Violent Thunder Tiger

[Merco 61]

Hello,

Will try to respond to your posts as briefly as I can.Tried both alternative prop nut fixing order.No difference.Swapped over to SC, nuts,no improvement.De-rated engine with shims,ordinary 5/16x32 nuts now remain in place.Engine is now rather nice.Even runs on straight castor which I use for ancient two strokes.Supplied plug is stamped Japan RF.I,ve tried some cool Rossi short reach,but to no advantage,so back to OS F.Some extra care with tank and more messing about with washers including changing steel prop washer for a very thick alloy one has brought an improvement.I will get there, determined to.Seems undeniable that most model 4st will loosen their prop nuts if provoked by lack of attention to detail at startup.Surely,the more powerful the engine,the more critical this becomes.The TT has proved that a high compression engine on startup delivers too much torque for a 5/16" nut to contain. when carb is out of adjustment.An analogy:electric lawnmower blade only retained by spring-castellated grip washer and plain nut but has electronic soft start. 150cc four stroke mower has only a plain bolt but the main shaft is protected by a huge boss made of friable cast alloy designed to shatter if (when) you hit something,thus protecting main shaft.I often use the big Super Tigre 2strs and have never felt threatened,likewise petrols,big softies really.I have been hurt by tiny.25 diesels,cut by Comp Specials,and nearly emasculated when kneeling before a Laser,accidentally starting it full throttle when the prop flew off and cut me acroos both thighs.OS 61 and 90 have come loose-the list goes on.I,m sure Jon will tell us Laser.s rationale for safe prop retention.I keep looking at the video of the 120 so the kid,s inheritance is under real threat. Re. old Enyas 35s,I,m pretty sure mine came with two cyl heads,one for straight and one for up to 10% nitro.Air bleed carbs on these and early Mercos were easy and worked well.

[Engine Doctor]

Posted by Martin Harris on 08/09/2017 10:14:53:

I'm not sure that the split flange nut isn't equally effective both ways round as any movement of the combination in either orientation will allow the prop to slip - i.e. the locking relies on compression of the flange onto the thread to prevent any movement - and all pictures/diagrams published by TT that I've come across show it the "wrong" way round. OS and the other clones have flanges on the larger nut for the drive washer so cannot be used in the TT orientation but reversing it was one of the first things we tried with the 91.

The larger nut can be tightened properly where as the smaller nut or locking nut will distort if tightened enough to hold the prop , probably why OS and others recess the nut into the washer to prevent incorrect fitting. the locking mechanism may be just as effective eitherway but if its not tightened enough in the first place it will allow the prop to slip.

[Jon H]

Posted by Merco 61 on 08/09/2017 13:43:04:

.I,m sure Jon will tell us Laser.s rationale for safe prop retention.

Put the nut on....go fly.

Thats the way we do it! we dont even supply a locknut as we have never found them necessary. The only engine i ever had that threw props regularly was my enya 120 fs but that was in part due to the very smooth ali backplate on the spinner i was using as it behaved just fine without it. I do tend to give it the big one when tightening it up and use a minimum of a 250mm spanner to get the required torque.

For your fuel merco i wouldnt go for 20% castor. It will make a nasty mess of the engine

Good to hear you are making progress though. Although i am slightly sad i dont get to play with it now!

Edited By Jon Harper - Laser Engines on 08/09/2017 14:09:29

[Martin Harris - Moderator]

Posted by Engine Doctor on 08/09/2017 13:45:46:

Posted by Martin Harris on 08/09/2017 10:14:53:

 

I'm not sure that the split flange nut isn't equally effective both ways round as any movement of the combination in either orientation will allow the prop to slip - i.e. the locking relies on compression of the flange onto the thread to prevent any movement - and all pictures/diagrams published by TT that I've come across show it the "wrong" way round. OS and the other clones have flanges on the larger nut for the drive washer so cannot be used in the TT orientation but reversing it was one of the first things we tried with the 91.

