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Wrist pin diameter and rpms

General engine tech -- Drag Racing to Circle Track

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ptuomov
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Re: Wrist pin diameter and rpms

Post by ptuomov » Sat Nov 30, 2019 12:38 pm

“Very compact piston and a rigid H beam rod, there would be NO point using a longer skinnier pin, as your asking the rod and piston to HOLD the pin straight, which is a terrible idea. Is this subaru type piston even a good design....well, maybe yes. Not for every app, but for some yes.

You haven't yet mentioned anything about the rods, but IMO it does matter. It's all a system. And I actually believe that for long service from plain bearings one key is matching the flexibility oi the parts to obtain more even loading.”

The rods need to connect the new piston to the stock crankshaft. The stock rods are massive, 860g. They are great under compression but can’t handle rpms. The rod bearing area is also huge, 52mm journal diameter, 24mm rod bearing length. This is with 25mm bore offset and 27mm rod big end width.

Since we need to spin the engine higher in terms of rpms, the plan is to go with lighter forged steel rods and shorter bearings originally designed for Toyota Supra.

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Re: Wrist pin diameter and rpms

Post by modok » Mon Dec 02, 2019 3:17 am

nice.
And one more thing to consider, regarding..."surface speed"
the pin spins. tho, we don't really know how much, but, it picks up SOME spin at tdc and it may continue spinning of it's own momentum, so, the moment of "zero spin" may not actually happen at all. Sometimes I've wondered if floating pins really are better than press fit. Experience leans to floating is better.... and maybe that's part of why.

I'm thinking about pins too, my brother got ahold of some OLDer wiesco pistons for a miata, look to be kind of a prototype.
They are supplied with a 2.185 length tapered ID pin, in what appears to be a piston blank sized for a 2.5 pin, It looks like the pin is so short it only engages half the pin bores of the piston.
wondering if this pin is strong enough, and I'm thinking no. But the tapered ID would make perfect sense if the pin was longer.
Good thing pins are cheap. CP-carillo doesen't even sell tapered ID,
says "straight wall design to reduce galling" :lol: polite way of saying -tapered ends are stupid idea-

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Re: Wrist pin diameter and rpms

Post by MadBill » Mon Dec 02, 2019 8:38 pm

Somewhere I've seen tapered pins with sort of internal 'flanges' at the outboard ends, to reduce ovality under loads...
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Re: Wrist pin diameter and rpms

Post by ptuomov » Mon Dec 02, 2019 8:55 pm

I think these brand new M139 wrist pins are tapered from the factory, at least as far as my eye can see:
221A52AA-5A79-472E-B262-BFCAFEC9C6A0.jpeg
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Re: Wrist pin diameter and rpms

Post by MadBill » Mon Dec 02, 2019 10:25 pm

Looks very short to me, in fact I'd call it a chamfer rather than a taper. Maybe that's the most they can get away with, short of encountering the aforementioned ovalizing issue..

There also appears to be a chamfer on the lower edge of the small end bore.
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Re: Wrist pin diameter and rpms

Post by modok » Mon Dec 02, 2019 10:34 pm

ok, we'll call that a chamfer.
Very nice

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Re: Wrist pin diameter and rpms

Post by David Redszus » Wed Dec 04, 2019 10:42 am

ptuomov wrote:
Wed Nov 27, 2019 6:36 am
Do high rpms put any requirements on the piston wrist pin diameter? I am thinking surface speed and oiling, assume that the piston assembly weight is given. Fully floating wrist pin.
Bearing loading must be viewed from several perspectives including wear, load, size and speed. Once pressures and velocities have been determined, the proper bearing material can be selected. The last step is to determine the loading and deflection of the wrist pin.

The required inputs are:
Load, lbs
Bearing ID, mm
Bearing length, mm
rpm
stroke, mm
rod length, mm

from the above (and an estimate of combustion loading) we can calculate the following:
bearing pressure, psi
rotational velocity, ft/min
oscillating velocity, ft/min
rotational PV, psi*ft/min
oscillating PV, psi*ft/min

Rotational values are used for the big end, oscillating values for the small end.

Once PV values have been determined, we can select which bearing material is capable of withstanding
the expected loads and speeds. Most high performance applications will use manganese bronze

The formulas and bearing specifications can be found in bearing mfg engineering handbooks.

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