How many rings per piston?

[noel shelleyParticipant @noelshelley55608]

This is an interesting thread ! In full size IC engines I always worked on a gap of 3 or 4 thou per inch of bore for water cooled engines. The bore temp of a steam engine may well be in the same ball park so I would say 4 thou is right for your bore. One benefit of using O ring packing is that there is NO gap and so this issue resolved. Noel.

[Arthur Jones 1Participant @arthurjones1]

Thanks Dave.

>> Do a web search for Piston Ring Tool.

Good idea, but in fact I’d already been looking for piston ring compression tools for fitting the ringed piston to the cylinder, and none seem to go down to 1.125″ which is (I realise) very small by normal standards.  Smallest I can find is 1.5″ – cost is very low but no good if it’s nearly 50% too big.  (Even if it’s huge by model aero standards!).  I’ll keep looking but I may improvise with tinplate and a jubilee clip!

I’ll try improvising some pliers – they only need exert an ounce or two of force so don’t need to be high-grade steel.  Or maybe a simple wedge or gag will do the same job?  My concern remains the risk of deforming the ring.

>> An ounce of practice is worth a pound of theory.

Sound advice.  I have clearly spent too much time (35 years) teaching engineering theory but am learning the engineering practice the hard way! (I’m not a time-served craftsperson and my parents were not into engineering).  I guess the flipside is my father’s saying “Experience is a good school but the fees are high”, and the fees here might be a lot of broken or distorted rings which took a lot of time to make!  I’ll start practising with the out-of-spec ones.

Thanks for clarifying on the ring-fitting, Duncan, makes more sense to me now.  So what do you recommend – gently spring the rings open over the piston, or wind them spiral-wise into the groove from one end of the ring?

Again, any thoughts on the other points (clearances, change of gap with machining etc.), anyone?

Thanks

Arthur

[noel shelleyParticipant @noelshelley55608]

cut a strip of tin from a metal can, must be flat not ribbed 1.5 times the circumference of the piston, on the bottom edge using narrow pliers bend out in 3 or 4 places,wrap round the piston and rings put a worm drive hose clip round to adjust the fit and the little tongues will stop the tin at the bore and allow the piston and compressed rings to enter. A VERY small  chamfer on the lead in edge will help. This a variation of a commercial ring compressor. Noel.

[Brian Baker 2Participant @brianbaker2]

Greetings Arthur, I have used a cable tie to close the ting and hold it so.  After part of the ring is inside the bore the tie can be slipped off or cut.

Regards

Brian B

[duncan webster 1Participant @duncanwebster1]

Once youve got the ring in the groove, push the piston down the bore until the ring is sitting on the top face, wrap a bit of cord round the ring and pull both ends to compress the ring, then using your third hand push the piston in.

[bernard towersParticipant @bernardtowers37738]

Why wouldnt you make a proper piston ring clamp of the right size, could save on broken rings!!!!

[duncan webster 1Participant @duncanwebster1]

Most likely time to break a ring is getting it onto the piston, ring clamp wont help.

[bernard towersParticipant @bernardtowers37738]

fitting a piston ring over a pistin is doomed to failure if not done in a controlled manner ie ring pliers it all depends on how precioyus your rings are

[Arthur Jones 1Participant @arthurjones1]

Hi all,

Thanks for all your advice.  Well, I think I was making a lot of fuss about nothing in terms of getting the rings over the diameter of the piston.  Following Dave (SOD)’s sensible advice, I tried fitting an “out of spec” ring over the machining jig mandrel (which is about +0.002″) and it went on without breaking or distorting.  The “frozen” gap was unchanged, even when I tried one where the gap had narrowed to 1/16″ during machining.  Thanks anyway for your advice.

Noel, thank for your reassurance over the gap after fitting.  (And thanks for grinding those drills at MMEX 2022 – I was the exhibition visitor who thoroughly and cheekily remedied your lack of drills to demonstrate your drill-sharpening machine!  They are excellent – which was certainly not true before your efforts, with most of them being sold to me by a well-known outlet with negative clearance so they only rubbed on the back edge).

Bernard:

>> Why wouldnt you make a proper piston ring clamp of the right size, could save on broken rings!!!!

Good point – but please can you be a bit more specific?   Looking again for 1.125″ ring compressors has got me nowhere, but I did find this website selling tapered-bore rings which I think would be fairly easy to replicate at home for the required size:

PISTON INSTALLATION TOOL

Was this what you meant, Bernard, or something else?  My only reservation is that the rings may tend to catch and try to rotated in their grooves when they start in the tapered bore as the rings are very shallow, but perhaps the tricks with string, cable ties, worm-drive clamps lined with tinplate etc. may be used to “start” the ring in the tapered bore.  I’m probably over-thinking this (again) but will have a go, material permitting.

So the only loose ends seem to be:

– how much side clearance in the groove?

– how much bottom clearance in the groove?

– can someone be more specific about the fit of CI piston in CI bore to avoid either leakage or seizing in a steam loco i.e. can you elaborate on “normal running fit” and “a good fit”? 1 thou undersize? 2 thou undersize? Or as close to dead size as will avoid interference? I guess that 1 thou undersize will cope with a piston at 100 deg C in an ambient-temperature cylinder (an extreme case I guess) – I’d be worried about anything closer seizing during startup, but perhaps I’m being paranoid again.

Thanks again for all your patience and answers!

Arthur

[bernard towersParticipant @bernardtowers37738]

Arthur, Make your ring compressor from a piece of 25 thou shim by cutting a strip of the width to suit piston and bend round piston blank then fold the two ends leaving a small gap and hey presto!

[noel shelleyParticipant @noelshelley55608]

Glad the drill bits worked Arthur, and good luck with the rest. Noel.

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