 

The larger nut can be tightened properly where as the smaller nut or locking nut will distort if tightened enough to hold the prop , probably why OS and others recess the nut into the washer to prevent incorrect fitting. the locking mechanism may be just as effective eitherway but if its not tightened enough in the first place it will allow the prop to slip.

Certainly, my instinct would be to go with the OS convention but the TT locknut is significantly larger than the OS one compared with the main nut - and they see fit to show it that way round both in their publicity photos and exploded diagrams. We normally advise to go with the manufacturer's recommendations...

One thing nobody has mentioned is how to tighten a locknut...my own technique (right or wrong) when assembling a propeller attachment (bearing in mind the slight compressibility of the hub) is to lean on the spanner for the first nut until my eyeballs start to bulge and then tighten the locknut (smaller spanner) while still holding the prop. The theory here is that holding the torque with the longer spanner for the larger nut against the locknut's one may allow the main nut to relax its torque fractionally as the two snug together.  Right or wrong, I honestly can't recall the last time I had a prop come loose.

Edited By Martin Harris on 08/09/2017 15:07:03

[graeme jones]

The TT instruction booklet has a photo of all 4 engines in the range on the cover, and inside. That's 54, 75, 91 and 130. The 91 is the only one shown with the large nut first, the other three show the small, slotted nut first. All 4 exploded diagrams show small nut first. In any case it's only the hex part of the nut that is "small" or "large". The small one has more threads because it has the male part of the cone. The large one has only a couple of threads because it has the female part of the cone, into which the male part fits and is supposed to lock. As I said before though, this does not prevent the prop coming off the shaft, it locks together as one piece and might just as well be one big nut.

Model Stationary and Marine Steam Engines by K N Harris is the only one of my collection of model engineering books that mentions lock-nuts. It says "nine times out of ten if lock-nuts ARE used in a model (and they are rare enough anyhow) they are wrongly fitted, the THICK one should be outside, it takes the weight!" Now this is in a different context and is talking about plain nuts, but was one of the reasons I tried mine the other way round. It still came off, allowing the prop to fly off as well. Tellingly, it could be screwed back on the shaft still in one piece. By the way, the spanner I used was a bit bigger, a 400mm adjustable, but the torque that can be applied is limited by the size of the prop not the size of the spanner.

The TT nuts are simply badly designed and do not prevent the prop flying off the shaft. Replace them with an OS set and at least you won't be hit by the prop, but it will still come loose. I also tried the method of having a helper switch on the glow plug once the starter was already turning the motor, with mixed results. From memory I did at least get a couple of flights before the engine stopped in the air because the prop had worked loose.

The glow plug supplied with these engines is TT's own, a Red Line RF. Substituting an OS F made no difference. The fuels I tried were MT Special 4 Stroke 5% and Castor Straight, again making no difference. I couldn't get the recommended 11 X 10 prop for running-in so used a 14 X 8.

[Merco 61]

Graeme, I,m with you all the way other than the 400 shifter.Too much leverage for my nerves.Jon says a 250 mm spanner which is a lot more than I,ve been using.In general terms I,m not at all convinced that a lock nut is guaranteed to work in this context,especially,as you say,when they come off as a jammed pair and can be replaced the same way.Where is the locking?Also I dont like the click when you spanner them apart.Tortured threads? I,ve binned the TT ones anyway,not enough thread on female to do any good,but sufficient perhaps to damage crankshaft.I was going through my prop drawer.The three blade 14x7 Graupners and MK fibreglass 13x10.5-all glow props- are all drilled for 4 bolt fixing.Is this telling us something we,d rather not hear? Sorry for repetition-I did some bench tests.The breakaway point was between prop driver and prop every single time,so in the absence of a bolted hub,some weapons- grade lateral thinking is called for,not just this engine but any biggish four stroke.I,m going to dig out a torque wrench and find out the poundage were putting on these shafts.

