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Can a compression ratio upgrade be performed in the field? (Narrow deck)

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00Dan

Well Known Member
I have an O-320-A3A running 7:1 pistons currently. I’m familiar with the discussion regarding the longer hold down studs and the hold down plates, but I’m still a bit fuzzy on my understanding of cylinder compatibility.

My basic question is simple: can the compression ratio upgrade be as simple as pulling the cylinder, replacing the piston and perhaps wrist pin, then reassemble with the old cylinder and rings?

As a follow on to that, has field experience shown the hold down plates to be strictly necessary if only going to 8.5:1 CR?

Does type of cylinder come into play?

Am I a fool for considering this instead of just waiting for overhaul time?
 
Well I know of an instance of 10-1compression pistons being used in a ND 320 without hold down plates. (short stud case). So yes YOU COULD swap 8.5-1 pistons and it should work ok. I would be leery of full power at sea level but 3,000 ft and above she should work ok. It’s never going to get Lycoming’s blessing but there’s guys flying behind Corvair engines in experimentals �� ! If you get my drift.
 
From my research, cyl's are the same. the only differences are the addition of hold down plates (and longer studs), a different piston, and thicker piston pins.

It does seem like that is a lot of beefing up for a 7% increase in power, however, I would probably not do the pistons without the other stuff until someone in the know told me it was overkill. I am guessing that Lyc stretched the 320 (150) to near it's parts margins (probably taken from the 290) and that was the reason for beefing up on the 160 variant.

I don't think you are crazy for considering it, but be aware it is not a small job. When I pulled jugs on my 6 (for pin plug replacement), it took me more time to pull the baffling off than to remove the cylinders.

Larry
 
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lr172 said:
From my research, cyl's are the same. the only differences are the addition of hold down plates (and longer studs), a different piston, and thicker piston pins.

Larry
Click to expand...

The 150 hp and 160 hp ND cylinders are different. The 150 hp cylinders used without hold down plates have spot-facing around the stud holes to allow the cylinder nuts a nesting place in the radiused area of the flange. No such spot faces in the 160 hp units as the top of the hold down plates provide a flat place for the nuts while the bottom of the plates mirror the radius of the cylinder flanges.
Talked to a noted aircraft engine hot rodder and he said he’s done it several times with no issues. Go for it but as Larry mentioned use the thick walled piston pins.
Duff
 
If new pistons and the thick-walled pins are it this seems like a no-brainer.

A couple questions about the job itself that my research has not clarified for me. When pulling a cylinder for a piston replacement, can the old rings be reused and everything just reassembled without a new break-in period? I’ve seen somewhere this was possible but I’ve also heard the other end of the spectrum on this calling for honing the cylinder, even if very lightly, and using new rings and breaking it in again. Part of my confusion is on where the rings are seating, the piston or the cylinder wall, which I suspect will answer my original question.

If I don’t have a record of cylinder measurements last time they were off, does that mean I’d have to pull the cylinders first, measure, and then order the piston (and potentially rings) based on that?
 
00Dan said:
If new pistons and the thick-walled pins are it this seems like a no-brainer.

A couple questions about the job itself that my research has not clarified for me. When pulling a cylinder for a piston replacement, can the old rings be reused and everything just reassembled without a new break-in period? I’ve seen somewhere this was possible but I’ve also heard the other end of the spectrum on this calling for honing the cylinder, even if very lightly, and using new rings and breaking it in again. Part of my confusion is on where the rings are seating, the piston or the cylinder wall, which I suspect will answer my original question.

If I don’t have a record of cylinder measurements last time they were off, does that mean I’d have to pull the cylinders first, measure, and then order the piston (and potentially rings) based on that?
Click to expand...

The break in is between the ring and the cylinder wall. You should be able to reuse the rings, but I wouldn't. One of the more common reasons for top overhauls is wear on the ring/cyl wall leading to low compression. As long as you have the cyl's off, this is a cheap way to potentially forestall a top overhaul down the road, depending upon how many hours the engine has and current compression ratings. A ring set is $100 and you can buy a 220 git honing tool that works in your drill for $25. Do research on method for getting proper cross hatch orientation. Need to match up/down movement with drill RPM.
 
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theduff said:
The 150 hp and 160 hp ND cylinders are different. The 150 hp cylinders used without hold down plates have spot-facing around the stud holes to allow the cylinder nuts a nesting place in the radiused area of the flange. No such spot faces in the 160 hp units as the top of the hold down plates provide a flat place for the nuts while the bottom of the plates mirror the radius of the cylinder flanges.
Talked to a noted aircraft engine hot rodder and he said he’s done it several times with no issues. Go for it but as Larry mentioned use the thick walled piston pins.
Duff
Click to expand...

DIdn't know that. Couldn't you still put the hold down plates on the 150 cyl's though and get 90% of the benefit? The plate is 3/16 or 1/4 and should distribute the load more evenly across the base flange (100-110* of it anyways). I suspect the spot facing creates a weak point in the flange, due to eating into the radius at a point where stress load is highly concentrated, and one of the reasons they wouldn't allow extra power on them. The hold down plates should successfully even out and move that stress away from the compromised area.

Larry
 
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lr172 said:
DIdn't know that. Couldn't you still put the hold down plates on the 150 cyl's though and get 90% of the benefit? The plate is 3/16 or 1/4 and should distribute the load more evenly across the base flange (100-110* of it anyways). I suspect the spot facing creates a weak point in the flange, due to eating into the radius at a point where stress load is highly concentrated, and one of the reasons they wouldn't allow extra power on them. The hold down plates should successfully even out and move that stress away from the compromised area.

Larry
Click to expand...

As I understand it part of the compatibility issue is case stud length - the models that shipped with the hold down plates had longer studs to accommodate the additional thickness.
 
You are correct. It is the cylinder stud length that prevents the hold down plates from being used.
 
Pictures always make things clearer when you can't quite understand the text.
I never used this setup, I was only test fitting a 150HP cylinder with the extra fin and the cylinder plates. I used brand new Superior cylinders.

In the first pic you can see the spot face on the one without the nut.

The second pic just shows how the plate fits on a 150HP cylinder which has an extra fin compared to 160 HP cylinders.

The 3rd pic is the castle nut that was used prior to the internal wrenching nut on very early models of the 320.





IMG_1104.jpg

IMG_1111.jpg

IMG_0908.jpg
 
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