Tool steel / heavy wall wrist pins.

[Ohio Tom]

Was asked about in a different topic. Thought I would share:

My experience with wristpins.

As an hot street and drag motor builder, I was always looking for ways to lighten the reciprocating weight.

One of the things I "used" to do was use stock thinner walled wrist pins on smaller,, higher revving N/A motors. OK for your 1600-1776 screamer.
This seemed to work just fine. No problems.


For my personal 2332cc drag TURBO motor, I just used the wristpins that came with the Weiseco pistons. (as many folks do with no problem).

One thing I started to see on my personal Drag motor was I was damaging the wristpin bushings on my rods. Every tear down showed tapering of the bushing. The wristpin was tight in the center ,but had a wobble to it. Like a teeter-totter. The bronze bushing was getting smashed and even protruding out of the rod... But still tight in the center...
Thinking this was normal wear and tear for 300+HP turbo motor, I started just replacing rods at service intervals.

Then.. I pulled a wristpin out of a Weiseco piston at 7,500rpm. Darn near scattered my $10,000 drag motor.

I spoke to a local V8 Race shop about the issue. Right away he knew what was up.

He said, "as you approach 100hp/hole, Wristpin flex becomes an issue.... "

I had a hard time believing that something so hard and strong could "flex" at high rpm.
I mean, you see motors that blow up and the wristpins look perfect. They are soo strong. Never seen one break or bend.

but.. they can flex like a spring.

I did some research on the subject and sure enough, when approaching "100hp/hole" the wristpin load can start to cause flexing.

The solution is to go to a thicker walled wristpin..
And even a "tool steel" wristpin for greater strength.

There is a weight penalty, but keeping a 330HP turbo motor alive needs a few things to be built strong.

What's crazy is I have a 250hp Street turbo motor (in my street car) that get LOTS of miles and has normal wristpins in it. I service this motor often as it is a Sales tool and needs to look run good all the time.
Teardown shows it's rods to be just fine. Nothing special about its wristpins.
So, 250hp, 7,000rpm.. No problems..
15yrs now on the same crank, rods, pistons...

So, My conclusion is that if we are approaching 100hp/hole and higher rpms, then wristpin upgrades are a good idea.
Less than that seems to be fine with supplied wrist pins....

[GARRICK.CLARK]

Good info.Thanks. I have a turbo motor , i'll check the small ends next time its down.

[slalombuggy]

Are the tool steel pins you are using the same thickness as the "stock" pins or are they the light weight thin wall pins?

I had a set of light weight tool steel pins I had in my N/A motor, but took them out and put the thicker pins back in when I went to boost. I'll leave it for now, but man I'd be all over shaving some weight if the lighter pins would stand up to 8-12 psi.

brad

[vwracerdave]

Thanks for the 100 HP/hole reference.

Real curious how titanium wrist pins would hold up and if that would be a beneficial solution or just a foolish money waste.

Have you noticed any difference between a shorter wrist pin used in slipper skirt pistons vs, a full length wrist pin used in a regular round style piston?

Just to point out to all the folks building a 150 HP street engine the wrist pins supplied with the piston kits are plenty good enough. Lets hope the 150 Hp street engine people don't get so ate up with this that now everybody is wanting $500 super special wrist pins.
_________________
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[RWK]

What type of bronze bushing in rod? Try one of the Ampco or aluminum bronze's, (18,21 or 954) at least double in mechanical properties over standard bearing bronzes, probably have to make your own, difficult to machine. Stuff makes great guides too!
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[Alstrup]

There is a reason to why some high rpm motorcycle pins are tapered
I have never seen it that bad though, but I dont build any 9000 rpm engines either.
But I am sure if you ask people like say JP Motorsport and others in that "group" of builders they have experienced similar.

[modok]

yep, aware of it.
IMO a high boost piston and pin should look more like subaru.
23mm heavy wall pin, closely spaced pin bosses tapered on the sides.
And yet even those can be bent once north of 100 hp/cyl.

[[email protected]]

That Hot VW’s 1915 that Dave Kawell built bent a Mahle pin when trying to push it past 30psi.

I seem to remember some one over on the UAC, or CLF mentioning that JE had a hd VW pin that was .240 wall.

[Nightshift]

H-13 tool steal tapered pin with Teflon button on my builds. I have them made by Diamond Pistons. Never a problem. They also make my pistons to order.