[Martin Harris - Moderator]

Posted by Merco 61 on 08/09/2017 17:27:07:

some weapons- grade lateral thinking is called for,not just this engine but any biggish four stroke.I,m going to dig out a torque wrench and find out the poundage were putting on these shafts.

To be quite honest, my regular dealings with a wide range of four strokes up to 180 size have been very straightforward over the years - my props have stayed in place quite happily using the method detailed a couple of posts back. The last time I had any real trouble was well over 10 years ago when I tried using lightweight wooden props which encouraged pre-ignition and an old OS40 Surpass did throw a couple of props when leaned out too far.

[Jon H]

Posted by Martin Harris on 08/09/2017 18:01:13:

Posted by Merco 61 on 08/09/2017 17:27:07:

some weapons- grade lateral thinking is called for,not just this engine but any biggish four stroke.I,m going to dig out a torque wrench and find out the poundage were putting on these shafts.

To be quite honest, my regular dealings with a wide range of four strokes up to 180 size have been very straightforward over the years - my props have stayed in place quite happily using the method detailed a couple of posts back. The last time I had any real trouble was well over 10 years ago when I tried using lightweight wooden props which encouraged pre-ignition and an old OS40 Surpass did throw a couple of props when leaned out too far.

I have an os 40 surpass. got it used/2nd hand, only run it twice, on its 3rd prop nut already. Its in the naughty engine drawer now

[Engine Doctor]

Posted by Jon Harper - Laser Engines on 08/09/2017 18:54:12:

I have an os 40 surpass. got it used/2nd hand, only run it twice, on its 3rd prop nut already. Its in the naughty engine drawer now

Hello John . The 40 and 48 surpass could be real so and so's . They are easily cured with the head shim. I think they were from an era when raising the comp ratio was thought an easy way to get more power..I had an Enya 60 FS that was virtually unusable due to throwing the prop . When stripped it even had a bent gudgeon pin due to the pre ignition ! Ituned to run flat out it would throw the prop when the throttle was closed and visa versa.. New piston , gudgeon pin and a head shim and it runs beautifully . It now lives in a twice size Lady bird .

[Jon H]

ahh the os48. I learned to fly pulled by one of those. My dad did keep a supply of spare nuts and props in the box! My 40 was acquired from the estate of an deceased club member and its fine as long as you don't go too lean on too small a prop. its fine as long as you are cautious but I can easily knock up a shim for it. As I don't need it just now its not a worry

[Bob Cotsford]

The OS48 in my Eros is a little peach running a 13" APC prop. Shall we say it is 'more than adequately' run in though. It still hauls a 7lb 84" vintage model around with ease.

As regards "some weapons- grade lateral thinking is called for" I'm with Martin, I've run up to 38cc petrol four strokes with single bolt hubs. Neither these nor ASP180s, Laser 160V or Evolution 35 petrol have needed anything more than a nut to hold the prop on. I think that the problem lies in the engine timing and compression ratio, not the prop fixing. Have you tried a different fuel. That might be worth a shot.

[Jon H]

Posted by Bob Cotsford on 09/09/2017 10:38:29:

The OS48 in my Eros is a little peach running a 13" APC prop. Shall we say it is 'more than adequately' run in though. It still hauls a 7lb 84" vintage model around with ease.

As regards "some weapons- grade lateral thinking is called for" I'm with Martin, I've run up to 38cc petrol four strokes with single bolt hubs. Neither these nor ASP180s, Laser 160V or Evolution 35 petrol have needed anything more than a nut to hold the prop on. I think that the problem lies in the engine timing and compression ratio, not the prop fixing. Have you tried a different fuel. That might be worth a shot.

I have some new engines in my head for Laser development and so far I have plans up to about 120cc. Trust me when I say I will be avoiding multi bolt prop hubs as long as I can as my experience with them so far has been poor.

