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Oil drainback holes in pistons

General engine tech -- Drag Racing to Circle Track

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jdperform
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Oil drainback holes in pistons

Post by jdperform » Sat Nov 16, 2019 8:02 am

Does anyone here have any oil ring opinions on efficiency differences between split oil return holes vs all the return holes in the root of the oil ring grooves? I’ve had a couple of bottom scraper rings get caught on the split return holes. Therefore bending them hence oiling the spark plug a tiny bit. These are 2.0mm ultra low tension oil rings. We get away with using them because we have a 7 stage oil pump with an air oil separator provision. If standard drain backs will suffice problem solved. There is NO support ring utilized.

DCal
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Re: Oil drainback holes in pistons

Post by DCal » Sat Nov 16, 2019 10:28 am

How much vacuum does your system pull? And how many split holes per piston?

jdperform
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Re: Oil drainback holes in pistons

Post by jdperform » Sun Nov 17, 2019 10:50 am

Unregulated vacuum, 2-.062 breather holes.
4 on major and 4 on minor thrust sides.

Mark O'Neal
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Re: Oil drainback holes in pistons

Post by Mark O'Neal » Sun Nov 17, 2019 4:19 pm

Chamfer the oil hole where it intersects the groove.

DCal
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Re: Oil drainback holes in pistons

Post by DCal » Tue Nov 19, 2019 6:00 pm

Pm me, I'm not willing to drop this.

jdperform
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Re: Oil drainback holes in pistons

Post by jdperform » Thu Nov 21, 2019 7:46 am

Pm sent

DCal
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Re: Oil drainback holes in pistons

Post by DCal » Sat Nov 23, 2019 10:57 am

jdperform wrote:
Thu Nov 21, 2019 7:46 am
Pm sent
Didn't get your PM please try again, I've been working on this and have some info.

jdperform
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Re: Oil drainback holes in pistons

Post by jdperform » Tue Nov 26, 2019 7:47 am

I’ve tried to get pm sent to no avail. Anyway I’ve done much research on this matter and the conclusion is that the holes are to large .125 and 3/32. Exacerbating the issue is holes are evenly divided above and below the bottom shelf of the oil ring groove. A large span of 1/8 inch for the scrapers to get caught on. We’re going to move holes up .025 or so and use 3/32 instead of .125 hole to feed the pins. Also use .075 holes on thrust surfaces. This seems to only be an issue with 2mm oil rings.

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Re: Oil drainback holes in pistons

Post by DCal » Wed Nov 27, 2019 3:21 pm

jdperform wrote:
Tue Nov 26, 2019 7:47 am
I’ve tried to get pm sent to no avail. Anyway I’ve done much research on this matter and the conclusion is that the holes are to large .125 and 3/32. Exacerbating the issue is holes are evenly divided above and below the bottom shelf of the oil ring groove. A large span of 1/8 inch for the scrapers to get caught on. We’re going to move holes up .025 or so and use 3/32 instead of .125 hole to feed the pins. Also use .075 holes on thrust surfaces. This seems to only be an issue with 2mm oil rings.
I drove up to N Wilksboro where my former co-worker has a bunch of piston samples. What we agree on is that some companies are doing exactly what you described, holes too big and radius or chamfer under the oil ring groove is too large. When the ring expands out to the bore the bottom rail is not properly supported. We feel the ring is being distorted too much and it's trying to roll right out of the groove and the larger drainback holes are giving a void for it to twist/roll into. Attention has to be given to the diameter of the land above ring also. Too small of a diameter there will cause even less support particularly on the down stroke. CP uses an .093 tool located high in the groove so that the unsupported part isn't so big.
I've seen it posted on here that the 2mm and 3mm radials are the same but Ric and I saw 2mm rails that were smaller and that's the problem. I just got my CP pistons in and I had ordered them with a 10 degree X .020 chamfer before you even posted so I was in prevent mode several months ago.

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Re: Oil drainback holes in pistons

Post by fdicrasto » Thu Nov 28, 2019 9:17 pm

50 years ago when I worked for a Chevy TransAm team as "engine monkey" Al Bartz supplied our best engines. When I freshened them the Venolia pistons had 12- .125" holes drilled just below the oil ring groove, 6 per thrust side at 45 degree angle. Of course oil ring tension was lower than we were used to running but never any issues and very competitive. I then had all my drag pistons done by my machine shop and he wanted to kill me for talking him into doing several sets but again, no issues as the oil ring groove was not compromised. Of course piston designs today might not be able to stay stable if done that way now, not to mention how much simpler it is drill the drain back holes at 90 degrees to skirt and then cut the oil ring groove. Thought that might be interesting for you to know.

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