[Nightshift]

TiNitride is the coating you need for Titanium wrist pins or you will see fretting on the piston pin bores. Other coatings will work also. They do work well and reduce the reciprocating weight substantially. Been there done that. LOL

Pin price is around $300 per piece.

[modok]

The best way to make a pin lighter is use a shorter pin.
Only needs to be about 2.2" long.

the pistons we Do use having such WIDE spacing between the pin bosses, allows all kinds of sin with alignment, but it's just wasted space and excessive weight.

[bugguy1967]

[Nightshift]

Pin length is determined more by pin boss span and tower design. Boss span is determined by rod width. Be careful when you shorten the pins as undue stresses are transferred to the boss. Yes, shorter and thicker will not flex as much. That being said you will need to redesign the boss and tower for the short pin.
.

[TomSimon]

There's a lot of good stuff in this post. I often wondered when the rod centerline being offset (not centered in the bore) would catch up. Answer: 100hp/hole !

I had this precision engineering prof that used to say: "Nothing is infinitely stiff/rigid, you've got to stop thinking like that. We live in a rubber world... everything moves given enough force... in fact, very strong things move just a little with surprisingly small forces"

Another answer is less clearance between the piston boss and edges of the small end of the connecting rod, less span = less bending. The acvw stuff has a huge gap. If custom pistons aren't feasable, thicker pins will do it. Berg wrote that crank flexing was a big contributer as well, introduced a angular component into the connecting rod, side scuffing the piston in it's bore, wearing the pin and retainer clips, burring up the retainer groove, and bellmouthing the rod bushings.

I am looking at a rod/piston combo out of a 300hp/per hole engine on my desk. The pin is dual tapered, 3/16" wall in the center. The big observation in my mind is the comparatively small gap between the small end of the rod and piston boss, it's only .08" per side. I've observed the same thing with top fuel piston/rod combinations I've looked at, but everything is thicker/heavier for 1000hp/hole lol.
_________________
2-time NASA Pro Racing Champion, Bonneville 130 Club and 150 Club Memeber, BRS Pro Gas racer

[Nightshift]

Piston guided rods can be a very close tolerance between the rod and boss/tower. It is not recommended for street use. I typically set my rod to boss clearance at .050" on a steel rod and .060 aluminium. Always remember to leave enough clearance over the top. .035 for steel and .050 for aluminium.
Cir-clip to pin.008" per side.

[mark tucker]

tool steel nascar pins(dell west, brooks,cp, etc) DLC coated on flebay cheep. cut to fit. and can be gotten in over size too. Ive used tool steel pins for well over 40 years in most engines. no reason not to and if your ordering pistins it's stupid not to have them. mahles new pistons have coated pin bores as well as tapered machined bores...not honed straight bores. taper bores keep the pin flex/deflection down according to mahles testing in F1& nascar, and other forms of motorsport. the dls coating is a added bennifit especially since these engine have dry sumps that operate under a vacuum( hoover). they have pin oilers but that hoover is hard to over come and pin scoring is still a issue for some. some nascar pins are almost a solid rod. screw the weight you need reliability. I have thin delwest pins for mine I think they are .170 wall. long ago I was using .090 or .120 wall tool steel some tapered some not and .150 tapered for big block stuff. taper is good way to shed a little weight and keep the pins stiff. I may taper my delwest pins ...if I ever get to put all this stuff together. on my std vw/mahle pistons I have always used the cb tool steel pins.but they do run on the small side....I also dfl coat them. witch helps with them being a little small.I think they are sized when still warm.

[Nightshift]

Del West is a good company. I have been using their Titanium valves valves for years. They wont steer you wrong.

[fl59bug]

[Nightshift]

Those pins are made by a company called Trend. (good product) You can buy them direct or from Diamond if you like. This topic has gone of the rails and I may be to blame.
Honestly it is not worth the $ for 4 pins. I stopped using them in my Funny cars and dragsters years ago as after a few passes they would be replaced as would the rods. 5 to 7 passes and my rods are sold to the guys running low HP alcohol blower motors in boats.

[fl59bug]

Thanks.

Lol I was being cheeky when I posted my response. Dave is right, unless there is evidence to the contrary or the engine has that 100hp/hole thing going on the as-supplied pins probably work just fine.