Wristpins

Ape · Post by **Ape** » Sun Jan 29, 2006 6:05 pm

Motorman 407 wrote: Any Super Stock builders here. It also seems like the trend is going towards straight wall pins, some say they wont gall the pistons as much as the tapered pins.

Give mahle ( but i wouldnt know how their Us support is) a call, i would assume that for nongalling you could get nonlinear tapered pins to take the deflection into consideration. I do know though that they offer serious slipper pistons with very short pins and large diameter.

Ape · Post by **Ape** » Sun Jan 29, 2006 6:15 pm

Motorman 407 wrote: Is there a way to figure out the minimum pin strength required other then trial and error?

As there are some knowledgable people on this board the topic of Finite element analysis has been covered, i would assume knowing the specific used material (lifecycle), the treatment stage, the max rpm, number of covered rpm, and other engine related (a.a.ep. crank throw) specs. should get somebody with thorough FEM-stress analysis pretty far.

MadBill · Post by **MadBill** » Sun Jan 29, 2006 7:13 pm

Regarding tapered vs. non-tapered pins, there is another alternative. (although I don't know who makes them) The problem with tapered pins of course is the possibility of galling the pin bores due ovalizing at the ends, because of the thin wall. It is possible to machine a taper up almost to the end, but then leave a 'flange' of the original wall thickness, thus retaining most of the strength of a straight pin with the weight of a tapered one.

383Malibu · Post by **383Malibu** » Mon Jan 30, 2006 1:07 pm

Motorman 407 wrote:...
talked to engine builders and piston, rod and pin manufacturers and they all suggested stronger (rather than lighter) pins
Stronger then what? If there is no reference that doesn’t really say anything...

Motorman – We were using a .927”x2.500”x.140” tapered wall tool steel pin as a baseline and were thinking about changing to a .866”x2.250” titanium pin. However, several of the folks we talked to suggested a .990” pin (and nobody recommended titanium). This is for a 381” sbc (4.165”x3.500”) shifting at 9500 rpm (~5540 fpm) with ~20” vacuum.

dbusch · Post by **dbusch** » Mon Jan 30, 2006 1:29 pm

i was pondering this deal myself too. i just recieved some JE pistons for an 850-875hp SBF that are 2.5" .927" .150 wall at about 117 grams. I usually run a .130 pin at 107 grams. I was going to send the .150s back, but now i might keep them. The lighter pins often beat up the pin bores in the piston, making it hard to get them out on rebuilds. Maybe the .150 pin wont hurt engine acceleration much and also keep the pin bores in the rods and pistons more round...

I noticed Erik K. talking about super light pins and how they should be avoided...

melsie68 · Post by **melsie68** » Mon Jan 30, 2006 5:01 pm

I wonder what anyone might say about the piston-guided rod set-ups I know cup teams are using now? They are obviously using a much smaller wrist pin in the neighborhood of 2.000" and very small diameter pin bores as well. They are using these in conjunction with forgings like the JE X-forging in order to maintain piston stability and rigidity. The idea with these guys is to free up as much weight and as much friction as feasible with their 12:1 screamers. They constantly going to a smaller wrist pin and smaller crank rod journal. I am not even sure how small they are now- maybe something like .787" on the pin OD. I believe it is some Mitsubishi 1.800" size on the rods now not to mention the piston guided deal uses a smaller rod width than standard SBC .940". Pretty much everything is fully custom but their may be some ideas their to apply to other motors such as the one being discussed on this thread.

thundair · Post by **thundair** » Mon Jan 30, 2006 5:43 pm

I will chime in for a few points

I made a lot of the carbon wound steel shell wrist pins in the 80's and although I proved the strength I could not interest anyone to help with testing reliabilty. I tested some .927 that I had made compaired them to stock pins.

The stock pins broke a 9,267# (not psi) and weighed 160 grams. The ones I made broke at 17,800 and weighed 84 grams..

That being said I did a lot of research on pins and looked at a lot of broken ones.. The break was almost always axial although some were dogleged from nitro burning monsters.

The tappered ones were flexing in an oval shape at very low pressure I dont remember the number but you can make a jig to put the pin in double shear and start the hydraulics.

If anyone wants to know thew process on making the carbon ones email me @ thundair@hotmail.com

MadBill · Post by **MadBill** » Mon Jan 30, 2006 5:56 pm

I bet it was your pins that I referenced in my earlier post, thundair! I recall a pic in one of Vizard's books, showing a steel pin on one scale, and next to it one with three carbon pins, with the same total weight.
It's a great shame they didn't fly. Lighter/stiffer pin = lighter piston and rod, lighter crank, etc...

