Just got some Speed Pro H859CP pistons and a set of Eagle H-beam rods. When I installed the wrist pins through the bushed rod end, it seems tight. I was able to get them in by hand, but it was tough. Had to tap them to get them out. Are they supposed to be so tight? The wrist pins slide easily into the pistons.

 

if you arn't sure i'd find the spec and measure. for a floating pin it should be about like the fit and feel on the piston. doesn't just fall in, has to lined up right then it'll go, no need to hammer on it. make sure you install the keepers, snap rings, or spiral lock right ,someoen messed that up here a week or two ago.

 

Did you try putting a little oil in the bushings first, and on the pins? While being a "floating" pin, they are close tolerance and need to be lined up perfectly to just "slip" together.

And by ALL means, like ede said, make sure you have ALL of the spiro locks on the pistons and that they are facing the right way.

 

As usual, Ed is right. Two tenths is not technically an interference fit, but can be difficult to assemble by hand. It would probably be classified more as a push fit. The term "floating" to describe piston pins is commonly misunderstood to mean it should just fall in and out of the wrist pin hole. Sounds like what you have is normal.

 

They should fit fairly tightly, not so tight that they require hammering necessarily, but tight enough that they don't ordinarily push in easily or fall out. Next time you do this, heat the rods a little bit by putting them in a 200° (warm) oven or boiling water before installing them, and put the pins in a freezer. Use light motor oil as lube.

Pay close attention to whatever retainers go in the piston, whether they're C-rings or Spirolox. Alot of pistons require 2 in each end, so if 32 of them came with your pistons, and it takes 2 at each end of the pins to completely fill up the groove, then put them in that way. There should be very little tolerance, if any, between the ends of the pins and the reatiners; the pin should be held pretty tightly in between them.

 

Thx for the replies guys.
When I mentioned I had to tap the wrist pins out, that was with the pistons installed on the rods. I was trying to push with the end of my finger inside the wrist pin (ouch!) and was unable to simply push them out that way.

Today I coated them with a little ATF tried pushing them into just the rods. They went really snug but not too bad. (they definately wouldn't fall out)
Is the rod/wrist pin clearance supposed to be .0002"? If so, they're probably OK.

The wrist pin slides thru the piston considerably easier than the rod.
I measured the wrist pin to be .9274", but I don't have any way of measuring the rod end or pistons that accurately (+/- .0001").

I had originally assumed the wrist pins would rotate within the rods during normal ops, but it seems the wrist pins will be rotating within the pistons.
Once the piston gets hot and expands, maybe the wrist pin/piston clearance will decrease gripping the wrist pin and forcing the wrist pin to rotate within the rod.?.?

The term "full floating" is definately a little misleading here, at least for a novice eng builder like myself..