To me, those bores look pretty bad. If the pistons are free in there, then they must be a very poor fit and you must be getting a lot of leakage past them. The 'oil' grooves look far too deep as well. Have you got the plans for the engine? Might be worth checking to see how the pistons were intended to be sealed. Seems odd to me that an engine of that size would be designed with plain pistons with no packing or rings.
Thank you both for your replies - I thought that the bores looked wrong! The question now is what to do about it. Does anyone have any ideas about what could be done to (1) fix the bore problems; and (2) stop them recurring? That could well be wearing down my pistons.
As to the pistons themselves, I am fairly sure that all the smaller Stuart engines are designed to have oil grooves rather than piston rings (according to their plans), although, of course, piston rings can be fitted if one wants. Given the comments above, I think that I really ought to have rings fitted - should I fit small rings into each of the existing grooves, or machine all the grooves together and fit one enormous ring?
Last Edit: Aug 28, 2008 19:14:08 GMT by jamespetts
I think what I would do is lap out the bores with a small cylinder hone - the spring loaded, two-stone ones use for brake cylinders. Then slightly enlarge the three grooves in each piston and fill 'em with teflon tape twisted into a string.
Well, to be honest, I'd make new pistons, with one, larger groove and fill that, but ....
All the smaller Stuart engines do indeed have oil grooves rather than rings.
However, it seems to work OK. I have seen many Stuart engines, but never one with bores that badly marked. My own ST10V is now well over 50 years old, has been run many many times, and the bore is as good as new.
Which raises the question, why is yours so scored?
Water type? Lack of oil? Poor quality of the original casting? (They did sometimes vary.) Or....?
I would be more interesting in finding a possible cause at this stage, rather than just fixing it, as the same thing may happen again.
Alan, I agree - I need to understand what is going on with this. The water that I am using is locally collected rainwater, filtered with coffee filters. Occasionally, I have used tap water, which, in this area, is quite hard (hence using rain water mostly). There should not be a lack of oil, as I have a displacement lubricator in the steam line, which seems to work well. The engine certainly appears to be oiled. I have also been sucking oil back into the cylinders after every run to try to avoid rust.
The Stuart castings, especially recent ones (and this is a recently built engine), tend to be of good quality, and I cannot see any evidence anywhere else of problems with them. The cylinders are not pitted, though: the markings face outwards, not inwards: in other words, they are bumps, not troughs. The substance is very hard, though.
Incidentally, how have you managed to avoid rust accumulating on your old Stuart 10v? Do you run it on steam, or just air?
James, I'd bring the bits down to the club and get some advice. Cleaning out the bores is likely to mean that the piston doesn't fit so well, and neither will the covers. Could be interesting getting it lined up again afterwards. John
thank you for the suggestion. I think that I shall do that. I had better not disassemble it again until after the Ascot show, though, given the amount of time that it takes me to put it back together again!
Hi James For what it is worth I have quite a few Stuart models (small to large in size) and apart from the Beam engine that has piston rings fitted, all the rest have Graphite Yarn/String on the pistons and I have never had a problem like this. I use steam oil via displacement lubricators or mechanical oil feed pumps, they are messy but lubricate well. The only problem. once, was when I forgot to feed neat oil into the Beam cylinder after a steam up and the piston rings "just caught". Luckily I spotted it before any major seizure occurred. I was told years ago that if Graphite Yarn was used it helped hold the oil and was better for lubricating the cylinders. I see you say the markings face outwards on the cylinder bore, my two thoughts on this are that firstly the pistons are very slack in the bore allowing excess steam to pass the pistons, and secondly if the markings face outwards that is helping take away any gaps between piston and cylinder. Good luck anyway.