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OS 1.20 Surpass Valve to Piston Tolerance

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OS 1.20 Surpass Valve to Piston Tolerance

Old 07-02-2015, 07:33 PM
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CLBetten
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Default OS 1.20 Surpass Valve to Piston Tolerance

I am working on a first flight crashed 1.20. It hit the ground running. Both pushrods and tubes were seriously bent. I removed the pushrods, reinstalled the head and checked for cylinder leakage. Both valves leaked, especially the exhaust. I removed the valves. I can see no contact marks on the piston or valves. I can't perform any test that shows them bent or damaged. I also believe the the valves cannot contact the piston at full open. Is this a "free running" engine? Meaning can the valves contact the piston regardless of cam timing?
Old 07-03-2015, 05:40 AM
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I don't think that the valves can hit the piston if the engine was running and the valves are in time. Even bending the push rods should shorten everything but could have pushed the rocker up a bit I suppose. Anyway here is the way to check, put the piston at top dead center and then see how far the valve will move before hitting the piston. That will give you a real good idea of how much movement can be tolerated.
Old 07-03-2015, 07:23 AM
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CLBetten
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Since it had no compression I figured the pushrods were pinched in the tubes enough to hold the valves open. If that happened they weren't in time. I was surprised when I had no compression with the pushrods removed. To get the valves to touch the piston at TDC the stem is so far in the guide I don't see how the spring and retainer could allow it.
Old 07-03-2015, 11:15 AM
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If the valves won't seat now you can try to lap them in just a bit. Here is what I have done before, cleaned the valve up with red Scotchbrite pad so the contact face is clean and bright, then take some toothpaste and put on the valve and seat area and drop the valve into the head. I used a pin vise to grab the valve stem and then very lightly twisted the valve back and forth a few times then cleaned the valve up and inspected the seat area on the valve. If you see a lightly matt circle on it you have probably done enough, if not do it again until you do. Then reassemble and check for leaking. Also be sure the ring isn't stuck in the ring groove causing no compression.
Old 07-03-2015, 05:32 PM
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CLBetten
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It is a new engine. The top of the piston isn't even discolored yet. I definitely have valve leakage. I am trying to understand how it resulted from the crash. I'll see if they lap easily.
Old 07-04-2015, 03:38 AM
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Jennifer Curtis
 
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Remove the valve and spin it by
the shaft. If you can see any wobble
in the face, the shaft is bent, and no
amount of lapping will fix it.

Jenny
Old 07-04-2015, 05:39 AM
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I agree with Curtis on checking the valves, I usually chuck them in a drill by the very end of the stem and spin them to see if they are straight. I had another thought about low compression and valve leakage, have you fixed the bent push rods and reinstalled them? If you are trying to turn over the engine without the valves working it will never have any compression feel to it. Same can be said if you don't open the throttle a bit while cranking. You have to have something to compress in the chamber. Since it is brand new the ring has likely not seated much so a few drops of oil down the glow plug hole should help make the ring seal better and see if the compression is improved. Just a few thoughts and hope you get it going.
Old 07-04-2015, 05:56 PM
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Got it handled. Thanks for the attempts to help.
Old 07-05-2015, 06:02 AM
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Want to share with us what you found, it might help someone in the future with a similar problem.
Old 07-05-2015, 11:03 AM
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Yeah, please, don't keep it secret.
Old 07-10-2015, 11:20 PM
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Sorry, I needed my new parts to be sure of my conclusions. This engine does have enough tolerance between the valves and piston that no contact can be made at maximum lift even if the piston is at top dead center. The valve leakage was being caused by grains of minerals ingested during impact with the salt flat preventing sealing against the seat. After I cleaned everything up I checked the valve to seat contact by marking the valves with a water color marker. They were perfect. I reassembled it. It has no valve leakage, great compression and runs perfectly.
Old 07-15-2015, 09:17 AM
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Some new engines have a tight valve guide and the valves don't seat well causing compression to be soft. But usually they run OK just a little harder to start. The example I have did not do that when out of the box, but after the first run or tow did that, then it quit after some run time. Could be that this is what happened and it had nothing to do with the crash. If it has enough compression to start I would run it on the ground a tank or two and then fly it if it runs OK.

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