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How to tell if I have a "decked" case?

1flyingyogi

Well Known Member
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=106jJoV5f-DijA1PboIimnD-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.
 
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.
 
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.

Jim- the 4.775 confuses me a bit as I just measured 3 engines (cases assembled, flange to flange) and come up with
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=10Am_G62Ai1EtxnL3QV5-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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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.

If in fact my case was decked, I already have high compression, even with the stock 8.5:1 pistons. I don't want to go higher.

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.
 
Jim- the 4.775 confuses me a bit as I just measured 3 engines (cases assembled, flange to flange) and come up with
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?

Ralph, which model 320 was it that you measured? It's a pretty big difference between your number and Jim's. This would more than account for a difference in piston heights of different compression pistons. For example, a 9.5:1 piston is only .06" taller than an 8.5:1 piston.

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?
 
Ralph, which model 320 was it that you measured? It's a pretty big difference between your number and Jim's. This would more than account for a difference in piston heights of different compression pistons. For example, a 9.5:1 piston is only .06" taller than an 8.5:1 piston.

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?

You would think so but it's not listed in the Lycoming table of limits.

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.
 
Case overhaul limits.

You would think so but it's not listed in the Lycoming table of limits.

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.

James number must have been a typo. 4.475? is the minimum for certified cases. Uncut cases measure 4.500? measured from the case parting line to the cylinder pads. This can be measured with a standard set of dial calipers on a bare case if done carefully. When the case is overhauled it is cut a maximum of .025? per side to remove fretting and to shrink the main bearing journals and cam journals so they can be line bored. Even with this small amount the idler shaft pads hace to be moved to keep the correct back lash on the accessory gears.
I?ve seen cases decked on the cylinder pad side to increase the compression ratio from some biplane racers at Reno win the class required the old 290 Lycoming. It?s a royal pain to remove and reinstall all the cylinder hold down studs to accomplish this and with at least 2 after market manufacturers of high compression pistons there is no need anymore. Interesting side note Lycoming makes a 10-1 piston for a helicopter version and they were highly coveted by Hot Rodder?s back in the day.
 
Poor Mans deck height measurement.

I would take a stock piston and pin and hang it on the rod. Then take a tablespoon size chunk of modeling clay and put it on the center of the piston dome. Spray some lube inside the cylinder head ( to keep the clay from sticking) then reassemble the cylinder on the case. Rotate the engine through a couple of times then disassemble. Take the plunger side of your dial calipers and measure the thickness of the squished clay and voila you have a poor mans piston deck height measurement.
With that measurement and the number you shared for the 9.5-1 piston you should be able to interpolate the aprox?mate comp. ratio with the stock pistons.
 
Deck Height

Jim- the 4.775 confuses me a bit as I just measured 3 engines (cases assembled, flange to flange) and come up with
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?

I'm sorry the correct minimum at ECi was 4.477 not 4.775. Divco and others may have a similar minimum to certify after lap and line bore.
 
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.

DId you gap the rings? I just bought 6 superior cly kits and on each one, I had to shave a good 10-20 thou off the rings to get the minimum gap up in the choke area. If I didn't do this, I would have had the symptoms you describe.

That said, if the cyl mounting decks are milled, you definately don't want high compression pistons.

Larry
 
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Check this out: Ask Paul: Wide deck vs. narrow deck engines

Basically, look for an "A" suffix in the serial plus hex nuts on the cylinders.

Thanks! Looks like mine is a wide deck - I'll confirm when I get to the hangar, but I'm quite sure it's a wide deck based on the the link you gave.

So... does anyone have a number for what the deck height is supposed to be for a WD O320? The numbers given so far are for ND cases.
 
Paul?s article on narrow verses wide deck engines is interesting but contrary in some respects to what I was told. On the IO540 at least the narrow deck engine is preferred by racers and hard core aerobatic guys because it is viewed as stronger and more tolerant of raised compressions. I confirmed this with Barrett engines. They prefer narrow decks for their high performance versions. I wonder why the difference in thought?
George
 
ND vs. WD

Is there a difference for narrow vs. wide-deck O320? They say these numbers are for a narrow deck. Mine is a D2J O320 - is that narrow or wide?

The deck heights are the same for both ND vs. WD . You have a WD engine based on the picture you posted. Keep in mind the same connecting rod is used in both versions ie same rod length.
I?ve even seen a factory Reman engine that utilized ND cylinders on a WD case. Apparently they don?t have any new ND cases any more so the simply took a new WD case and drilled it for the smaller circumference ND stud pattern. Apparently when your the factory you can do such things ! 🤪
 
High Performance ND engines

Paul?s article on narrow verses wide deck engines is interesting but contrary in some respects to what I was told. On the IO540 at least the narrow deck engine is preferred by racers and hard core aerobatic guys because it is viewed as stronger and more tolerant of raised compressions. I confirmed this with Barrett engines. They prefer narrow decks for their high performance versions. I wonder why the difference in thought?
George
One of the reasons ND engines are some time preferred is because contrary to what you might think the ND case holds up much better. Ask any case OH company and they?ll tell you they crack far less.
One example is the National record holder biplane ran a narrow deck 360 for many years that produced over 260 hp at over 3500 rpm.
 
Subject change.

I have a later model wide deck case which was factory machined as a narrow deck. For a few years Lycoming furnished replacement wide deck castings for older engines that were machined with the narrow deck bolt pattern. I was lucky to find an NOS one when overhauling my IO360.

GM
 
Thanks for everyone's input. I measured my deck height last night and found it to be 4.46". It's not easy to measure precisely, but I'm certain that it's not more or less by 0.01". From what you guys are saying, this does in fact look like it's been shaved down.

Just to test the clearance (and verify that the piston topping out was in fact the cause of the prop's limited travel), I put a .04" sheet of steel between the pad and base of the cylinder as a spacer to test the movement of the prop and it moves freely as it should.

So if the height difference between my stock 8.5:1 pistons and the CT 9.5:1 pistons is .06", and the deck has been shaved by .04", when I use my stock pistons, my compression ratio is about 9.2:1.

My question is this: I never knew the deck was shaved and that I had high compression. I've been running the standard ignition timing and the engine has been running great and as far as I can tell, no detonation. At this point, should I leave the timing alone or should I retard it?
 
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