Wrist pin replacement!!!

[helmutholderbenz]

I am replacing a wrist pin in one of my wiseco pistons due to a cir-clip failure. These pistons are old from the 80s era xt series. My new wiseco pin came in and I notice that it is a little different. The original pin is of the same thickness all the way through but this new pin looks to be tapered, thin on the outside but gets thicker in the middle on the inside of the hole( if anyone can make sence of that!) Now I'm worried that using this new pin it will throw my motor out of ballance. Do you think I should use it or go get the pistons ballanced? Thanks for your help

 

[londonboards]

I'm sorry you haven't had any response from a more technical member but I know what you mean. I have wrist pins that, on the inside, are thicker in the middle.

I can only imagine that they should really all be the same. Same pistons and pins. Before assembly, people often weigh them together and check they are all the same weight. They then file little parts off the inside of the pistons to get them all to weigh the same. I just don't know how critical this all is to the average road machine.

 

[TxGSrider]

I think I'd weigh the new one and the old one (if your job has a postage scale they are pretty accurate... of if you have a local drug dealer they should have a scale...) and see how close they are in weight. Then call up Wiseco and get their opinion. If its way off, then just buy 3 more.

 

[Nessism]

No worry with the newer pins. You will be fine as long as all the pistons/pins weigh the same within a few grams.

 

[Guest]

It would be a cheap way out to just order 3 more new pins.

 

[Doug]

Many newer design wrist pins are tapered to the outboard ends to save weight for quicker revs. Looking at the distribution of stresses on the piston and rod it's easy to see that all the combustion forces get transferred to the rod and therefore that area on the pin is so designed.

Why back when, sometimes rods would break in Hemi race engines do to the rather heavy weight of the piston stopping and reversing direction at the top end of the stroke, not from the combustion forces pushing down.

I have a good friend who runs a NHRA prostock bike with the S&S Harley style engine. The bore is 5.25" and two cylinders make well over 300 hp. He uses tapered pins and if the pistons he buys have straight pins, he machines the tapers into them. The pins are around 60 Rockwell and not that east to cut but he owns a machine shop.

As mentioned, if the weight of the straight and tapered pins is about the same, fly'em. Even putting a second tapered pin in one of the other pistons (one up and one down, opposite on crank) might help the balance.

 

[JTGS850GL]

Agree with Jay. Order 3 more of the newer pins and be done with it.

 

[helmutholderbenz]

I work at a machine shop, I took the pistons to work yesterday and weighed them. Three weighed within a gram of each other 237/238 gr. one was down at 235 and it wasn't the one with the new pin. I'm glad I ate crow and went back to work this past Thursday, its so hard to buy motorcycle parts without a job! and the perks of the job are PRICELESS!!!

 

[gs11ezrydr]

I agree with Jay and the rest . Just get 3 or 4 to be sure they weigh the same of some new matching pins and problem solved, Don.t forget circlips never reuse them I have been told. Make sure you don't spring them and place the gap at either the top or bottom, not front or rear. Could pop out when installed front or rear in high R.P.M. applications due to circlip collapsing. Seen it in 2 strokes. No problem here likely better safe then sorry though. Also I have seen tool steel pins that were straight all the way through and a little thicker, black and not chromed, but they were used for forced induction applications, like turbochargers, blowers, nitro and nitrous motors where max strength was needed. When balancing the pistons(without the pins installed) I have weighed them and took the lightest one lightened it a little then made the other 3 weigh the same by grinding off a little on the insides of them. Sorry just Rambling.