Lightweight Pins worth it?

[MoPar_Kid]

Rebuilding my 408 SBM. Cast crank, I beams, KB hyper pistons. Considering replacing rod pins with lighter tool steel. Its a difference of 200 grams and $70. Motor wont see north of 5500 or 600hp (unless I go solid roller). Obviously overkill for street, but Id like to think over a 1/2lb of rotating mass would make a respectable improvement on free rev. Only problem is, crank is already balanced for the heavy pins, hate to waste the mallory but weight is weight.

[Charliesauto]

You will not see any difference in performance by changing to a lighter pin.

[David Redszus]

Design theory says you should run the lightest weight wrist pin that you can as long as it does not bend under load.

Put a wrist pin between V blocks in a hydraulic press with a dial indicator under the pin and not how easily a wrist pin will bend.
When it bends, it flexes the piston skirts and its only a matter of time before failure.

[digger]

Everything bends under load. You want it stiff enough such that the piston bosses don't flex enough to fatigue or affect ring seal

[1972ho]

David just a question seems as though you know something of wrist pin flex,I install a set of tool steel wrist pin that had a gram weight of 138 with a wall thickness of .175 and when installed they slid right into the piston and rod but after some use the pins had to be forced out with a little mallet force.Would that be something normal or did the pins bend a little into the piston they move freely in and out of the rods.Thanks

[David Redszus]

David just a question seems as though you know something of wrist pin flex,I install a set of tool steel wrist pin that had a gram weight of 138 with a wall thickness of .175 and when installed they slid right into the piston and rod but after some use the pins had to be forced out with a little mallet force.Would that be something normal or did the pins bend a little into the piston they move freely in and out of the rods.Thanks

Very common.
In a typical SBC at 8000 rpm we can expect to see a piston G force of about 4000 G, which becomes about 6000 lbf at the wrist pin.

A 22mm wrist pin, depending on wall thickness and length, will deflect about 0.24mm under load.
When the pin is center loaded, the pin end deflection wears a groove in the pin boss and leaves a ridge.
The newly created ridge makes it difficult to remove the pin, even though it may spin easily.

If in doubt, go with a stiffer pin rather than a lighter wrist pin, unless your pistons are disposables.

[ptuomov]

I’ve been reading some books on piston design and, at least to me, it seems that only the person who designs the piston really has all the information to make the wrist pin decision. Yes, the pin bends, but what I think really matters is what are the implications of this bending to the piston and how the piston and the rings function in the bore when the wrist pin bends. Perhaps this just reflects my inexperience and incomplete understanding.

[piston guy]

The other affect of a thin pins is loss of power since energy is absorbed by the flexing wrist pin. Heavier , stiffer pins transmit more energy to the rod and so forth.

[BOOT]

Can the $ be used to make the car faster in another area?

I think if the tool steel pins are same strength or stronger even tho lighter and you don't mind the balancing then it'd be worth it if you had nothing else that was better bang for buck to make it faster. I mean that is the point of tool steel and chrome moly right?

Not that I have any experience with them.

[ProPower engines]

2 things with those cast cranks is do not over tighten the drive belts it will snap the snout off the crank. I have seen 7 in the last year that guys have used and broke the snout and all were cast cranks.
They are internally balanced and there are several that grind the section between the main journal and the rest of the snout very thin which is where they break right where the crank snout meets the main journal.

The other deal is depending on what brand of crank you may need to ad a bunch of heavy metal with a light bob weight.

If this is a new build return the crank and get a forged crank. I have seen eagle and scat and ever the mopar over the counter crank break off the snout.
FWIW The local dealers told me the Mopar shaft is by Callies.

When you say 200 grams is that per pin or a pair of pins??

[DCal]

Rebuilding my 408 SBM. Cast crank, I beams, KB hyper pistons. Considering replacing rod pins with lighter tool steel. Its a difference of 200 grams and $70. Motor wont see north of 5500 or 600hp (unless I go solid roller). Obviously overkill for street, but Id like to think over a 1/2lb of rotating mass would make a respectable improvement on free rev. Only problem is, crank is already balanced for the heavy pins, hate to waste the mallory but weight is weight.

I was curious if the rotating assembly had anything to do with the reason for the rebuild. If not I would leave the piston/pins alone because they've already made any adjustments needed for compatibility. Piston manufacturers have been pushing thicker pins for decades and its good to see some on here understand why. As an added bit of info CP is making "not round" pin bores on purpose for several OEM piston customers. This lessens the friction and helps stabilize the piston in the bore. Everything helps when they have a 200,000 mile warranty on a 700+ motor.

[user-23911]

If you were starting off with a japper turbo engine, stock pins are thick wall and never give trouble.

When you upgrade / downgrade to forged pistons from Ross, JE, wisec etc, you end up with the thin wall pin.
They bend and break and are unfit for use,
The fix is to go back to the stock pins.


Or better still, stick with the stock pistons and tune it so there's no knock.

[MoPar_Kid]

2 things with those cast cranks is do not over tighten the drive belts it will snap the snout off the crank. I have seen 7 in the last year that guys have used and broke the snout and all were cast cranks.
They are internally balanced and there are several that grind the section between the main journal and the rest of the snout very thin which is where they break right where the crank snout meets the main journal.

The other deal is depending on what brand of crank you may need to ad a bunch of heavy metal with a light bob weight.

If this is a new build return the crank and get a forged crank. I have seen eagle and scat and ever the mopar over the counter crank break off the snout.
FWIW The local dealers told me the Mopar shaft is by Callies.

When you say 200 grams is that per pin or a pair of pins??

Interesting... Were these all new/recent castings from same supplier? All Chrysler applications? Thatd be a bumber, but at least its the snout and not a web. I'll have to be careful with ac compressor, a lot of leverage. I guess theres no future for a blower.. Crank is Scat neutral balance with 2 sticks of metal. I really cant afford or justify a steel crank. If it was a solid tappet high comp race motor Id consider, However I wouldn't consider LW pins for something of that level. 200 is the total for all eight.

Since I'll be running KB hyper pistons, I wont chance over stressing them. They scare me more than the crank.

[MoPar_Kid]

Can the $ be used to make the car faster in another area?

I think if the tool steel pins are same strength or stronger even tho lighter and you don't mind the balancing then it'd be worth it if you had nothing else that was better bang for buck to make it faster. I mean that is the point of tool steel and chrome moly right?

Not that I have any experience with them.

Thats where my notion was. Why, not, its apart.. But seems like one of them situations of too much risk for little return. I called KB for their opinion and they didnt think it was worth it even for the aspect of rev ability. They also couldn't give me a general RPM or HP limit either.

[Mark O'Neal]

If you were starting off with a japper turbo engine, stock pins are thick wall and never give trouble.

When you upgrade / downgrade to forged pistons from Ross, JE, wisec etc, you end up with the thin wall pin.
They bend and break and are unfit for use,
The fix is to go back to the stock pins.


Or better still, stick with the stock pistons and tune it so there's no knock.

What's stopping you from running a lighter piston with a heavier pin?