Piston to rod small end distance

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John@RED
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Piston to rod small end distance

Post by John@RED »

Hi,
in a crank guided setup, how much should be the distance between piston and rod small end at the pin?
Is that gap functional to pin lubrication piston side?
Assuming a 4032 piston and steel rod, would there be any issue due to different material expansion?
Would like to increase small end width to the maximum permitted for better support, without piston guiding.
Thanks
miniv8
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Re: Piston to rod small end distance

Post by miniv8 »

The oil ring in the piston generally supply's the oil to the pin inside the piston.
Where the oil comes from actually to lubricate the small end of the rod, has to be a combination of the two, oil seeping from the sides of the pin inside the piston, along with windage spray from the big end.

A lot of connecting rods in Diesel production engines have a considerably wider support under the pin, so the small end of the rod tapers towards the center.

This configuration allows for added support of the pin during the power stroke with minimal weight penalty, and allows for more meat in the piston to support for the same loading. It also allows for the pin to expose some of its hopefully rotating surface for lubrication.

One of the 100 year old debates is whether the floating small end of a connecting rod should be drilled at the base, or at the top. I'm not sure that has ever been resolved.

Regarding actual clearance numbers, it might depend on your bore size and piston to wall clearance, but I cant imagine putting anything less than ten times the big end side clearance at the crank.
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Re: Piston to rod small end distance

Post by PackardV8 »

John@RED wrote: Sun Oct 24, 2021 4:45 amWould like to increase small end width to the maximum permitted for better support, without piston guiding.
Would help to understand the application and the goals.

How does one increase the connecting rod small end width?

And why would piston guiding then become an undesired result?

FWIW, the Packard V8, designed in the early 1950s, was the last engine I've ever seen with a wide rod small end; 1.50" wide. The goal, as stated in the SAE design paper, was to stablilize the equally long (3.250") piston pin. Very stable, but much heavier than would be considered necessary today.
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DCal
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Re: Piston to rod small end distance

Post by DCal »

As a rule I like see .060 per side on a forging minimum because of forging draft which is about 1-1.5 degrees. So the pin tower spread narrows as the the compression height gets shorter. Tell your piston salesman the width of your rod and the clearance can be done in their shop.
John@RED
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Re: Piston to rod small end distance

Post by John@RED »

PackardV8 wrote: Sun Oct 24, 2021 1:39 pm
John@RED wrote: Sun Oct 24, 2021 4:45 amWould like to increase small end width to the maximum permitted for better support, without piston guiding.
Would help to understand the application and the goals.

How does one increase the connecting rod small end width?

And why would piston guiding then become an undesired result?

FWIW, the Packard V8, designed in the early 1950s, was the last engine I've ever seen with a wide rod small end; 1.50" wide. The goal, as stated in the SAE design paper, was to stablilize the equally long (3.250") piston pin. Very stable, but much heavier than would be considered necessary today.
DCal wrote: Sun Oct 24, 2021 6:05 pm As a rule I like see .060 per side on a forging minimum because of forging draft which is about 1-1.5 degrees. So the pin tower spread narrows as the the compression height gets shorter. Tell your piston salesman the width of your rod and the clearance can be done in their shop.
miniv8 wrote: Sun Oct 24, 2021 12:44 pm The oil ring in the piston generally supply's the oil to the pin inside the piston.
Where the oil comes from actually to lubricate the small end of the rod, has to be a combination of the two, oil seeping from the sides of the pin inside the piston, along with windage spray from the big end.

A lot of connecting rods in Diesel production engines have a considerably wider support under the pin, so the small end of the rod tapers towards the center.

This configuration allows for added support of the pin during the power stroke with minimal weight penalty, and allows for more meat in the piston to support for the same loading. It also allows for the pin to expose some of its hopefully rotating surface for lubrication.

One of the 100 year old debates is whether the floating small end of a connecting rod should be drilled at the base, or at the top. I'm not sure that has ever been resolved.

Regarding actual clearance numbers, it might depend on your bore size and piston to wall clearance, but I cant imagine putting anything less than ten times the big end side clearance at the crank.
Thanks for the info.
Better explanation is that I found some pistons made for rods no more on the market, so I'd need to cnc reduce some std rods small end width to fit
That's why I asked for a suggested minimum gap
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