The main reason the OEM's use pressed pins is cost. The piston and conn-rod assembly automation is much less expensive than the tooling required to install floating pins and to make sure that the clips are installed properly.

The floating pins are definitely better than fixed pins in theory for these reasons:

1) The bearing surface in the conn-rod adds additional load bearing area.

2) The floating pin can rotate during operation so that the wearing surface is around the entire pin, not concentrated toward the top of the pin as in a press fit application. This is what actually makes lubrication of a fixed pin more critical. This allows the floating pin to survive without scuffing in hotter, more highly loaded environments without the already minimal oil film breaking down.

3) The pin will wear longer since the wear is not concentrated in one spot.

4) If the small end of the rod is designed properly it can accept a bushing with no loss of strength. Even though babbit plating the rods in the DZ302's was expensive, it was cheaper than buying a new set of forging tools for a new conn-rod forging with beefed up small end wall thickness.

5) Most of the time floating pins are used in turbocharged or supercharged environments where the piston is exposed to very high cylinder pressures and temperatures. The diesel industry almost exclusively uses floating pins. My bet is that GM installed the floating pins in their 3800 supercharged applications so that they could survive, and having already spent the money on the required automation, determined that they could use the same line to assemble the bread and butter piston and rod assemblies as well and get a longevity benefit.

Do you NEED floating pins? No. I said in theory they're better but there are millions of engines out there that never had any problems with the fixed pins they were assembled with. If you're going with aftermarket rods, I'd spend the extra money for the bushed style and go floating pins. If you're staying with stock rods I'd stay fixed pin.

 

i wonder if the rod has to have a bushing installed or just honed out.

Seems like i read a racing book were they just said to hone them.

 

Most modern day race engines run the piston pin directly on the steel rod BUT, the use Casadium coated pins (if you have to ask you can't afford it, about $600 for a set of coated pins.) Most aftermarket rods like Manley Sportmasters and Crower Sportsman have Ampco 45 bronze bushings in them already. I have never had anyproblems with "Spiro-lock" type pin retainer in street motors and most pistons have this type of pin retanier although I just looked at a set of Mahle street/strip forged pistons with round wire lockas and I am pretty sure Federal Mogul uses them in most of the newer TRW line of forged pistons (I am not a big fan of FM pistons.) JE, KB and Wiesco all use Spirolocks in their pistons. My 72 LT-1 has Wiesco pistons with Spirolocks and they have over 14K miles at the present time with no signs of problems.

 

An engine I play with a lot comes in 2 flavours, both having alloy rods & floating pins, but one having honed small ends so that the pin runs in the alloy & the other having a phos-bronze bush, honed to size. Both have an oil feed via a countersunk hole to the pin drilled from the top of the rod & get lubrication from splash feed from the flywheel & oil drain holes in the bottom ring groove. The larger, more powerful engine, doesn't have the bushes & in over 20 years of building/fixing these engines I've NEVER seen a small end worn, except in cases where Bubba has damaged it while "rebuilding" it, or it's been starved of oil. This includes fixing many that have holed pistons due to detonation as the owners never allowed for leaded fuel being taken off of the market.

I'm the 1st to admit that I've got no experience of SBC small ends, but the principle is the same (with all the usual allowances for material types, coefficients of expansion, relative hardness, the all important lubrication, etc). I have, however, had a lot of experience with m/cycle engines, including 4 pots that'll run at 12 000rpm all day, NOS & forced induction (all of which put incredible loads on small ends). They've all had floating pins (though a lot of people don't think so as they are a press fit in the pistons, the pistons requiring heating to insert the pin. Once cooled they grip the pins, but under operating temps the pins are free to rotate in them). A lot of these bikes have done high mileages at stupid rpms & the floating pins have given no problems. One engine blew apart at high revs after many years use (the owner got lazy with servicing it & it let rip). It was a GSX1100 tuned to the limit & it burst when being ridden well above the red line (as usual). There wasn't much left of it, but the few parts that were unscathed were in remarkably good condition, including the small ends (unfortunately a couple weren't connected to the big ends anymore ).
So, in theory, I agree with Lars. If the pin is a press fit then the load will be more evenly distributed. If a pin is a sliding fit then the load will be theoreticaly concentrated on the point at which it touches the rod (if no oil was present), but I've never seen evidence of it. Maybe in the real world it's one of those theories that doesn't get a chance to prove itself? A bit like all the additives in modern oils that supposedly keep an engine free from corrosion, acid damage & everything else for 250 000miles - who knows? How many cars make it that far & avoid being turned into dog food cans or having an engine rebuild?

Who mentioned teflon buttons? Last time I looked, my mates who still build insane bike engines invariably use teflon buttons in anything that's going to produce copious power and/or run at high revs. The main reason for using them is to avoid using circlips (snap rings?) that may break under these conditions (although I've never known one to break - plenty of other parts have broken though!). Teflon buttons are alive & well, but never ever agree to help sombody replace the cylinder block on a CBX (in line 6) if they're using them

ttt