Pistons and floating pins

imshaggy2000 · Feb 2, 2004

I just purchanced some TRW 8.5:1 compresion dish top forged pistons for my 69 351w. I basicaly bought them to stay on my budget for the turbo engine. They where $175 for the set. They didnt say if they had floating pins or not, I would guess that they probably press fit. What is the advantage to floating pins? If my pistons arn't designed for them can I add them? My dad said you need them if you want to rev the engine really high, my engine will only be going to 6,000 max, but should make about 550 hp and 550 lb/ft torq right there.

D.Hearne · Feb 2, 2004

Most TRW forged pistons can either run floating or pressed pins. Do they have spiro-loc grooves in the pin bores? If not then, you will either have to run pressed pins or find someone who can cut them and bush your rods to float the pins.

dolfan87 · Feb 2, 2004

It's basically a friction thing. If the pin floats, you will have a more easily spun rotating assembly. On an engine that is going to be turbo charged, I wouldn't run without them floating.

Anything over 400hp, really should be a floating pin in my opinion...but that's just me.

slapper · Feb 2, 2004

dolfan87 said:
It's basically a friction thing. If the pin floats, you will have a more easily spun rotating assembly. On an engine that is going to be turbo charged, I wouldn't run without them floating.

Anything over 400hp, really should be a floating pin in my opinion...but that's just me.

I'm not trying to be a pain, but I disagree.
Floating pins just make it easier to replace cylinder and piston assemblies on some engine configurations, such as radial and opposed, when compression results reveal below minimums.
The floating pins probably actually increase the friction from the nylon end-plugs against the cylinder walls.
It's just different design styles for piston pins.

rbohm · Feb 2, 2004

actually dolfan87 is right to a point. reduced friction is one reason to use floating pins, and the nylon plugs are virtually frictionless. another reason for floating pins is ease of assembly and disassembly, and one other thing, time. if the pin siezes in the piston or the rod you dont break a motor straight away. you have to to finish the race, or get to the pits to prevent further damage.

dolfan87 · Feb 2, 2004

slapper said:
I'm not trying to be a pain, but I disagree.
Floating pins just make it easier to replace cylinder and piston assemblies on some engine configurations, such as radial and opposed, when compression results reveal below minimums.
The floating pins probably actually increase the friction from the nylon end-plugs against the cylinder walls.
It's just different design styles for piston pins.

Well I hate to argue with you, but what in the world are you talking about? Nylon plugs?

The rod end is bushed with a bronze bushing, and the piston pin is held in place with spirolocks at either side of the piston. There is no nylon plug anywhere.

It is a WELL known fact that friction is reduced in a floating pin situation. This is why turbo charged, nitrous, and super charged engine builders strongly recommend using the system.

*Edit* Were you talking about the Top Fuel guys using TEFLON instead of the spirolocks? First off, that's top fuel racing, where they run it one time, and then break em down. Second the use of the Teflon in street applications is a no no.

slapper · Feb 2, 2004

dolfan87 said:
Well I hate to argue with you, but what in the world are you talking about? Nylon plugs?
The rod end is bushed with a bronze bushing, and the piston pin is held in place with spirolocks at either side of the piston. There is no nylon plug anywhere.
It is a WELL known fact that friction is reduced in a floating pin situation. This is why turbo charged, nitrous, and super charged engine builders strongly recommend using the system.

A TRUE full floating piston pin is NOT held in place in the piston pin bosses with spirolocks, or anything else. The pin FLOATS in and out and is prevented from causing damage to the cylinder wall by nylon,(or now-a-days teflon coated aluminum), plugs. Does plug contact the cylinder wall?? Yes.
Spirolocks are unique to the automotive industry as far as I know, and yes, they are a pain to get installed, but the pin is not allowed to travel past the spirolocks
WEAR is reduced in a full-floating pin design as the pin is allowed to rotate within the piston and connecting rod bosses, and the same radius of the pin is not continually subjected to the impact load of combustion, and the applied load of reciprocation.
The assembled tolerances of full-floating pins are obviously greater than semi-floating, or press fit, which is endorsed as a allowance for piston thermal expansion from forced induction or nitrous. That is why engine builders favor the design for forced induction/nitrous applications.
Why would you get a easier spun rotating assembly as you mentioned?
The angular motion of the piston and rod assembly around the pin is not impeaded from a press fit anymore than from a semi, or full floating
Remove the piston rings, and yes, you'll get a easier spun rotating assembly, but not from piston pin type design
As mentioned, piston and cylinder removal and replacement is much easier and can be performed in the field, and the wrist pin can be removed, and the piston and cylinder assembly lifted from the engine without every removing the piston from the cylinder, or unseating the rings. The reverse is true as long as the rod bushing has not been elongated or scored beyond serviceable limits. Of course this does not apply to the V8 configured engine, as the cylilnders are not removeable.
The full-floating piston pin has been used as early as the late 1800s

