Hey everybody. I've shearched and searched and cant seem to find a definite answer. Everybody will tell you if your rod or main clearances are too large, you wont have sufficient film and spun bearings and seizure will insue, however, when it comes to a full floating setup, specifically the piston bore to wrist pin clearance, if the clearances were way too big for exaple, and you lost the film between your piston and wristpin causing seizure, wouldn't everything kinda just be fine? Like the wrist pin would be seized in the piston sure, but not in the rod end.

So why does it matter if your piston to pin clearances are off?

88BB8B said:

Hey everybody. I've shearched and searched and cant seem to find a definite answer. Everybody will tell you if your rod or main clearances are too large, you wont have sufficient film and spun bearings and seizure will insue, however, when it comes to a full floating setup, specifically the piston bore to wrist pin clearance, if the clearances were way too big for exaple, and you lost the film between your piston and wristpin causing seizure, wouldn't everything kinda just be fine? Like the wrist pin would be seized in the piston sure, but not in the rod end.

So why does it matter if your piston to pin clearances are off?

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Wrist pin seized in the piston? Think about that for a minute.

pauleyman said:

Wrist pin seized in the piston? Think about that for a minute.

Click to expand...

I think I understand what he's saying. Technically in a full floating set up the wrist pin would still rotate in the small end bore of the connecting rod if it seized to the piston.........

However with too much wrist pin clearance the result will more than likely not be seizure of the wrist pin to the piston. If this did somehow happen there are no provisions for oil getting between the rod and the wrist pin and with both seized then your in real trouble.

If you run with too much clearance the most likely original scenario is it knocks and eventually wallows out the wrist pin hole in the piston. What happens next could be several things
1) The piston just breaks
2) Excessive heat build up causes some type of seizure
3) The piston contacts the valves

Manufactures have specs for a reason. I would not in any way recommend you run out of spec clearances.

What specs are you looking at and what specs are you getting? Most people don't have the proper tools to measure this clearance.

I haven't encountered any loose wrist pin clearances of my own, I was just curious, because I was thinking you would have to have both the connecting rod and piston bores out of spec in order to cause seizure in both places. But say you had to big or too small a clearance between your rod and wrist pin, and they ended up seizing, would that then just behave like a press fit? In Theory, could you run with one or the other seized, and not even notice?

97egl said:

I think I understand what he's saying. Technically in a full floating set up the wrist pin would still rotate in the small end bore of the connecting rod if it seized to the piston.........

However with too much wrist pin clearance the result will more than likely not be seizure of the wrist pin to the piston. If this did somehow happen there are no provisions for oil getting between the rod and the wrist pin and with both seized then your in real trouble.

Click to expand...

How do you mean that there would be no provision for oil to get to get between the rod and pin if the pin seized to the piston. All the pistons and floating style rods I've seen have holes and sometimes channels to direct oil to the joints. If the pin seized in any of the holes, oil should still be able to get where it isn't seized, shouldn't it?

I hadn't thought much about the issue of heat, but of course it could definitely generate a lot and break stuff, but again, ideal scenario, if the pin were to seize in the rod, but not the piston, my instinct tells me that the floating nature of the pin in the piston bores wouldn't provide much traction to turn the pin hard enough within the rod, as it seized to generate a dangerous amount of heat.


Also side question, I think I already asked somewhere, but the more opinions the merrier. Why aren't press fit rods ever bushed? Would it be possible/functional to have a setup with a custom bushing that interfered with the wrist pin, forcing everything tight, just like the stock rods but with an extra layer. I heard that this wouldn't be acceptable because a bushing material would not be suitable for such an application. What if the bushing was made of a more suitable material?

There would be no point in installing bushings in press fit rods, the pin is made to be seized in there and locked, press fit rods are made in bulk and made cheaper, installing bushings would require hand machine work that would cost money for no reason, press fit rods are not made for performance they are made to be cheap and still get the job done.

I see what you're saying. But I'm thinking of a situation where somebody whats to use a bushed connecting rod with a piston which does not have grooves for spiralocks. Instead of machining 4 pistons in 8 spots. Would it not be easier to rebush the rods with interference bushings instead? Is there a reason that wouldn't work?