OldSStroker · Post by **OldSStroker** » Mon Jan 30, 2006 6:15 pm

Motorman 407 wrote:I to ended up with pins from PPPC. I choose their 9310 material Casidiam coated 2.750 long .150 wall tapered pins weighing 120 gram and they charged me $412 for a set of 8 pins. Even though I don’t agree with the tapered pin design I choose them in this case to get the weight down and I haven't had any issues with them so far. I also wanted the longer pins to get more surface area for the pins in the pistons.
talked to engine builders and piston, rod and pin manufacturers and they all suggested stronger (rather than lighter) pins
Stronger then what? If there is no reference that doesn’t really say anything.
I started out heavy and worked down to lighter pins.
Also you can arrive at, in this case 850 hp, in different ways. Take a 310ci comp eliminator engine making 850 hp @ 9500rpm and for example the engine masters winner making 850 hp @ 6400 rpm. Both has roughly 4750 ft/min piston speed. Assuming the piston and small end rod weights are the same would the 9500 rpm motor create more stress on the pins? Is there a way to figure out the minimum pin strength required other then trial and error?

Thoughts:

Look at inertia or g-loading on the pins from the mass of the piston and the max acceleration at TDC. Use the exhaust stroke because you don't have cylinder pressure helping to unload the pin. Pin g forces are proportional to the square of the rpm, so 9500 is more than twice the g-load on the pins than 6500. That's why EMC engines aren't challenged in pin-strength. A 310 is right around 5000 gs at 9500, depending mostly on stroke and a little on rod length.

F=Ma so getting the piston lighter should be a major concern. If you aren't discussing piston weight (mass) and rpm in the same breath as pin stength, you don't know what pin strength you need.

If you plot or diagram the bending loads on a beam (the pin) loaded in double shear, you might notice why a tapered pin works. Moving the pin towers closer together and narrowing the rod does lots of good for the pin loads and allows shorter (lighter) pins. This has been the focus on F1 and Cup pistons recently, especially when F1 went to a 2-race engine. If you are limited to a min. piston weight, or combined piston and pin weight, put the material where it does the most good. Sounds obvious, but getting there is more than half the fun.

If you are DLC (Casidiam) coating the pin, you are doing it to eliminate the bronze bushing in the rod so you can make the rod smaller/lighter on the little end, right? A pin that changes shape under load tends to seize in the piston or the rod. If you need to increase the pin-to-bore clearance to keep from galling, the pin isn't strong enough. DLC isn't meant to be a bandaid for flexing pins.

$50+ per pin isn't cheap, but neither is it very expensive for very well made pins. If one were to make just 8 excellent pins from scratch, twice or three times that would be closer.

user-9274568 · Post by **user-9274568** » Mon Jan 30, 2006 6:21 pm

I just want that 310 that makes 850!

383Malibu · Post by **383Malibu** » Tue Jan 31, 2006 7:03 am

I was told by a pro stock team a couple of years ago that they were investigating the use of “waffle” pins (presumably in reference to the supporting structure in the hole of the wrist pin). Does anyone know anything about this pin design and whether it is being manufactured and in use today?

BRENT FAY · Post by **BRENT FAY** » Tue Jan 31, 2006 5:10 pm

Try going to www.wiseco.com/TechTips.aspx Forged piston tecchnology 101 for some more info for inertia force due to stroke/rod length/piston weight. It has 49 slides of formulas and graphs. I found it very informative along with 2 ring info. Brent

OldSStroker · Post by **OldSStroker** » Tue Jan 31, 2006 5:41 pm

BRENT FAY wrote:Try going to www.wiseco.com/TechTips.aspx Forged piston tecchnology 101 for some more info for inertia force due to stroke/rod length/piston weight. It has 49 slides of formulas and graphs. I found it very informative along with 2 ring info. Brent

Thanks for the good link, Brent.

Motorman 407 · Post by **Motorman 407** » Tue Jan 31, 2006 8:44 pm

I just looked at the formulas in the Wiseco page and it seems like Oldsstroker pretty much nailed it . The high rpm short stroke combo created way more load than the long stroke lower rpm combo even though they both have the same piston speed. (using the same rodlenght and weight # )
Thanks for good info