Piston Ring Gap

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Piston Ring Gap

Postby rhaipol » Thu Mar 14, 2019 8:39 am

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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Fleet Engineer
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Re: Piston Ring Gap

Postby Merlyn » Sun Mar 17, 2019 5:52 am

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.

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