OS46 AX problem

[Keith Berriman]

Bit of help please.  Engine 4 years old and had regular runs but Sunday morning test run and it suddenly through the prop and spinner.  Spent Monday stripping down for a full check as noticed exhaust oil with traces of black in it. Engine all good inside with a little carbon deposit on crank. So rebuilt and tried a test run again yesterday rechecked main and slow running jet settings as instructed started first time tick over okay went for throttle loud click and prop gone again. Have changed the engine for now so more investigation can take place.  I have another os46 and no problems 

[Jon H]

Very strange for a 2 stroke to throw a prop. It sounds like pre ignition but again that is unusual for a 2 stroke. 

 

Did you take the backplate off the engine when you dismantled it?

[Keith Berriman]

Good Morning Jon.  Yes I removed all parts just to make sure of the main bearing was running smoothly and check for any wear on the back plate all good

Did notice the liner was not polished chrome but all dark as if burnt but piston ran smoothly up and down 

[PDB]

If you're using the same glow power source for both engines probably not the issue, however my OS46 was throwing props and it turned out to be an intermittent glow connection to a power panel.

[Jon H]

54 minutes ago, Keith Berriman said:

Good Morning Jon.  Yes I removed all parts just to make sure of the main bearing was running smoothly and check for any wear on the back plate all good

Did notice the liner was not polished chrome but all dark as if burnt but piston ran smoothly up and down 

 

Hmmm. If the liner was dark in patches it might be suffering from the OS liner plating disease. I would whip the top off and grab a photo. 

[Andy Stephenson]

The liner problem with the OS 46 is a well known problem, I had to have two replacements for mine before it would run properly but no prop throwing though.

[Nigel R]

Get a photo of the liner up here, the stripped liner can be easily spotted.

 

Would a bent rod cause this? 

[Paul De Tourtoulon]

I have never had a prop thrown on a 2 stroke, but as suggested look around the exhaust port to see if the chrome is missing, if so you will see the brass coming through, I had this with my Os built (? ) Irvine .53,,,

[Keith Berriman]

Thank you for your responses tomorrow I will strip it down again and take some photos 

[Ace]

A long shot - have you change the glow plug for a long reach 4s* ?

[Engine Doctor]

Cant think what would cause a 2s to lock enough to throw a prop. Have you checked the crankshaft for an cracks ? Cylinder should be shiny not black . If carbon is present then that can grip a piston when it gets hot .

[Ace]

Posted 3 hours ago

A long shot - have you change the glow plug for a long reach 4s* ?

Edit time passed 😥

Long shot - Have you recently accidently replaced the std glow with a long reach 4* plug - that would increase compression and advance the timing. 

[Nigel Grant 1]

I recently got an OS 46axII that had only been run a couple of times from a clubmate. I saw he had fitted a No.8 plug to it whereas they are supplied with a No.6, which is a hotter plug. When I ran it on the bench on Morgan's 10% it definitely rattled away with pre-ignition and threw the prop a couple of times. Fitting a No.6 made it come right.

I had the same problem with an OS25SF a while ago. An OS F plug would upset it.

[Nigel R]

20 hours ago, Engine Doctor said:

lock enough to throw a prop.

 

I've had one thrown prop from a two stroke. Happened with a brand new piston/liner. The fit was very tight, from the ABC 'pinch'. First run, within a few seconds of starting up it stopped and threw the prop.

 

Bizarrely (or maybe not, given the pinch on the piston/liner), same engine, a few minutes of run time later, I had closed the throttle to stop it, and it stopped on compression. Within (maybe) fifteen seconds, it had restarted itself. I assume that was caused by some local hot spot on the piston or liner being just enough to set off the fuel that remained in the engine.

[Andy Stephenson]

It's not unknown for a glow engine to start spontaneously after a run, I heard about a bad accident at a pylon race meet where this happened. A suspected glowing ember of carbon was thought to be the cause.

