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Saito 125 "exploded" why??

Old 07-28-2014, 11:31 PM
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ALO 111
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Default Saito 125 "exploded" why??

I have a saito 125 gk with less than 20 flights on it,always runs great . So the next time when i start it ,it starts a little more difficult than normal but idles ok. As soon as try to open the throttle it wont really pick up revs much and dies if throttle is fully opened. I do all the normal checks at the field " glow plug fuel lines valve clearance all seems fine. Try to start again and while the starter is turning it over BANG !! the cylinder is blown clean off . It has broken on no particular place either. Aside from the piston skirt that broke a small piece off after hitting the block there is no other damage.
Unfortuantly i had this motor lying NIB for a few years before fitting it a couple of months back so the importer is shrugging his shoulders.
Any idea as to what happened ?
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Old 07-29-2014, 01:51 AM
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Mr Cox
 
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Sounds like a "hydrolock" i.e. too much fuel in the engine (and the fuel is not compressible).
Old 07-29-2014, 02:48 AM
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1QwkSport2.5r
 
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That is some pretty serious carnage for just a hydraulic lock. Looks to me like it had probably had a stress crack or other anomaly that fatigued the metal and perhaps under the right circumstances caused the metal to crack further and separate in a violent matter. Ouch.
Old 07-29-2014, 02:50 AM
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Rudolph Hart
 
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Alo i think the rod bent and the piston went down to far,when it came back up cockeyed it gripped the bottom part of the barrel and heaved your cylinder upwards.It looks like heavy score marks right at the bottom of the cylinder? dave what do you think i'm just guessing,anybody else? i'd be interested to know.cheers pete
Old 07-29-2014, 03:29 AM
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ALO 111
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Thanks for the reply,s
I just checked again the marks are only on the photo "light playing tricks" conrod is perfect no damage at all. This happened at idle speed so i doubt it could hydro lock while running. ??II am convinced it is a casting flaw but welcome all input . If it is a casting flaw its my view that this should never occur no matter how old the motor.
Old 07-29-2014, 03:51 AM
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I think that the simple explanation would be a casting flaw in the base of the cylinder head. A crack formed and it turned into the weak spot for failure.
Next would be that it seemed like some sort of a detonation occurred. Maybe too much nitromethane or something wrong with the fuel where it had aged for a long time (stored in poor conditions) or something and was going bad.
Since the Op stated the rod and piston are OK, then I don't think it could have been a hydraulic lock though, as that generally damages the connecting rod too.

So we have a cylinder head that cracked at the base and was bent or angled out of position slightly, causing the engine to run poorly as the valve gaps would change. The OP then when starting got too much fuel into the cylinder, almost a hydraulic lock. The cylinder head started to come off and the glow plug ignited the fuel and "bang" it blew off.

Last edited by earlwb; 07-29-2014 at 03:53 AM.
Old 07-29-2014, 04:22 AM
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Since this ocurred while the starter was turning it, it is almost surely the result of hydro-lock. A previous crack would be discolored and obvious.
Old 07-29-2014, 06:42 AM
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1QwkSport2.5r
 
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I stand by the point of this being a manufacturing defect of some sort. Nobody is going to tell me a 21cc piston and rod could hydraulic lock so hard it blows the cylinder apart without any other damage. Does the cylinder bolt to the crankcase at its base? If so, could it be possible loose cylinder bolts allowed the cylinder to move thus jarring and stressing the base of the cylinder? Maybe I'm too left field on this one.
Old 07-29-2014, 07:21 AM
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That's a clean, random break all the way around.

Hydrolock does a lot of damage. I've seen a piston crown dented and deformed. And, this one fired the fuel charge.
Old 07-29-2014, 09:14 AM
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ALO 111
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Originally Posted by 1QwkSport2.5r View Post
I stand by the point of this being a manufacturing defect of some sort. Nobody is going to tell me a 21cc piston and rod could hydraulic lock so hard it blows the cylinder apart without any other damage. Does the cylinder bolt to the crankcase at its base? If so, could it be possible loose cylinder bolts allowed the cylinder to move thus jarring and stressing the base of the cylinder? Maybe I'm too left field on this one.
I agree, cylinder bolts are all in there place and tight. I find it strange that the conrod has no damage at all. The motor turned over a few times before it went bang so how could it hydro lock ?
Has anyone ever seen this before ? Hydro locks occur often but i have not found one other case like this.
Old 07-29-2014, 09:49 AM
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1QwkSport2.5r
 
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Originally Posted by ALO 111 View Post
I agree, cylinder bolts are all in there place and tight. I find it strange that the conrod has no damage at all. The motor turned over a few times before it went bang so how could it hydro lock ?
Has anyone ever seen this before ? Hydro locks occur often but i have not found one other case like this.
Hydraulic locking doesn't usually blow the whole cylinder off. Usually as has been said, the conrod and/or piston suffer the most damage. A 2-stroke is more likely to run a little bit and hydraulic lock if all circumstances are right. Even at that point, most starters won't be able to turn the engine over unless the user is over lowering the starter or using a freakishly high torque starter.
Old 07-29-2014, 10:14 AM
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A few years ago someone had reported that their Saito engine blew off its cylinder head in a similar manner. But I don't remember the details now. It might be way back in the old history files someplace too.

Someone else mentioned that it was possible to still have a hydraulic lock occur, at or near TDC. In that case it may not damage the piston or rod. There just might be the right conditions to get the fuel to ignite in that situation too. Or the cylinder head came off and as it was getting pushed off the air fuel mixture ignited causing the bang.
Old 07-29-2014, 04:35 PM
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How about the valvetrain? Is that damaged?

