Marko Posted October 20, 2009 Share Posted October 20, 2009 Hi All ....ive been flying my chipmunk the engine is an OS 91 4 stroke its been fine ,starting first time but now ive had my first major engine failure engine started and it was fine l had a fly and landed. fueled up and started the engine but the prop just spun without starting there is no compression on inspection l found the inlet valve spring to be broken lve got a new set from justengines but l dont know how to remove the collets from the spring to remove it .......... ls there a tool for this job....... ls it a job you can do without much knowledge of repairing engines or ......... should l send it to justengines to fix any help would be great and save me money cheers........................mark Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 20, 2009 Share Posted October 20, 2009 You couod use a proper small spring compressor, but frankly, its usually possible to do with a couple of flat blade screwdrivers or similar levers... an assistant is useful for spotting where the collets shoot off to after you prise them offSome engines use a circlip to hold the spring top in place on the valve stem. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 20, 2009 Share Posted October 20, 2009 The tool is known as fingers... With the cylinder head removed, hold the head of the valve with your thumb and use the 2 "Churchill" fingers (weaker hand) to depress the collar at the top of the valve. There are 2 split collets round the head of the valve stem which you'll need to ease off with a small screwdriver or similar. Some people do this job in a clear plastic bag in case everything pings... Reassembly is the opposite procedure easing the collets into place and releasing the spring pressure so that they're gripped by the collar. Fiddly but you'll get there in the end! Quote Link to comment Share on other sites More sharing options...
001 Posted October 20, 2009 Share Posted October 20, 2009 Mark, when dismantling and refitting. put the cylinder head in a large clear plastic bag, work with your hands inside it so that nothing goes astray, make sure everything goes back together the same as it was dismantled and clean everything. Loosen and tighten the bolts holding the head down in a diagonal pattern, Or, if in doubt, personally I would send it to John Haytree in Devon to fix. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 20, 2009 Share Posted October 20, 2009 Ah yes thanks Martin...I had forgotten the name of the special tool there for a moment Quote Link to comment Share on other sites More sharing options...
Marko Posted October 21, 2009 Author Share Posted October 21, 2009 Ok thanks guys l have fingers when the head bolts are out and the push rods out is there much force needed to remove the head l dont want to force it and damage it or bend the push rod covers would a flat blade screwdriver do the job to prize it off ? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 21, 2009 Share Posted October 21, 2009 No prising necessary - the head should just lift off but there's probably a hidden bolt under the rocker post. It's good practice to remove the rocker post (1 bolt) at TDC on the compression stroke and you should do this before slackening any head bolts. The only thing then that might hold it is the rubber pushrod tube seals but you can just ease the head away from the pushrod tubes .Edited By Martin Harris on 21/10/2009 00:22:43 Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 21, 2009 Share Posted October 21, 2009 be cafreful not to score or otherwise damage the head if you use a screwdriver - why not remove the head screws before anything else, and with a plug fitted, turn the engine over, and the compression stroke should usually pop it off. A little oil in through the exhaust port may aid this by increasing compression. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 21, 2009 Share Posted October 21, 2009 I'd be very surprised if a tool was needed - I've worked on many OSs and clones and haven't known one to need any tool to remove it . Please try to avoid using anything if you can. Perhaps if it's been run for years on castor....? Never managed to get the head off a Saito, though Edited By Martin Harris on 21/10/2009 00:36:38 Quote Link to comment Share on other sites More sharing options...
Marko Posted October 22, 2009 Author Share Posted October 22, 2009 Hi all job done the engine is now fixed l even made a tiny spring compressor all l need now is a nice day to start it hopefuly l should get a few more rpm aswell thanks for all your advice .................... mark Quote Link to comment Share on other sites More sharing options...
Glenn Philbrick Posted October 22, 2009 Share Posted October 22, 2009 I've have just replaced the valve springs on my OS 91. If you put a small block of balsa under the head so that it fits within the dome you can press down on the valve collets by resting it all on your bench or suitable surface that way you can push down quite hard. You may even need to use something just to free the collar off as it can get quite tight but once the taper is loosened then you should not have a problem. Refitting them is quite easy but watch out those collets are very small and can go ping. This is the second time my OS has broken springs each time it was the exhaust, I think it is over revving in a dive so now have fitted a larger pitch prop. Glenn Quote Link to comment Share on other sites More sharing options...
Glenn Philbrick Posted October 22, 2009 Share Posted October 22, 2009 I've have just replaced the valve springs on my OS 91. If you put a small block of balsa under the head so that it fits within the dome you can press down on the valve collets by resting it all on your bench or suitable surface that way you can push down quite hard. You may even need to use something just to free the collar off as it can get quite tight but once the taper is loosened then you should not have a problem. Refitting them is quite easy but watch out those collets are very small and can go ping. This is the second time my OS has broken springs each time it was the exhaust, I think it is over revving in a dive so now have fitted a larger pitch prop. Glenn Quote Link to comment Share on other sites More sharing options...
Marko Posted October 22, 2009 Author Share Posted October 22, 2009 Hi Glen look up all done now matey thanks my old os 91 is a mk1 lts out of my blackhorse chippie so its not even working to hard i thinks the springs time was up lve cleaned the valves and inside the head with fairy power spray (liquid gold) everything went back together nice and clean cheers .......................mark Quote Link to comment Share on other sites More sharing options...
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