Press fit vs. Floating piston wrist pins

[novadude]

Can someone explain to me why you'd chose one or the other? I know most OEM SBCs (with few exceptions), had press fit pins, but I've noticed that even "budget" pistons (like KB hypers, etc) seem to be coming with spiroloc grooves these days. Any reason to choose floating over press-fit (or vice-versa) when building a mild 300-400 hp basic street 350 Chevy?

[rfoll]

If you buy new rods, the bushing option for floating pins is about $50. The cost of pressing pistons on rods is about $50. I have twice had to take assemblies back to the machine shop because a piston was on backwards. The downside of floaters is the possibility of losing a retainer, it happens.

[GOSFAST]

Can someone explain to me why you'd chose one or the other? I know most OEM SBCs (with few exceptions), had press fit pins, but I've noticed that even "budget" pistons (like KB hypers, etc) seem to be coming with spiroloc grooves these days. Any reason to choose floating over press-fit (or vice-versa) when building a mild 300-400 hp basic street 350 Chevy?

Hi "nova", the "positive" sides to "floaters" far outweigh's the "negatives"!

Just a couple, you can assemble and disassemble the pistons/rods even with the 4 lock ring setup (if necessary, carefully) for any add'l machining after the initial mock-up, flycutting the pistons for instance!

On initial mock-up you can hold the pins in position with 2 strips of simple masking tape in place of the permanent locks. You don't need any "special" equipment, pin-press/rod heater, during final assembling!

As mentioned above, you get one pressed on "backwards" this COULD really open the proverbial "can of worms". It takes some special attention to R and R a pressed pin deal without destroying a piston?? To add also, I've never "lost" a lock ring, but I will say some years ago, when most locks were the "Waldes" type (these are the one R and R'd with special pliers) occasionally a lock would find it's way out. But it usually could be traced back to improperly installing the "Waldes" ring. One side was "flat" and one side was "rolled" on this style lock, the "rolled" side had to go against the wrist pin, the "flat" to the piston groove! Many had gotten installed incorrectly in the arena!

Thanks, Gary in N.Y.

P.S. One add'l feature is you "free-up" some add'l internal frictional HP, may be a small number, but it is on the "plus" side. This reason by itself is a "major" plus over here!

[Matt Gruber]

another plus for floating,
it's easier to try more compression in the future by changing pistons yourself, no trips to machine shop.
u could do crazy things, like run more cr on the rich cylinders. if it didn't work out, easy to swap back.

[novadude]

Thanks. I didn't know if there were any downsides to floating the pins (besides the extra cost of bushed rods).

[stock z/28]

You can take this for what its worth, I am certainly no expert.

In my opinion under high loads and/or speeds the floated pins in the rods add the capability of the pin moving in the rod, and possibly avoiding galling in the pin bores if the piston is distorting a bit.

I think it adds some power/durability under some conditions.

On press fit pins, I have a bunch of pins I have ground a bit to allow them to slide though the rod eye of a pressed pin for taking measurements or general mock up. Im sure most builders do as well.


A big disadvantage to floated rods is that they cause me pain and on occasion some blood.

[GOSFAST]

(Quote) On press fit pins, I have a bunch of pins I have ground a bit to allow them to slide though the rod eye of a pressed pin for taking measurements or general mock up. Im sure most builders do as well. (End quote)

If you have access to a small lathe you can "build" some tooling to deal with the pressed-pin issue of having to do a pre-assembly with the "pressed" components.

Requires a few small pieces of 1.000" round aluminum, for the SB's and BB's, for larger pins, you'd use 1.250" round!

I'll place a photo below to help "explain"!

Thanks, Gary in N.Y.

P.S. Here's a shot of the SB, the BB, and a BB (R-RB) Mopar, makes preassembling/mock-up really simple. I'll just add another "tip" here, for the SBC only you can use a length of electrical conduit, fit's perfect with just some minor polishing!

[Nick Campagna]

What about pin buttons. They were popular at one time. No wire, flat or round, to hold the pin in place.