Just speculating, but if the valvetrain failed, causing the exhaust valve to not open, wouldn't that cause excessive compression?
Old 07-29-2014, 05:31 PM
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It's a pretty good picture but not quite focused enough on the break itself all the way around to make a judgment. If the crack pre-existed, you should see some evidence, slight polishing visible under magnification, then a sharp brittle fracture the remaining way around (as the crack progresses, eventually gets to the point where there is not enough area left to support the load and you get sudden failure). Also, if there were casting defects such as voids or inclusions, you should be able to see them by examining the broken surface closely.

Greg
Old 07-29-2014, 10:08 PM
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ALO 111
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Valve train has no damage at all. I might have another photo just trying to find it in my unsorted files. Unfortunately the motor is at the importer and has been for ten weeks but iam getting no answers or help whatsoever.
Old 07-30-2014, 02:24 AM
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Wow! I am glad you were not hurt as that looks like it could have thrown pieces. I have never seen anything like that before.
Old 07-30-2014, 03:54 AM
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You've never been in my shed at night watching a bench run?
Old 07-30-2014, 06:38 AM
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Originally Posted by Old Fart View Post
You've never been in my shed at night watching a bench run?
hahaha. Somewhere on RCU from about 10 years ago is a video of Norwegian guys who put a shot 2 stroke on the bench in the garage, set up the camera, cranked it wide open with no prop, and ran inside until it blew.

An engine will certainly turn over when hydrolocked if one keeps the starter going. It just breaks and reshapes the parts. And maybe blows up if the fuel ignites.
Old 07-30-2014, 08:32 AM
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Could it be hot fuel? Say it had 40 or 50% nitro in it. It is somewhat clear that detonation occurred. That means lean mixture. Just the right circumstances (fuel air mixture just right for it to occur) and detonation at the right time and spot. Where it broke may be the weakest point and the cylinder just could not take the overpressure. Very similar to hydraulic lock failure. No matter, that engine is toast now. Things happen!
Old 07-30-2014, 08:38 AM
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When you enlarge the photo you can see the white oxidation on the top of the cylinder head. This tells me that during the casting process something went wrong. I would contact Horizon Hobbies directly and demand a replacement based upon this evidence.
Old 07-30-2014, 10:17 AM
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Originally Posted by wcmorrison View Post
Could it be hot fuel? Say it had 40 or 50% nitro in it. It is somewhat clear that detonation occurred. That means lean mixture. Just the right circumstances (fuel air mixture just right for it to occur) and detonation at the right time and spot. Where it broke may be the weakest point and the cylinder just could not take the overpressure. Very similar to hydraulic lock failure. No matter, that engine is toast now. Things happen!

I wouldnt have thought that engine was toast. good catch

Lean engine wont do this......

just looks like it wasnt heat treated correctly and was to brittle.
Old 07-30-2014, 10:26 PM
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The fuel was 10% nitro and I had already used about half the 5 litre can, I have since used the rest with no problems. I am sure there was a crack at first because in hindsight I remember when I first started it and it would not rev it sounded strange 'like a leaking exhaust. I don't run a silencer so it was a bit more difficult to hear. I have also being flying my gas planes alot so maybe not quite in tune with the four stroke sound. What I must say is it did fire before it blew,I guess it turned over about 3 or 4 times before it went bang.
Old 07-31-2014, 07:26 AM
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The lighting in that pic does not give good contrast for the break area, but unless some extreme and overriding cause overstressed the metal, failures of this magnitude are the result of a chain of factors. Engine orientation, past event history or even plug type.

I may have missed you mentioning motor mount orientation. If beyond horizontal you run the risk of injecting a slug of raw fuel into the chamber already presented as Hydrolock. A mismatch of fuel tank height to carb, either from construction or environment, (how the aircraft was sitting prior to starting) could exacerbate the likelihood fuel could siphon into the intake. If inverted that fuel would not drain away and remain available the next time you turned the crank. That's why I try to teach my students it's always prudent to flip the prop over several times by hand, (with glow heat OFF!) before attempting to start. Much easier to determine if anything amiss ahead of time.

Prior partial or full hydraulic lock occurrences may have stressed the casing already weakened from a manufacturing fault, (and perhaps hidden under the paint layer) or during its use. However, I have to agree there should be evidence, if even microscopic, of where the failure initiated. The one single catastrophic event mentioned before strong enough to cause that failure should also be evident in other ways. The piston, con rod and even the cylinder fasteners would show equal force applied. Did you check to see if those retaining bolts were loose or stretched?

Been a while since I had either of my 125's apart and I cannot state if the jug wall thickness should be uniform. Unless there is a step in the break area opposite the camera, seems yours decreases substantially opposite the pushrod tubes.

Last edited by Cougar429; 07-31-2014 at 07:30 AM.
Old 07-31-2014, 10:55 PM
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The motor was mounted inverted like all my motors. The tank however will be lower than the carb when stating as it was mounted in a P51 'taildragger'
There is no evidence of stretching or loosening of the cylinder bolts. What I find strange as to hydro lock in this case is the motor started, ran for a few seconds then died. Glow igniter was not removed. I immediately tried again and it fired for what I guess were about three or four cycles then went bang.I have sent an email to Horizon for what it's worth as the importer here does not even contact me back.
Old 08-01-2014, 04:44 AM
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Being as it is inverted the muffler could have been full of fuel and flowed back into the cylinder. Even a half revolution backwards could let that happen.

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