 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
| HomeDiscussion Groups General ModelsRailroadsRocketsRadio Controlled Air ModelsHelicoptersLand ModelsWater ModelsModelGeeks.com Contact UsLink To UsSearch & Site Map |
Model Forum / Radio Controlled / Helicopters / April 2005Blown pistons
Hi guys, Just wondering if anyone could explain what might cause a holed piston. A friend of mine has gone through 3 pistons in 2 engines. The first was an OS 50 SXH had about 1 gallon of fuel (MT Sport 5) through it. The second engine, an OS 50 Hyper went through 2 pistons both after about only half a gallon of Wildcat 30%. The engines were run rich and I believe all were run on Enya no3 plugs. On this last one he said that just before it happened the engine went to running perfectly. I've had a similar thing happen with a ringed 32sxh. It was running spot on for about 3 minutes then bang. It sounded like the exhaust had come off. On the 32 the top edge of the piston had broken off and then gotten chewed up. The pistons have nice big holes pretty much in the centre of them. Any ideas? Thanks Andy Sounds a lot like over compression, were the plugs long reach or fitted with an idle bar or have the head shims been lost/removed when the engine was stripped? What were the engine temperatures like when running and did they run hard or rough? Chris > Sounds a lot like over compression, were the plugs long reach or fitted > with [quoted text clipped - 5 lines] > > Chris Hi Chris, The plugs were Enya no3's. I don't believe that they have idle bars (not sure what they are, but the plugs just have the element inside, nothing else). Also, they are the same length as the OS no8's. The 50 hyper was put back together by Irvine (we assume, they didn't say they sent the engine anywhere else) so we can only assume that it was done correctly. Saying that, the engine blew a piston from new. On the over compression, is that possible with ringed engines? I know that there was no difference in power ratings between the 32sxh ABC and the ringed version. The engines never ran hot, cold to warm. Can you explain hard and rough to me? Thanks Andy Hi Andy, yep you can get overcompression in any type of engine, you just need to find a way to reduce the head clearance to produce it (i.e. remove the head shims, there is usually a single aluminium shim fitted between the button and the top of the piston liner maybe more depending on the manufacturer and the nitro level designed for) or put something into the combustion chamber (e.g. a longer plug or one with the copper washer removed). With a ringed engine you'd just get more blowback past the piston than in an ABC or AAC engine. The hard or rough running is what the engine sounds like... normally they are quite a sharp sound, when over compressed they sound harsher, harder it's difficult to explain but easier to hear. You can also feel more vibration through the airframe, like a misfire in a car or an engine running 1 cylinder short of normal. Have you stripped any part of the engines to check the rest out for damage like main and front bearings and the rod bearings? The only other thing that'll cause damage apart from the piston locker that's been mentioned is: do you use any additive or after run in the engine? It is possible that you got hydraulic lock at some time or the after-run/additive reacted with the fuel somehow to damage the top of the piston. What does the button look like (the aluminium disk the plug screws into)... is it black or brown as you would expect after running a decent ammount of fuel through the engine. If it is clean indicating the carbon has been burnt away during combustion and not been cleaned up then the engine head has been running hot, that will also show up in the plug element - it will be slightly pitted maybe a bit grey and pushed back into the plug body and not smooth and silvery like it should be. If the plug element is fine and not distorted (the pushing back inside the body is the dead giveaway of overcompression) and the button looks ok (scratches from metal debris are expected as they're bits of the piston) then you probably got a bad batch of pistons in your engines. I have seen engines in the past where the skirt has broken away caused by a bad batch of alloy (I think it was too much carbon that made them brittle)... 2 x .21 race car engines at £300 a pop, run in and running well, about a gallon of fuel through each engine, piston and liner shot, debris also took out the main bearing and pitted the rod and button. Chris If your friend used pistonlock tool for setting the fan, its make a weak spot on the piston Regards Chris > Hi guys, > [quoted text clipped - 18 lines] > > Andy Hi Chris, No, myself and my friend both use a plastic toothbrush pushed into the carb hole (with the carb removed obviously ;) ) to tighten the fan. Thanks Andy > If your friend used pistonlock tool for setting the fan, its make a weak > spot on the piston [quoted text clipped - 25 lines] >> >> Andy > Hi guys, > [quoted text clipped - 18 lines] > > Andy Hi again, I've been doing alot of reading on this and wanted to know if I'm yet in the right ball-park. I am slowly but surely coming to the conclusion that it's the plugs that have been killing the engines. Here's how I think it works: The enya no3 is a hot plug. The more nitro in the fuel, the lower the flashpoint of the F/A mixture. The hot plug is advancing the timing and detonating the F/A mix, as opposed to the prefferred 'burn' of the mxture. The detonation (same effect as taking a hammer to the top of the piston) is causing an eventual failure of the piston at the concentration point of the detonation (which on the 50 hyper is right in the middle I believe). So I'm thinking that for the Wildcat 30% being used, an Enya no5 or no6 would be most appropriate. Comments on the above would be most appreciated. Also, I'm going to run colder plugs in my own 32sxh ABC that's running on Wildcat 15%. This engine has run the smoothest of all at the field all over the winter and now in spring has started to vibrate and hunt alot, at the same time it's running much warmer. I'm going to try out an enya no4 first and may even try a no5 to see which the engine likes better. Have fun Andy Running lean is what causes pistons to blow. This can be compounded by using piston-lock tools, but you said you don't use them. Hot plugs are GOOD for these engines. Both the OS50 and the OS32SX love to run with hot plugs. What they DON'T like is to be run too lean, as they'll cook and then pop. The Enya #3 is a perfect plug to use on both of these engines. Lean runs can also be induced by plumbing problems or a loose muffler. Clunk lines tend to disintigrate over time, which causes pinhole leaks that make the engine run lean. You always want a margin of safety (i.e. don't run the engine at the perfect lean mixture) just in case you get a bit of junk in the filter and the diminished fuel flow causes a lean run. Leif > Hi guys, > [quoted text clipped - 18 lines] > > Andy |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.