An obvious sign of pre-ignition is the harsh crackle you hear at full throttle. We were on the field when someone was running a heli engine giving the worst crackle I've ever heard and the inevitable happened, it stopped with a bang. When examined, it had blown a hole in the top of the piston.

[Nigel R]

Pre-ignition sounds awful, you know it is doing something nasty to the internals. That frying egg noise.

 

My case, I was running in, most definitely avoiding any preignition or pinking... varying between peak and slightly rich; very short peak, richen for a few seconds, back to peak, back to rich etc. Gradually lengthening the peak periods until it could hold full throttle.

 

For reference, Brian Gardner liner in OS 61, which if I remember right Andy, you have one?

[Engine Doctor]

 

2 hours ago, Nigel R said:

 

Bizarrely (or maybe not, given the pinch on the piston/liner), same engine, a few minutes of run time later, I had closed the throttle to stop it, and it stopped on compression. Within (maybe) fifteen seconds, it had restarted itself. I assume that was caused by some local hot spot on the piston or liner being just enough to set off the fuel that remained in the engine.

 

Re the re starting on its own. Some years ago when the OS32 SX was current i was priming my engine on a very cold morning . A few chokes and a few flips to get engine wet and ready for starting. No glow plug connected and it started ! This particular engine did it a few times and was treated with caution when turnning over. Then the chrome fell off the liner and it was returned under warranty . New cyl and piston that lasted about two months and plating came off again . After being repaired a second time it never started without a battery to plug . 

Another point to consider . Has the prop been tightened properly using a correct spanner of suitable length ? I regularly repair or service engines  where the prop nut has obviously been tightened using pliers or rounded off from using an incorrect spanner or clapped out adjustable ! The owners nearly always  deny this but the chewed up nut doesnt lie 🤥😄.  Also what typer of prop or spinner is fitted can cause issues ? Wooden props will crush and settle eventually becoming loose. Some makes of prop or spinner could also be affected by heat fron engine and loosen. These types of props/ spinners have too be checked for tightnes regularly or better still avoided. 

Edited May 5, 2023 by Engine Doctor

[Martin Harris - Moderator]

Mention of the propeller reminds me that a very lightweight prop e.g. wood, can promote the tendency to throw a prop due to the reduced flywheel effect.

[Keith Berriman]

[Keith Berriman]

Various photos of engine parts just the liner a darl colour

Fuel used Model Technics 5%

[Keith Berriman]

Standard APC prop 11*7

Quality spinner metal back plate 

Correct 10mm spanner used on nut 

 

 

 

 

[Andy Stephenson]

1 hour ago, Nigel R said:

For reference, Brian Gardner liner in OS 61, which if I remember right Andy, you have one?

Nigel,

Brian Gardner produces new piston and liners for OS 61RF/Hanno and SF engines as well as 61VF an FSR.

He needs to sign up 50 or so potential customers before he will do a run for a specific engine. I have two P&L sets for my RF engines, they are superbly engineered and not cheap. These engines are treasured by their owners, as replacements are difficult to come by. I have to say though it probably wouldn't be worth going this route for an OS 46 as there are many alternatives available. For a while Just Engines supplied a well respected Chineese replacement for the OS46 liner but these have long since run out.

[Andy Stephenson]

1 hour ago, Engine Doctor said:

Also what typer of prop or spinner is fitted can cause issues

With an APC 10 x 6 prop on an OS 32, I discovered that the prop driver wasn't big enough to fit against the prop hub. The hub on this prop has a recess in it such that only a small surface area is in contact with the drive. The result is that, over time, the driver sinks into the material of the prop and it comes loose, this happened to me twice before I discovered how it happened, fortunately whilst in the air and not on a ground run in front of me.

[alan p]

Regarding the plating stripping is it viable to get the liner cleaned up and replated with hard chrome, assuming you would have to hone to suit your piston if it is serviceable?

[Jon H]

57 minutes ago, Keith Berriman said:

Quality spinner metal back plate 

 

Does it throw the prop if flown with no spinner? Ali spinners can slip if the knurl on the backplate does no play well with the prop driver or prop. Its a similar situation the the one Andy describes above.