ST40. Help needed on fitting new ring.

[Martin McIntosh]

A friend keeps coming up with old motors he inherited. I can normally get these back to as new running condition but a very old ST40 is giving huge problems regarding fitting a new ring. Have done this countless times before on other motors.

The motor is well and truly gunged up with burnt castor on the outside but internally is very clean with no bearing wear etc but the ring was very much stuck in place. After lots of soaking in various potions and heating I managed to remove it bit by bit. Got the very last one from Motors and Rotors but despite the piston groove being spotlessly clean and the ring thickness being the same as the old one I simply cannot compress it enough to fit the barrel (the ring does go into the groove all the way round). The problem seems to be that it has very high tension. Difficult to get over the piston crown in the first place. Tried twisting s/st wire round to compress it but that did not work. Obviously with no more available I dare not risk breaking it but have run out of ideas.

[Denis Watkins]

Have succeeded using a jubilee clip Martin

But use it very carefully

[Martin Harris - Moderator]

Sadly, Mick is no longer with us to advise (although he’d probably have fitted it for free) but Dave might have inherited some tricks and wrinkles from the ST maestro - worth a call to Motors and Rotors to see what he might have used?

[J D 8]

         How about using a plastic zip tie to hold ring in place, available in small sizes.

[Jonathan W]

Have you first tried to insert the ring it into the bore on its own, without the piston? This will show you if there is enough of a ring gap. Just needs a thou or two. if there is no gap, or a negative gap, it will jam in the bore.

[Martin McIntosh]

I dare not remove it from the piston because I know what is likely to happen. When I compress it by hand there is still a visible gap so I do not believe that that is the problem.

Thought about a jubilee clip but the part where the screw is bulges out so one would not fit. Plenty of cable ties so may try that idea.

Dave checked that it is certainly the correct ring since I took the motor there with me. The only difference from the original is that instead of each end being scalloped out to clear the locating pin, this one has a piece cut away about 1.5mm from one end. It fits over the pin OK.

Thanks for your replies so far.

[Jon H]

can you fit the piston and ring into the bore but out of the engine? Again this would at least prove it all fits together. 

[Engine Doctor]

As John has just said , but did you test fit and gap the ringing in  the cylinder BEFORE fitting to piston ? Insert in the bottom or unworn part of cylinder and check the gap. Set to about 4 thou per inch of bore. 

R8ng were often sold ungapped and need gapping .  As its a pegged ring in two stroke engines you may have to file the back of the ring to accommodate the peg.

[Peter Christy]

I'm no expert on this, but I have a vague recollection that some STs had a "pin" in the groove to stop the ring rotating. The pin has to be in the gap in the ring, otherwise the ring won't sit properly in the groove.

 

Its not one of those, is it?

 

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Pete

 

[Engine Doctor]

As Pete says the ring gap MUST align with the ring locating peg/ pin.

The pin is to stop the ring rotating and getting caught in the ports. The Gap still needs to be big enough to allow the ring to fit the cylinder with space enough to allow for expansion as engine heats up.

[Martin McIntosh]

Please see my post above which explains all of this, and why I cannot now check the ring in the sleeve. I do not believe that it is a gapping issue.

Spoke to Dave W. today and he more or less agreed that these have very high tension and can be difficult. Would not care so much if it was my own motor but has already cost my mate 25 quid!

[Peter Christy]

Martin, can you still rotate the ring around the piston? IIRC (and I may not!) if you look down on the top of the engine, with the crankshaft at 12 o'clock, the pin referred to above should be somewhere around 10 or 11 o'clock.

 

If you can rotate the gap round to that position, you should feel it snap into place.

 

If your piston has a pin to locate the gap, and the gap is in the wrong place, the ring will not drop fully into the groove, making it impossible to get the piston back in the engine.

 

If all else fails ask Dave to put you in touch with Paul Roberts. He is a wizard with these things!

 

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Pete

 

[Martin McIntosh]

Thanks Peter. The first thing I do is to ensure that the ring groove locates fully over the pin. The problem is the ring tension preventing me from engaging it fully into the bottom of the sleeve using my thumb nails. It nearly gets there but not quite and I can still see a ring gap. Dave suggested that I try a little thick oil rather than the WD40 I have been using, also heating the sleeve a little to expand it. Rather reluctant to freeze the piston and ring to shrink them in case the ring gets brittle and snaps, but it may work.

