What are Wrist-Pin Buttons?

April 27, 2018 / by Mike Magda

Most have heard of spirolocks and other methods of wrist-pin retention, but when it comes to all-out racing, wrist pin buttons reign king of strength and simplicity. 
When the application calls for a sturdy, full-round piston in a high-performance engine, wrist-pin buttons have long been an option for keeping the piston secured to the connecting rod. In some piston designs where the wrist-pin bore invades the oil-ring groove, these buttons can actually force a kink to the oil rings under extreme stress. But a new, patented wrist-pin button from Wiseco is helping to eliminate the impacts that destroy those rings and, therefore, extend the engine’s lifecycle.

Pin buttons are used in extrememly rigorous racing situations. They firmly support the piston ring and prevent the wrist-pin from moving.

“We’ve been using it on Pro Mods and Radials vs. The World cars for two, three years now,” says Wiseco technical sales associate Vic Ellinger. “Everyone who has them has alleviated a lot of issues that they had before with conventional buttons.”

Engine builders who service today’s high-horsepower racing engines on a regular schedule have always been grateful for wrist-pin buttons and the time they save in the shop or pits. Not only does the piston and ring assembly come apart and go together quicker, there’s also a lot less bloodshed by not having to remove and install four sharp-edged Spirolox retaining clips needed on every piston. Aside from the time factor, the pin can “plow” open the groove that supports the Sprirolox or cnap ring and increase the side clearance.

“Buttons maintain end play of the pin,” says Ellinger. “It lets the pin float freely and not bind against the clips. A design utilizing clips won’t float as easily if the stack-up tolerances of the pin, lock-ring spacing and clipsare on the tight side, and the parts are expanding rapidly during the course of a run. Asthe pin is hammering the buttons side to side, it’s not going to open up that side clearance more than a few thou during a season.”

There is a potential downside, however, to using pin buttons when the wrist-pin bore actually cuts into the oil-ring groove and interferes with a portion of the support surface that ring land provides. Unfortunately, this design option is needed on pistons with short compression heights where the engine builder is using the longest connecting rod possible. Or it’s used in high-boost or nitrous applications where the ring grooves are moved away from the heat in the piston crown, or the lower location is needed to increase the strength of the crown area.

Note how the pin button intersects the oil ring and has a machined notch to allow the ring to pass through. Because the button fills the wrist-pin cavity with material, it offers additional support to the piston.

The wrist-pin buttons designed for these applications are machined with a slot that matches the oil-ring groove of the pistons. The oil ring can be installed alone or with a support rail. These oil-ring support rails never come into contact with the cylinder wall and have no effect on lubrication. Their sole purpose is to stabilize the oil ring pack, and they are designed with dimples in strategic locations to keep them from rotating inside the ring groove. Problems, however, can arise when the wrist-pin buttons twist inside the pin bores and pinch the oil ring and its support rail.

“With nitrous, it shocks the button so hard, that it actually kinks the scraper and locks up the oil ring. It’s like you stuck them in a vice. Right where the button would hit the rail, it would have a big kink mark in it,” says Ellinger. “Just kicks it right up and it looks like you hit it with a chisel.”

Wiseco’s solution to prevent the buttons from twisting and damaging the oil ring is to machine a pair of dog-ear-shaped dowels—one on each side of the button. These dowels match up with a pair of receiver notches that are machined into the pin bore.

“When the button drops in, it can't move and rotate in the bore. Then you can also eliminate the oil rail support, which gives you an extra .030-inch worth of ring land to use wherever you need it, especially for boost or nitrous,” explains Ellinger. “That's basically the reason, it won't kink the scraper and you free up real estate on the ring lands."

Note how the tang on the wrist pin button fits into the notch on the piston. This design keeps the button from rotating and binding.

The new buttons with the D-shaped ears are machined from 2618 or 4032 billet alloy to match the construction of the pistons. The corresponding receiver slots are plunged cut on a hand mill. The buttons are designed to allow lateral movement as needed for the clearances, but the design eliminates any rotation within the pin bore. The recommended clearance between the pin, buttons and cylinder wall is dependent on a variety of factors, including bore size, pin size and piston material.