dolfan87 · Feb 2, 2004

slapper said:
A TRUE full floating piston pin is NOT held in place in the piston pin bosses with spirolocks, or anything else. The pin FLOATS in and out and is prevented from causing damage to the cylinder wall by nylon,(or now-a-days teflon coated aluminum), plugs. Does plug contact the cylinder wall?? Yes.
Spirolocks are unique to the automotive industry as far as I know, and yes, they are a pain to get installed, but the pin is not allowed to travel past the spirolocks
WEAR is reduced in a full-floating pin design as the pin is allowed to rotate within the piston and connecting rod bosses, and the same radius of the pin is not continually subjected to the impact load of combustion, and the applied load of reciprocation.
The assembled tolerances of full-floating pins are obviously greater than semi-floating, or press fit, which is endorsed as a allowance for piston thermal expansion from forced induction or nitrous. That is why engine builders favor the design for forced induction/nitrous applications.
Why would you get a easier spun rotating assembly as you mentioned?
The angular motion of the piston and rod assembly around the pin is not impeaded from a press fit anymore than from a semi, or full floating
Remove the piston rings, and yes, you'll get a easier spun rotating assembly, but not from piston pin type design
As mentioned, piston and cylinder removal and replacement is much easier and can be performed in the field, and the wrist pin can be removed, and the piston and cylinder assembly lifted from the engine without every removing the piston from the cylinder, or unseating the rings. The reverse is true as long as the rod bushing has not been elongated or scored beyond serviceable limits. Of course this does not apply to the V8 configured engine, as the cylilnders are not removeable.
The full-floating piston pin has been used as early as the late 1800s

Slapper, we are discussing Mustang engines here, not Volkswagon, or Motorcycle, or anything else.

Your saying a TRUE full floater is not held in by spirolocks...well that is just getting a tad more anal than I wish to go into it.

To me, when the pin floats in the rod, as well as the piston, that makes it a full floater, you can get all technical with the bushings if you want to, but I don't have time to sit here and argue removable cylinder engines.

The fact still remains that the floating piston design is better for Shaggy's application.

87

slapper · Feb 2, 2004

dolfan87 said:
Slapper, we are discussing Mustang engines here, not Volkswagon, or Motorcycle, or anything else.
Your saying a TRUE full floater is not held in by spirolocks...well that is just getting a tad more anal than I wish to go into it.
To me, when the pin floats in the rod, as well as the piston, that makes it a full floater, you can get all technical with the bushings if you want to, but I don't have time to sit here and argue removable cylinder engines. 87

I'm sorry. You're right.
I thought we were discussing piston pin design advantages, as shaggy first asked.
Next time I'll just post a short answer, as I intially did,...without going into reasons why.
I'm sure that will be easier to read without a lengthy anal explanation.
No flame intended.
Later.

dolfan87 · Feb 2, 2004

slapper said:
I'm sorry. You're right.
I thought we were discussing piston pin design advantages, as shaggy first asked.
Next time I'll just post a short answer, as I intially did,...without going into reasons why.
I'm sure that will be easier to read without a lengthy anal explanation.
No flame intended.
Later.

I guess I probably got a little too pissy with my response. I think I am from a different school than you when it comes to this type stuff.

My knowledge of engines is pretty much strictly in the automotive field, and specifically the high performance V-8.

When I said a more easily spun rotating assembly, I was referring to the engine doing it's work. When engine temperatures rise(especially in high performance, S.C.-T.C. or N2O applications), that pin can get a tad snug in the piston, and when it can move freely in the rod as well, it reduces the chance of busting a rod, or scattering a piston.

In Shaggy's case, he is wanting to turbo charge his engine, and I feel a floating pin is the only way to go.

I apologize for my "tense" response.

87

imshaggy2000 · Feb 2, 2004

I hope i didn't screw myself over on this one. I'm pretty sure they are press fit pins, I went with them because i was having problems finding pistons for a 69 block with the 9.48" deck, and they where pretty cheap for a forged piston I'm going to buy forged I beam rods any way so I can buy those bushed or press fit. Doe's any one have a solution for me?

D.Hearne · Feb 3, 2004

Did you look to see if there are spiro-loc grooves in the pin bores?

imshaggy2000 · Feb 3, 2004

They haven't arived yet, I just bought them. I know a machinest that might be able to cut them for me if they don't, but I would have to call him up and ask.

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