# Piston pin is off center



## borna (Mar 18, 2012)

Hello all,
Today I just made a piston for my new engine and as you can see from the picture, the piston pin is about .0625 off center. Now wondering how this will affect the operation? Do I need to make a new piston or still operational?
The engine is single cylinder 4 cycle slow running engine.

Thanks
Borna


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## moanaman (Mar 18, 2012)

Make a new one. An offset wrist pin will cause excessive piston wear.  It will probably run OK but only for a short time. I have tried offset crank shafts in racing motors but never sure the small gains was worth the effort. 

Barry G


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## kuhncw (Mar 18, 2012)

Hello Borna,

You said the engine is low speed. I'd give this piston a try. The offset will tend to make the piston cock slightly in the bore and bear a little heavier on the lower edge of the piston skirt on one side and perhaps against the edge of the top land on the opposite side. 

Offset pinbores have been used in diesel engines to reduce piston slap as the piston goes over top center on the power stroke. The piston tends to cock in the bore instead of riding against one wall of the cylinder and then smack the opposite wall as it goes over top center. In the diesels, despite minimal clearance at operating temperature, the sharp cylinder pressure spike at firing could make the cylinder liner ring enough to promote cavitation erosion on the coolant side of the liner. That is more than you wanted to know, but offset pinbores exist in some cases and for a reason.

In summary, I'd give this piston a try and after some running take a look at the cylinder walls and the piston. You may find there is no problem.

Regards,

Chuck


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## borna (Mar 18, 2012)

Thank you all. 
Ok since at least it will run, I'll give it a try and if time allows I'll make a new one.

Also anybody had this issue in a model engine and if so, what did you experiance?

Thanks
Borna


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## popnrattle (Mar 18, 2012)

I would use it but make sure the piston pin offset is OPPOSITE the thrust side of the power stroke. I would venture to say most all commercial piston pins are offset. A 1/16 may be a little much for a miniature as offset distances are probably a function of stroke and/or bore. Nascar has a maximum. My inline 4 cylinder tractor uses a 1/2 inch offset crank centerline to take advantage of the straighter rod during the power stroke but engines with multiple banks have to use offset piston pins. Later, Rick.


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## borna (Mar 18, 2012)

Thanks,
Would you please explain what you mean by 
"make sure the piston pin offset is OPPOSITE the thrust side of the power stroke."

Thanks
Borna


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## Mike N (Mar 18, 2012)

I would bore the hole over where it belongs & press in a couple of plugs & re-drill the hole on center.

It looks like you have a large enough wall thickness for a good plug.

I don't like to start over!


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## popnrattle (Mar 18, 2012)

Maybe a better explanation is to orient yourself to imagine the engine crank rotation clockwise and place the offset pin on the right side of the piston centerline. The rod will be slightly straighter during the power stroke with less skirt pressure. Later, Rick.


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## borna (Mar 18, 2012)

Actually maybe I can drill 2 holes 90 degree apart from the original holes (perpendicular) and make sure is straight this time.
Also what if I don't plug the 2 original holes?

Now I want to know why the 2 orgianlholes are not straight After examine the piston on the first hole I used center drill and then used 1/8" drill bit and drilled through the piston. I think when the drill passed the hollow part and as started to drill the second hole, the drill bit must have wander when hit the second wall and went off center. 
So maybe Once I drill the first hole, make it large enough so I can pass a center drill to start the second hole?

Thanks


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## AssassinXCV (Mar 19, 2012)

How about make 3 engines, one with offset, one with centered, and one with opposite offset. Run them for 3 days straight. And come back with some valuable info :big:

Ian


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## b.lindsey (Mar 19, 2012)

Borna,
My vote would be with making a new piston as it was designed or as you desired it to be if your own design. From the picture and what you say about drilling it, it would seem to be more angled than offset. If you decide to make a new one, try drilling the cross hole first and then boring out the inside of the piston afterward. 

