Drilling piston skirts

EarthHeavy

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Hey guys, one more question - I’m going to be running to 80mm 750 jugs and pistons on my 447 build and saw a video that described drilling two small holes, one above the other, through the piston skirts just under the last ring land to allow oil to splash onto the sides of the cylinders and reduce wear. Is this a common trick? Seems like a good idea to me but wanted to run it by the forum before I start putting holes in things. Thank for reading!
 
Never heard of it. Aren't the cylinder walls lubricated by splash from below? Some bikes have supplemental oil jets.

There were huge spaces in the piston walls on my Royal Enfield Bullet piston which was apparently an Indian solution to piston overheating, but I've replaced that piston.
 
Never heard of it. Aren't the cylinder walls lubricated by splash from below? Some bikes have supplemental oil jets.

There were huge spaces in the piston walls on my Royal Enfield Bullet piston which was apparently an Indian solution to piston overheating, but I've replaced that piston.

From what I’m understanding is that the area in the cylinders that contact the sides of the skirt are prone to holding more heat bc if the cylinder design, so adding a couple of small holes on either side of the skirt offer more lubrication to the walls and the piston…
 
Sounds like good way to weaken the piston, so the skirt could break off. I remember a time when some of the 2-stroke racers were drilling the back side of their pistons, to get more air/fuel mix into the crankcase and maybe go faster. I remember seeing some of those pistons with the skirts broken off too.
 
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There was a thread on here years ago where the piston skirt had lots of indentations from starting to drill a hole almost like a golf ball but more spaced out,
 
Yes, holes in the skirt to aid in lubrication and oil cooling is a tried and true method.... that's why the factory does it.
No, you adding more holes isn't a good idea, in my opinion.
1. however minute, the loss of metal (weight) is gonna affect vibration. Whether or not it's noticeable is an open question. Ask yourself if you want to find out?
2. holes in the skirt weaken it. It may be by a tiny amount, but it will

Here's the factory piston. You can clearly see the holes made at the factory.

1736972576239.png


1736972597174.png


And here's the piston from your 80mm big bore kit. Same thing, holes in the skirt to aid in cyl wall lube and cooling.

1736972679015.png


1736972694194.png
 
Yes, holes in the skirt to aid in lubrication and oil cooling is a tried and true method.... that's why the factory does it.
No, you adding more holes isn't a good idea, in my opinion.
1. however minute, the loss of metal (weight) is gonna affect vibration. Whether or not it's noticeable is an open question. Ask yourself if you want to find out?
2. holes in the skirt weaken it. It may be by a tiny amount, but it will

Here's the factory piston. You can clearly see the holes made at the factory.

View attachment 342501

View attachment 342502

And here's the piston from your 80mm big bore kit. Same thing, holes in the skirt to aid in cyl wall lube and cooling.

View attachment 342503

View attachment 342504
I agree with Gary. Not needed on the 750 pistons. They are Teflon coated to reduce friction.
 
I don't like it either In fabrication at least with steel materials there is a metallurgical texture
Heat treatment there can be internal stresses introduced at forging or casting ..And assuming the drilling is performed " Professionally "
even if ream it or polish even
there is another hole prone to stress intensity factors .
But worse --- should one get tensile stresses at the hole edges and or surface
A tensile stress that can reduce fatigue strength cracking

The Basic rule is don't drill and don't weld. Welding is another story because it has other benefits .but needs to be done by someone that is skilled
Aluminum is difficult Well into the 70 ies there was only one place more or less that welded Aluminum in Sweden
It is better now what I hear boys use people with established reputation .Which there are more of today
 
I heard of drilling piston skirts 50years ago just like XSJohns. Had forgotten about it till his thread.

Mike's 750 pistons Teflon wears pretty fast. Mate pulled the pistons after 10,000klm's and the black Teflon was scoured. Don't know if it was a problem related to his engine but quite surprised.

I don't think drilling divits in the skirt can hurt. For my thinking they shouldn't go to a point where the drill starts to cut in, only the taper on the tip, (if that makes sense), enough to pick up a little oil.
 
Could someone explain what real world problem the drilling is trying to solve?

IMO the best policy is if it ain't broke don't fix it.
 
Just wondering if the scuff marks are due to inadequate lubrication or poor oil filtering?
If it was a lubrication issue wouldn't the marks be spread more evenly at the point of contact?
The contact point is more evident on the second picture but as the skirt has to contact the bore isn't this normal?
What I'm seeing here is vertical scoring which IMO suggests it was caused by a hard object being pushed up or down between the piston and bore.
 
Look closely there are vertical lines in all of the photos of the jugs…
Yes a pattern of vertical scratches on the front lower skirt and liner about universal on XS tear downs.
The one set shows true "scuffing" (material transfer from piston to cylinder. I have a set that failed nearly immediately after an over bore. Those two sets both show wear at the diagonals, it corresponds with the cylinder stud location. overtighten studs, get cylinder liner distortion? Mebby. :shrug:
IIRC the vertical scratches are more common/prominent on the front side. wonder if those scratches are about the piston cocking in the bore a bit, so the lower edge scrapes off the oil film?
Do I vaguely remember a thing about piston pin offset to reduce side loading, this engine, or some other MC engine?
 
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Pin offset is to reduce thrust/anti thrust forces. Which way does the crank spin? At where does the maximum side load on the piston go?
Noise mostly, but wear/friction is another. Nearly all engines, depending on skirt/rod length.
cliff
 
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