaw chuck and skimmed out to match your newly machined rings which would be oversized width wise.
Then came the " Cord Rings Advent ".
These were designed for worn bores and did not ever be a lasting repair, they were a four piece ring, a thin top and bottom ring, a wavy reinforced bottom piece to provide the thrust to grip the bore and another distance piece to hold the top and bottom ring in place.
When you stripped the engine again after the cord ring fitment the tram lines visible in the bores showed clearly the extent to which the rings had gripped the bores.
Reamered it out almost.
All of the four ring parts were heavily cromed and unlike the cast iron rings were not easily breakable.
As the years went by and the lack of proper marine engineering skills together with the frequently in fitting piston/liner kits this practice seems to have almost died out, nowadays gapping the rings seems to be of a bygone era ( mine )
However I still well remember being taught it all, for donkeys years now.
No control room dweller type apprenticeship here!
Haven't measured a ring gap for years now, rings are all pregapped and already fitted onto the pistons in most kits supplied so not much point in taking them off anyway as you know the clearances will be correct.
In conclusion we must not forget that in a worn " barrel effect " engine the ring ( which must be at 90 degrees in the bore when measuring ) the ring gap will be greater midway than at the top or bottom of the stroke and gas such you are not only measuring the ring wear but the liner wear also.
The only way to measure it properly is in a new liner parrellel all the way down.
The burrs caused by wear on the rings ( which can slice your pinkies, still got scars on mine ) which show without measurement a badly worn ring present.
I think rings are made of various alloys etc now, however the problem still exists if you snap one, you have to buy a complete set on most engines be it compression, oil control or scraper ring.
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.
popeye62
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Re: Piston Ring Gap

Post by popeye62 »

Hello Rhaipol, you have it sorted by now but here is my input.
The piston ring end gap, butt clearance or however you refer to it is not particularly important during service. I do not know what engine you are working on but if it is an old large 2-stroke then the gap is important if you are fitting a new ring. It is crucial that the gap is not below the minimum stated at a given height in the liner whether it is new or old, and the ring end can be filed down if it is. If the ring ends touch during service there is the possibility of serious damage to the engine. If you are working on a modern 4-stroke then the gap is likely set and as the rings are harder they cannot be filed. I have never heard of a gap too small but anything is possible so always check the gap with the new ring in the liner. If on a 2-stroke and you are doing a piston ring inspection and the gap happens to line up with a port (piston rings do rotate, they don't all stay lined up in a staggered fashion) then you can check the gap but it doesn't really mean anything unless you have a previous measurement at the same place. You can't see the rings in a 4-stroke until you pull the piston. If you place one of the rings back in the liner and the gap is bigger then is this wear in the ring or in the liner? It is both. This is why the manual states to measure the ring width as well as calibrating the liner. Modern 2-stroke rings do not have flat ends and are often scarfed or use 'S Lock' ends of which the gap is difficult to measure. A large gap is not a problem for gas leakage because modern high power engines are designed to leak (see CPR or POP rings) to avoid the first compression ring taking all the load and sharing it with the other compression rings however, if the ring is wearing and the width is reducing there is the possibility that the gas pressure can deform the ring because there is less support from the piston. The most important gap on a piston ring is the one between the top of the ring and the underside of the ring groove. If this gap is too big or too small it is bad for the running of the engine. The sealing of the combustion chamber is not as a result of the tension in the ring it is the air and gas pressure acting behind the ring and pushing it against the liner and we want all the compression rings to do the same with an ever decreasing pressure (the pressure decreases because the ring pack acts like a labyrinth seal). If the gaps are too big or the grooves are full of carbon then the pressure cannot act behind the ring. If this happens then the gas pressure (Pmax) will force its way between the ring face and the liner pushing the ring back violently against the back of the groove; this is called piston ring pack collapse and it breaks piston rings. Cylinder lubrication is important, 2-stroke has a dedicated system but 4-stroke relies, generally, on splash lubrication and an oil control ring which coats the rings and the liner so the rings surf up the liner on an oil film (hydrodynamic lubrication) when the piston stops at the top and 30b+ of gas pressure is applied there is no hydrodynamic lubrication it is boundary lubrication meaning that the oil film is so thin to non-existent that metal-to-metal contact will occur and this is why you can see various 'ghosts' of ring impressions near the wear ridge. Keep the rings and the operation of the engine at optimum and they will last somewhere near what the book says. Do regular PMI checks. If you don't maintain the quality of the correct LO, I mean properly maintaining the LO or put the cleanest fuel in that you can and maintain all the air filters then the rings will fail prematurely. There is no formula. Regarding manufacturers' time between overhauls, they should say "If you have got this far without anything breaking, well done but it's time to change the part" It's all up to us, Rhaipol. Best wishes
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