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Piston Ring Gap--Tapered Cylinders

RomeoMike

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Question for the experts. I am building up an O-320 from all new parts. My cylinder diameters are measuring -0.0095" smaller at 5.5" from the cylinder base. This is causing my ring gaps too be excessive. Example: If I gap the Oil Ring to the minimum gap of 0.0075" when it is at 5.5" from the base it causes the gap to be about 0.039" in the non-tapered area of the cylinder (1.2 - 4.0" from cyl. base). The spec for the Oil Ring gap is 0.15 - 0.030" in the non-tapered area of cyl.

Superior SL # L04-01 states that "Certain cylinders with a large amount of choke may slightly exceed the maximum gap at the 1.2 to 4.0 location when gapped to .0075 at the top of the ring travel.

I consider 0.039" to be more than a slight amount above the maximum spec of 0.030".

What do you experienced engine builders think about this? Is it unusual to see this much taper in a new cylinder? Is it a problem or not? Will it be an oil burner if built up with the bigger ring gaps?

Thanks,

RomeoMike
 
Will it be an oil burner if built up with the bigger ring gaps?

Thanks,

RomeoMike

Not a builder expert on Lycs, but specifically to the gap affecting oil control. It would be a non measurable effect, IMO. Engines with poor oil control may have larger gaps, but the cause of loss of oil control is the ring itself combined with the remaining cross hatch, if any. As the ring face wears it gets wider and will begin to leave a thicker oil film on the barrel due to face width and pressures, the gap is secondary and more easily seen and measured.

Another thing - air-cooled, aircraft engines use ring gap and piston clearances much larger than liquid cooled or land based air cooled brethren. Imagine a hot engine entering a heavy rain and chilling the heads and barrels while the piston and rings remain hot and expanded. This real world condition is what drives the basic clearances.

I shall defer specific clearance recommendations to an experienced builder (with 10+ successful builds).
 
With the piston at the bottom of its stroke, the pressures are minimal.

As the factory says, size it for the top end where the "work" is done!
 
Choke

It?s known as choke. Chuck Ney was a proponent of removing it. I had the choke removed from my cylinders.
 
I am confused. You say that your cylinders are measuring almost .010" small. If this were the case, your pistons shouldn't fit. If I recall correctly, the piston clearance is only .003 -.0045." Are you using a dial bore indicator with a 5-6" mic? .010" is considered a HUGE error in cylinder work. Tolerence is .001" or less.

Be sure you are reading the correct manual. I thought that .030" max ring gap was for straight bore cylinders. I would expect a gap larger than .030 on a choke bore, if you are measuring outside the choke area. I mistakenly installed choke bore rings on a straight bore cylinder and the gaps were in the low 40's. When I replaced them with straight bore rings, they measured in the high 20's. The names for rings don't reference the bore style, making it difficult to know which part numbers are for choke vs straight barrels, but they do make different rings for each.

As others mentioned, .020 oversize is not really a big deal for the oil rings.

Larry
 
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Another thing - air-cooled, aircraft engines use ring gap and piston clearances much larger than liquid cooled or land based air cooled brethren. Imagine a hot engine entering a heavy rain and chilling the heads and barrels while the piston and rings remain hot and expanded. This real world condition is what drives the basic clearances.

When I built my 320, I found the piston clearance and ring gap specs almost identical the the Chevy 350's I rebuilt the prior year for the boat and with larger pistons as well. Not that a small block chevy is considered a tight engine and many water cooled engines do run tighter cliearances.

Larry
 
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I looked over Superior Service Letter L94-06 E again and my cylinder is within the dimensions given. The SL specifies 5.125 +.002 /-.000? measured at 1.2? from the base and 5.1155 +.0060 / -.0000? measured 5.5? from the base. So my cylinder dimensions agree with that. I am using a dial bore gage calibrated to an outside mic.

The Ring gap for the top 2 rings should be .045 - .055? for a choke barrel (measured at 1.2 to 4.0? from base). And it must have a minimum gap of .0075? at the top of stroke or 6.0? from the base.
I do not have a problem with achieving those gap specs. with my top 2 rings..

By the way the Lycoming Service Table of Limits shows .020 to .030? gap for a straight barrel for the top 2 rings. I do have a choke barrel.