May get the owner to come round before I continue so that he can call the shots.

[Martin McIntosh]

Done it at last! Between the two of us we finally got the piston in but it was very tight in the bore despite a ring gap being visible. When attempting to get the con rod to engage it fell out again. Abandoned the thing in disgust and went to the pub.

This actually gave me the opportunity to scour the bore with steel wool and some headlight polishing pads. Got it back in the bore, now freed up, and managed to engage the con rod. Compression is good and I see no reason why it should not run OK.

Thanks again for all your suggestions.

[Peter Christy]

Well done! STs are generally very good engines - I have quite a few! Even the ones that have been abused over the years still deliver top performance. A tip, though: they don't like nitro! Run them on straight fuel, if you can get it, or no more than 5% if you can't. Also, they were designed for long reach plugs - almost unobtainable today. I use 4-stroke plugs in them to good effect. I also do this with some of my other old European engines that were designed for long reach plugs (Webra Blackheads, etc).

 

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Pete

 

[Martin McIntosh]

Thanks Peter.

The first STX60RE was bought around 1980 for next to nothing. It went in a Dalotel, piped with copper tube S bend and may have been the best motor I ever had, first flick starting and straight castor fuel. Smashed to bits after nine years when the wing gave in.

Much later I got two from M&R built up from spares. I replaced the carbs with Webra Dynamix ones as before, the reason being that I used crankcase pressure via a TK regulator but the main needle would not close down sufficiently on the Mag carbs. However, I could not get them to run properly. Accidentally filled up with 20% nitro one day and bingo! Don`t ask me why this should be so.

[Peter Christy]

You can make STs run on nitro by putting extra shims under the head. This lowers the compression ratio a bit. However, this was not how they were supplied by MickW!

 

Strangely, I've even had a couple of Japanese engines that were very unhappy on the slightest wiff of nitro! One was an AirSupply 40 helicopter engine, and the other a GSM 36. I put the GSM in a Lark helicopter, and because it looked similar to an OS, filled it up with 16%. It was very unhappy, with a terrible tick-over, and very hot and spitty. I switched it to straight, and it ran like a turbine!

 

All my STs run very happily on straight. The power is more than adequate, and the fuel substantially cheaper! I've also had no problems with the bearings dying after a period of not being used, which you often get with nitro, no matter how carefully you drain them! Win-win! 😀 (yes, I am a cheapskate! 😆)

 

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Pete

 

[Martin McIntosh]

Very puzzling as to why two out of three would not run properly on straight fuel which is all I used to use on ABC motors.

[Peter Christy]

I wonder if Dave had added extra shims. Straight fuel is something of a rarity these days. I've had to order it specially - 4 gallons at a time - which does at least last me a while! Perhaps he shimmed them expecting you to use nitro.

 

Mind you, I've always used the standard carbs, which work pretty well on no or just exhaust pressure. They went through a period when they were prone to jamming, but then they changed the material (I recall being told) and I certainly haven't had one jam in a long time.

 

Not sure what is happening to Bekra straight (my preferred brew) now that Optifuel have taken over. Still have some left, though, so I won't have to panic anytime soon...

 

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Pete

[Paul De Tourtoulon]

I have never had any Super tigre engines but the clubs tug and Telemaster for teaching  had them, and at the time no one used Nitro ( 1980 / 1990's ) thee ST's used 12% castor and that was all, I only used Nitro when I started ducted fans around 1984 and that was in an OPS.

[Martin McIntosh]

I have had no reason to look at the head shims, did not think of that one but it is a possibility and would explain why I could not tune the top end on a pipe.

My OPS 60 Resonanzas all ran on straight, being of very high compression.

Dave now stocks Beckra 10 but I do not know what else.

[Martin McIntosh]

Tested the motor today, it is capable of going like stink and idles just fine. Laser 5 used because it was in my field box.

[Peter Christy]

Yep! That sounds like an ST! 😀

 

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Pete