“All that has to be taken into account. And we like to see, depending on the application, it could be as tight as .005-inch or as loose as .015-, even .020-inch,” says Ellinger. “It just depends on what the specs are of the motor, and the type of application. If it's Bonneville, that's going to need something different where they're chilling the engines to rather cold engine tempsand immediatelyputting them into extreme conditions. You have to take into account how fast the parts are going to grow.

“The bore size is going to determine what end play you have with the length of the button, so your stack-up tolerances are going to give you what your number is,” continues Ellinger. “But, obviously, the stack-up tolerances can cause some variance with the manufacturer of the pin, or what brand pin. Also, if you open up the bore to allow for more clearance for a high-horsepower application, that's something that needs to be known during the design phase. If you're plus-one, plus-two, three-thousandths over what the nominal bore size is, we've got to compensate for that with the pin bore.”

This wrist pin and buttons are mocked up in the same orientation as inside a piston. Note how the pin buttons sandwhich it, preventing any movement of the wrist pin.

The clearance for the buttons and pins is calculated based on the final bore size, length of the pin and a target side clearance. For example, a 4.600-inch bore matched with a 2.930-inch-long pin and a desired .005-inch end play = X clearance. Wiseco engineers can then calculate the required length for each button.

“The dowels are free floating, probable .0007- to .001-inch clearance. Most guys just install the piston upside down without the rod and give it a wiggle test. An experienced hand can tell if something is awry,” adds Ellinger. “Some actually assemble the parts and measure the overall length of the pin and buttons and make sure it has at least .005 less than the bore size for a measured length.”

Obviously the use of a wrist pin button requires a full-round piston so that the bottom side of the button has a shelf upon which to rest. Some engine builders may be leery of the extra weight that comes with pin buttons, so Wiseco rifle-drills a hole through the pin axis of the button.

“And we lighten them up on the backside, if we can,” adds Ellinger. “But typically, with these applications, they're more like diesels as far as the strength that's needed. The weight isn't an issue, because the rod and the crank are usually also fit for use. So, the rods are beefier. The cranks are better quality billet. So all that just kind of depends on just the power level, but typically the weight isn't an issue with those type of builds as they are with a naturally aspirated deal.”

Ring choice may be a little more flexible with the use of the new Wiseco pin buttons, but there are limitations due to the engine size and use.

 “You really want to lean towardshigher-tension oil rings just because of the swept area of oil volume you have to try to control, especially with a setup like 5.000-inch pistons and 5.875-inch stroke,” explains Ellinger. “I mean, there's just so much oil up there sloshing aroundthat they really need to be able to keep that oil off those cylinder walls as much as possible, and have it controlled. Because they're only on the gas for four seconds, three seconds, and they're really hard to control all that oil while it's surging up the back cylinder walls.”

In all-out, racing applications, there is simply no beating a wrist-pin button in terms of strength, and ease of assembly.

Although pin buttons are used most often on high-end, frequently serviced race engines, they’re also suitable for many other applications.

“No, you can use it in any application, really, motorcycle, car, boat. The highest horsepower stuff is usually where we put it but you could even use it in a bracket engine,” says Ellinger. “There are a lot of benefits to it because it's the closest thing you can get to a true, non-stroker style piston design for a stroker. We've even used them where we've gotten into the second ring groove. We had to machine a second ring groove and an oil groove into the button because we lowered the lands to increase the strength of the piston, like a Top Fuel part would be.

“Wrist-pin buttons are really simple,” sums up Ellinger. “There's nothing there other than getting the things to quit ruining really expensive ring packs. That's really what they've done is brought oil consumption under control and eliminated the need for an oil rail support to add .030-inch worth of ring land to where you need it most, up top.”

Topics: featured, PISTONS 101, ENGINE TECH, Tech

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Written by Mike Magda