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How to tell if I have a "decked" case?
Sorry if I'm using the wrong words, but if I heard correctly, Lycon said that my case was probably "decked" - meaning that someone in the past milled down the area of the case where the base of the cylinder sits with the purpose of increasing compression.
I had my cylinders ported and flow balanced and then installed high compression pistons (9.5:1 from Combustion Tech). When we put the cylinders back on (even before installing the pushrods), the prop would NOT turn all the way around. It gets stuck when any of the pistons moves to the top of the cylinder. (like the cylinder height is too short to allow the piston top to move any further) I called Lycon and sent them some photos and took some approximate measurements and they think my case has been decked so these high compression pistons, although only .06" taller than the stock pistons, will top out because they're too tall for the cylinder. And the reason why is because the cylinder sits on a shorter base because it was milled down (decked). So my question is, how do I verify? Is there some published dimension somewhere that I can look at and compare with mine? Would it be the distance from one side to the other where the cylinder base sits? (sorry I don't know what that's called). Look at the photo attached and you can see a "lip" at the base of cylinder #3. It is raised a little bit. According to Lycon, that's where the face height originally was and it was milled down from there to effectively increase compression. Does that sound right to you guys? At first we thought maybe we were sent the wrong pistons or that something was wrong with the cylinders or maybe they might have switched up my cylinders and sent the wrong ones. But I verified with Combustion Tech that it's the right pistons and Lycon said they definitely did not send me the wrong cylinders. So this is the only explanation. Or is it?? https://drive.google.com/open?id=106...imnD-FYW98tNyO |
This was the way to increase compressions back when there weren't anything other than stock pistons available.
I'm guessing if you went with stock pistons you'd be ok. Deck height is something the case overhaulers would know and you'd have to measure the case halves on a surface plate. |
Deck Height minimum 4.775"
I have the data from when ECi was a repair station in San Antonio that overhauled crankcases. The minimum approved deck height to certify Lycoming 320, 360, 540, and 720 crankcases was 4.775 there. That is measured from the parting surface of the crankcase halves to the cylinder deck.
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I know this is a long shot but are you sure its the piston hitting the top? The rings on these engines do have an upside and a downside. If you install the rings (any 1) up side down the piston will not go all the way to the bottom of the tapered barrel. You should have noticed this when installing/gaping the rings but just a thought.
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360 wide deck = 9" across 540 wide deck angle valve = 9 1/16" 320 narrow deck = 9 1/16" granted the measurement method was crude but result is a lot less than 4.775 X 2. Is there a raised surface between the two mating case halves that would account for the 0.275 x 2 dimension difference? |
Thanks for the replies. I'll measure and see what I get.
And yes, we made sure the rings are installed properly and carefully checked the gap for each one. I can see the piston top and feel it with the tip of my pinky through the spark plug hole. This is the absolute limit of how far up it can go. I also checked by taking the cylinder off and pushing the piston as far up into the barrel as I could and it's in this same position. This photo is of it on the engine and with the prop at the "stuck" position. https://drive.google.com/open?id=10A...5-Hxrz5owx0dNR |
maybe it's a case of milling a taper around the top edge of the piston for more clearance.
I remember something about folks swapping C-85 pistons into a O-200 for increased CR and having to taper the edges on some engines, or similar. If it turns out your case halves have been shaved, using the new pistons & tapering the edges may get it to rotate acceptably but than you may end up with a higher unknown CR that may not be a good thing. |
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Lycon told me to get exact measurements, and they can calculate what CR I have now with the stock pistons. They said just by the fact that the 9.5:1 are topping out, they think I'm at least 10:1 or 10.5:1 with my stock pistons (8.5:1). This would explain a lot. I always thought I was "lucky" to have an unusually strong engine. My engine turns a prop that's bigger and has more pitch than what's recommended for a 160hp engine. And I'm quite happy with my speed and climb. I get about 205mph WOT level and climb at least 2200fpm. However, I was greedy and wanted even more power so I went through all this which ended up being a huge hassle and very costly in time and money. |
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Isn't there a manual or some place where we can look this up and know for sure what that number is supposed to be? |
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As a data point, these guys list 4.475" min on their 8130s and it appears it's supposed to be nominally 4.5" for these crankcases. |
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