[onetenor]

Hi can I put my oar in here? Re the nuts remaining locked together.They are designed to lock on a taper machined into them. and thus become one nut in effect. This actually detracts from the effect it is designed to have. Their locking force thus becomes that against each other not against the thread. I suggest a wave or inwardly castellated washer is used either under a plain nut or plain nut and washer and a second plain nut.The lock washer going on first. Alternatively locking washer ,prop, either a plain or lock washer plain washer combo then plain nut as deep as you can get.And now a question I meant to ask a while back. Is the thread on the shaft a proper fit in the nut,i.e. not slack?..If not you need a replacement C/shaft.The friction between nut and thread is what holds it in place. When the torque of locking the two nuts together reaches locking point they become as I said before one nut and the force comes between them and not the thread . In my opinion FWIW two plain well fitting nuts are a better bet. Right I'm ready to be shot: blindfold at the ready last. cig about to be rolled: by better engineers than me.

[Merco 61]

Hello, onetenor,I dont think anyone will argue with anything you have written,certainly not me,a good analysis and sound theory I,m sure.The threads on the crankshaft are in good condition.I,m doing my utmost to keep them that way.I,m using nothing more than two ordinary plain nuts at the moment,as even SC ones are about £12 per pair and Ive lost two sets to date.A nylon spinner is fitted-for the starter and as a loose nut catcher. I think a spinner with an ally backplate might help too. With compression reduced and needles about where they should be,the prop no longer flys off but once in a while there is a squeak and the prop shifts on the shaft,just a little,when starter applied.Then essential that all is re-tightened or all will come loose at next attempt-guaranteed.The neighbours are used to my engine noises but are getting fed up with the foul expletives when this happens.Do you think there is any pssibility that the powerful starter is part of the problem? How do you set the prop to perform the nifty backwards flick that Jon does in his video?Thanks a lot for your interest everyone,I,m sure that all the good gen is appreciated by all four stroke users.

[onetenor]

Good questions Merco 61 First of all I would go back to absolute basics. Get rid of the leather .It has probably polished itself with the slipping.The flattening of the alloy grooves on the driver is most likely also down to slippage.Recut them as I said making them as sharp as you can. never mind the depth it doesn't need to be deep. Straight grooves will be O.K..Reassemble as I suggested before. Not sure about the prop position but I think it may be vertical as it comes up against compression in the clock -wise direction. Someone will correct me i'm sure. The spinner may be the problem too slipping against the driver face. .If it's alloy and you still need a nut catcher change it to a plastic one. The backplate will embed better into the driver grooves.. If you still need a starter but are prepared to risk more nuts you can get silicone inserts with grooves cut into them that catch the prop blades instead of friction on the spinner cone.. Remember about the washer (s) under the nuts. This allows more torque by allowing the nuts to move and put more force on the threads and hold tighter. t doesn't work quite the same the other way round..Just be careful. Just an afterthought about the bounce method of starting. It could make matters worse because of the inertia effect. caused by the sudden change of direction I see I missed the nylon spinner bit. Personally I would leave it but if you want to change it feel free.Again I await the firing squad

[Ron Gray]

@Merco61. I too was very interested in the backflip stating method used by some fliers. I recently bought a Weston UK 52 to go in a GP Revolver, my first foray into 2 stroke glow for 35 years or more (although I have several petrol 2vand 4 strokes). Now this engine is interesting in that it is almost impossible to turn it over by hand due to the tolerances of the ABC unit, so much so that I thought there was a problem with the engine, only to find out that they are supposed to be like that. Anyway, when I first started it I was resigned to use the starter which managed, eventually, to turn it over and get it running. I then thought about the reverse flick method so gave it a go, didn't touch the prop, just gave the spinner a quick flick of the wrist and she fired up straight away. I now use this method all the time. Prime the carb (wet finger), turn it over by hand (difficult!), attached glow power, flick and away she goes. Not only do I save my pinkies it looks super cool to onlookers!