piston clearances

[Philip BurleyParticipant @philipburley44197]

I have built a Stuart Turner No 1 , iron piston in iron cylinder , The piston slides up the bore with a light push . When assembled with out rings the engine turns over easily , but with the rings in it's extremely tight . The rings have a small gap and clearance at the bottom of the grove and free to move in the grove . Should I hone out the cylinder a bit more ? How much piston clearance should there be ? PS the rings a a Stuart Turner product

regards

[Philip BurleyParticipant @philipburley44197]

[nigel jones 5Participant @nigeljones5]

The piston sounds just right, I suspect there is a machining error in your ring fit. If you put the ring in the bore without piston and you have a few thou gap then its not the rings – it will be the slot in the piston which the ring sits in not being deep enough – done it myself.

[JasonBModerator @jasonb]

What is the existing finish of the bore? straight off the machine or honed? New rings can feel tight until they bed in.

I like to make a piston so it will drop through the cylinder when there is nothing over the end but if you block the bottom off with your hand the piston without rings/packing should just drop very slowly as the air escapes around it, push fit is a bit tight for me.

 

Edited By JasonB on 05/06/2018 20:33:06

[Neil WyattModerator @neilwyatt]

Tubal Cain bemoaned the fact that steam engine rings are almost always far too tight in the bore.

Neil

[Phil WhitleyParticipant @philwhitley94135]

this any help? admitted;y this will be for an ICE but if anything steam engines will run at least as hot. I knew there would be a "per inch of bore formula somewhere. Hope this helps.

Phil

[Phil WhitleyParticipant @philwhitley94135]

https://www.hastingspistonrings.com/tech-tips-faqs/checking-compression-ring-gaps

always helps if you include the link in the post!!!

[I.M. OUTAHEREParticipant @i-m-outahere]

Maybe something wrong with the rings , i would take them off the piston and sit them on a flat surface to make sure they are not warped / twisted . Then i would roll each ring around the groove in the piston to make sure there are no tight spots , you can check the ring groove depth clearance at the same time . When you checked the ring gap how hard was it to get the ring into the cylinder ? Did it feel like there may be an abnormally high amount of ring tension ? Is there a small amount of clearance between the walls of the ring groove and the ring ? (I would be aiming for 0.003 – 0.005 inch or so) I would then look for any taper in the ring and piston groove – to check the ring just give it a light lap on a flat surface ( surface plate if you have one ) with some fine wet and dry paper ( do both sides ) , look for any high spots or burrs and to check the groove in the piston use feeler gauges or a gauge block if you have a set . Lastly , in the ring groove is the a small radius where the sides meet the bottom of the groove ? If so check the inside dia of the ring to see if it has a small champfer to accont for this .

In the 30+ years i have been building full sized engines i have seen some weird things – a piston ring that was thicker one side than the other was an especially memorable event that had me scratching my head wondering why the piston was so difficult to get into the bore , once i stripped it all apart and rolled the ring around the piston ring groove i discovered the tight spot . Fortunately that was only a mower engine so the supplier excanged them for a different set .

Just to add to what Jason mentioned with cylinder finish , you can wrap some fine wet and dry paper around your hone and give the bore a finer finish ( don't polish it ) just a few stokes back and forth to knock the tips off the honing marks and you can also give the outside dia of the rings a light hit with some fine wet and dry if the finish is a little rough – once again don't polish too much as the fine scratches help the rings to bed in .

[Philip BurleyParticipant @philipburley44197]

I checked the clearance yesterday , 4 thos , its a 2 inch bore , the piston is in 2 halves , with 2 rings between them . The rings are free to turn when the piston halves are screwed together and clear at bottom of the groves . The rings do seem to excessively tight when pushed into the bore , , more like rings for a rebore of + 20 . I guess if I run it in on the lathe it will bed in . ( I hope ) These rings from ST are expensive , I might have a try at making some , I have a suitable piece of CI . PS the cylinder was honed with an expanding hone device and it's not undersize

Regards

[J HancockParticipant @jhancock95746]

If you even think those rings are oversize it would be worth a check before 'making' them fit by driving force.

Just make up a trial bore, exactly as your engine, say 1/4" long, push your piston+rings in and hold up to

a strong light.

That should show up any ovallity.

[JasonBModerator @jasonb]

What is the gap when they are not closed up?

I usually make my own mostly for IC engines and have never had to resort to motoring an engine to bed anything in, at that dia I would be looking in the region of 3/16" gap when relaxed.

[Philip BurleyParticipant @philipburley44197]

a bit more than that when relaxed , I think they are just too expanded ,

regards Phil

[Philip BurleyParticipant @philipburley44197]

I ran the engine on the lathe with plenty of oil down the cylinder , and now it will run OK . Needs plenty of air though, my little compressor will hardly keep up . Its satisfying when it first works isn't it !!!
regards

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