Bill


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## lensman57 (Mar 19, 2012)

Hi,
Please do a new piston, you will ruin the rest of the engine if you tried to run it even at slow speed. Your connecting rod will be twisted to start with or you will have to bore the connecting rod piston end to the same angle that the piston pin has been set. This is near impossible to get right and in the end would still ruin your engine. Thankfully a piston is not that difficult to redo.

Regards,

A.G


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## borna (Mar 19, 2012)

yes I agree with Lindsay, the hole angled than offset and I think is because the drill bit wandered from its original entry point. So as suggested I should make the hole first then bore the inside or temporary plug the inside bore and drill, remove the plug. 

I agree making a new piston is the best solution, but just wondering what if I use the same piston but drill new holes perpendicular to the original holes and leave the original angled holes alone and unused. What does that do in the operation of the engine?

Thanks
Borna


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## b.lindsey (Mar 19, 2012)

Borna,
Does the piston have rings? Also what type of lubrication does the engine have? It may be fine to leave the first set of holes open, but after plugging, re-drilling, removing the plug...it may be almost as easy to make a new piston. Then you are sure of what you have without question.

Bill


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## borna (Mar 19, 2012)

Hi Bill, the piston is made of cast iron and has one ring and the lubrication is mixed with the fuel.
Again I agree and decided to make a new piston, but now I'm just wondering what would be the affect of opertion with the first set of holes open?

Thanks
Borna


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## b.lindsey (Mar 19, 2012)

With the lubrication in the fuel probably little effect. If it was a splash type system then the extra holes would allow more oil to reach the cylinder walls, which may not be a bad thing but not applicable in your situation.

Bill


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## borna (Mar 23, 2012)

Made 2 new holes and this time they are centered. Ring seems to be working well and getting good compression.


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## michelko (Dec 21, 2018)

Hi its a common practice on ic engines to give the pin an Offset. This helps the engine to run smoother and quiter.
I don´t know the english Expression in german ist desachsierung.

Michael


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## Mechanicboy (Dec 21, 2018)

It's not correct: This helps the engine to run smoother and quiter.

It's socalled desaxe: If the offset is in the direction of rotation, it has the effect of increasing the leverage applied to the crankshaft during the "power" stroke, and reducing thrust wasted against the cylinder wall.

The crankshaft is offset to reduce thrust wasted against the cylinder wall.


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## lohring (Dec 22, 2018)

The industrial engines we modify for racing have their wrist pins offset 1 mm.  These 26 cc, 36 mm bore engines develop over 6 hp at 16,000 rpm and can turn 20,000 rpm.  As was pointed out above, piston offset is common in IC engines.

Lohring Miller


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## doughnut (Dec 23, 2018)

The industrial engines we modify for racing have their wrist pins offset 1 mm. These 26 cc, 36 mm bore engines develop over 6 hp at 16,000 rpm and can turn 20,000 rpm. As was pointed out above, piston offset is common in IC engines.

Lohring Miller

Now you have me interested. Model car racing?
BR, Paul


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## lohring (Dec 24, 2018)

I race RC boats.  I have modified some 26 cc engines for RC cars.  The main difference is the required power band.  Boats don't loose much rpm in the corners, but cars throttle a lot.  Even with a transmission this takes a broader power band.  The pipe design is mostly responsible for this.

Lohring Miller


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## popnrattle (Dec 29, 2018)

Hello, I haven't offset piston pins but I have read racing engine message boards that tout benefits of up to a 1/16" offset. I offset the cylinder in the miniature engines I build 1/4" above crank CL for less piston side load on power stroke. My antique Case tractor(4"x 5-1/2" bore/stroke) has very noticeable cylinder offset. Link below is a good read about offsetting cylinders. Later, RT.

https://www.fev.com/fileadmin/user_...pactonPistonAndPistonRingFrictionBehavior.pdf


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