The Lycoming Service Table of Limits shows .015 to .030? for the oil ring gap (measured at 1.2 to 4.0? from base). It does not distinguish between choke or straight barrels for the oil ring. This is the problem as I see it. The spec does not appear to be correct to me for a choke barrel. The oil ring top of travel is well into the choke area of the barrel. The math also proves that this spec is not achievable with the given choke and a minimum gap of .0075? at top of stroke.

Lycoming Service Table of Limits and Superior Service Letter L04-01 shows a max service limit for the Oil Ring gap of .047. Again it does not distinguish between choke or straight barrels for the oil ring gap. I do fit within the service limit, but seeing as I am using all new parts I would expect to fall within the tighter suggested new limits.

I was hoping Malon might be able to shed some light on this.

Thanks to all that have responded.

Please do your own research before building YOUR engine. Do not rely on my comments or any of the information in this post.


RomeoMike
 
The Lycoming Service Table of Limits shows .015 to .030” for the oil ring gap (measured at 1.2 to 4.0” from base). It does not distinguish between choke or straight barrels for the oil ring. This is the problem as I see it. The spec does not appear to be correct to me for a choke barrel.
RomeoMike

I would seriously question the vailidity of that spec. Granted, there is less heat on the oil rings and they won't take the land out if they touch, but I don't see how they spec a gap .020-.030 smaller than the gap necessary for a compression ring 1/4" above it. I speculate they forgot to include both choke and straight barrel specs for the oil ring.

EDIT: Bore circumferences (and therefore the outer ring circumference, including the gap) for the base and choked bore areas are 16.100 & 16.068, respectively. So, the .045 gap goes down to .013 in the choke and the .015 gap goes negative in the choke. Even the .030 gap goes negative. Ring gap should never go to 0, let alone negative and hot running conditions always result in smaller gaps than than cold static. That is why the gap is there. The hotter the combustion chamber will get the larger the ring gap must be.

Lycoming can get away with tighter gaps in the choke, because by the time the rings get hot, the barrel is also getting hot and expanding. However, consider the first stroke. It is ice cold and you must still have some type of gap or your oil rings will sieze and bend or break the ring lands, in the case of compression rings.

Larry
 
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I looked over Superior Service Letter L94-06 E again and my cylinder is within the dimensions given. The SL specifies 5.125 +.002 /-.000? measured at 1.2? from the base and 5.1155 +.0060 / -.0000? measured 5.5? from the base. So my cylinder dimensions agree with that. I am using a dial bore gage calibrated to an outside mic.

The Ring gap for the top 2 rings should be .045 - .055? for a choke barrel (measured at 1.2 to 4.0? from base). And it must have a minimum gap of .0075? at the top of stroke or 6.0? from the base.
I do not have a problem with achieving those gap specs. with my top 2 rings..

By the way the Lycoming Service Table of Limits shows .020 to .030? gap for a straight barrel for the top 2 rings. I do have a choke barrel.

The Lycoming Service Table of Limits shows .015 to .030? for the oil ring gap (measured at 1.2 to 4.0? from base). It does not distinguish between choke or straight barrels for the oil ring. This is the problem as I see it. The spec does not appear to be correct to me for a choke barrel. The oil ring top of travel is well into the choke area of the barrel. The math also proves that this spec is not achievable with the given choke and a minimum gap of .0075? at top of stroke.

Lycoming Service Table of Limits and Superior Service Letter L04-01 shows a max service limit for the Oil Ring gap of .047. Again it does not distinguish between choke or straight barrels for the oil ring gap. I do fit within the service limit, but seeing as I am using all new parts I would expect to fall within the tighter suggested new limits.

I was hoping Malon might be able to shed some light on this.

Thanks to all that have responded.

Please do your own research before building YOUR engine. Do not rely on my comments or any of the information in this post.


RomeoMike


What you have is common and will work fine. Just set the gaps so they make the min in the choked area and you will be fine.

Good Luck,

Mahlon
 
Thanks Malon and others who have replied. I plan to press on with my engine build. I have made a token attempt to contact Lycoming to suggest a change to the Table of Limits. If my conversation with them proves to be beneficial, I will share that with the forum.

For the archives, I would like to change the name of this thread to "Piston Ring Gap" but I do not know if that is possible.


RomeoMike
 
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