wrist pin wont fit in new bushing in rod

Rock2000 · Jul 12, 2014

This message is a reply to an archived post by rustred on March 08, 2012 at 15:44:55.
The original subject was "Re: wrist pin wont fit in new bushing in rod".

Hmmm, I seem to be having similar problems. It doesn"t really even seem like the pin will fit in the new piston, let alone the bushing. How can any of this need to be honed? Everything but the connecting rod is new and from a single supplied kit. If it doesn"t fit together, it sounds defective to me. I"ll have to check with YT, but taking this a machine shop sounds like BS to me.

rustyfarmall · Jul 12, 2014

(quoted from post at 13:33:42 07/12/14) This message is a reply to an archived post by rustred on March 08, 2012 at 15:44:55.
The original subject was "Re: wrist pin wont fit in new bushing in rod".

Hmmm, I seem to be having similar problems. It doesn"t really even seem like the pin will fit in the new piston, let alone the bushing. How can any of this need to be honed? Everything but the connecting rod is new and from a single supplied kit. If it doesn"t fit together, it sounds defective to me. I"ll have to check with YT, but taking this a machine shop sounds like BS to me.

If the pin will not fit the piston, then you've got some wrong parts.

NEW bushings in the rods ALWAYS require honing or reaming to the exact size for the pins. The reason is that the new bushings always compress just a bit when pressed into the rods.

Bob · Jul 12, 2014

The rod bushing has to be "sized" after installation, on a specialized machine to keep the wrist pin and "big end" bores parallel.

The video linked below shows the "big end" being honed, the procedure is the same for the little end.

The rods NEED to have the "big ends" checked, and be checked for straightness, as well. That is standard procedure when doing an OH.

I ASSUME the piston is aluminum... if the pins came with the pistons, they likely will fit correctly.

With the pistons WARMED and the pins lubed with light oil, they should pass through the pistons with thumb pressure.

You don"t WANT them looser than that.
Rod honing machine

D Slater · Jul 12, 2014

If its not a problem already answered in the posts it would help to know what engine you have.
Some IH engines had changes to the rods and piston pin size and bushings used on the same size engines.

Rock2000 · Jul 13, 2014

The pin will fit nicely into the piston when the piston is slightly warmed. I'll own up to being an idiot on that one.

I'm still not understanding the bushing part, although I'll trust you guys. I'm assuming the rods are a specific size. So why can't a bushing be bought that is made to fit the rod and pin? They know the pin size (it came with their kit), and assuming rods are made to careful tolerance don't they know the rod hole size? Where's the missing variable? I wouldn't expect the rod to wear, since I expect that's what the bushing is for.

It's a 1950 H.

Thanks
Rock

Nebraska Kirk · Jul 13, 2014

Since the bushing must be a tight press fit in the rod, it has to be made a few thousandths of an inch larger than the hole in the rod and when it is pressed in, the inside diameter of the bushing will shrink a little. That is why the bushing must be reamed and honed to a final fit after it is installed. The bushing has to be honed to a VERY precise fit to the pin, the specified clearance being .0005"

Rock2000 · Jul 14, 2014

But if the hole in the connecting rod is a known size, and the bushing ID and OD can be made as needed for the application (by the manufacturer), then why can't they determine the ID and OD needed so that when it is pressed into the rod it shrinks a known amount to bring the final ID to the correct size? I still don't see why custom work is needed, unless the rods weren't made to exact tolerances. It seems like all measurements should be known values: the rod hole size, the bushing ID and OD, the bushing material, and therefore the final bushing ID.

Even saying the bushing needs to be reamed, why can't the pin be pushed into the bushing before the bushing is even pressed into the rod? There is just so much more bushing material than would ever be needed. It is just going to require more reaming for no reason.

Thanks
Rock

Nebraska Kirk · Jul 14, 2014

The thing is that the rod hole is probably only accurate to +/- .0005 inch, plus the bushing dimensions may only be accurate to +/- .001 inch. Even if the hole in the rod was accurate to .0001 inch the bushing would still have to be accurate to .0001 for it to work as you describe. A bushing made that tight of a tolerance will cost $100 I bet.

When you are dealing with half a thousandth (.0005) of an inch, then the bushing MUST be pressed into place FIRST and then finished afterwards. There is just no getting around it. You may not like it, but that is the way it MUST be done. Engine rebuilding is a very precise and meticulous process, if the clearances are not right, then the engine will not last long.

Owen Aaland · Jul 14, 2014

Proper installation of the pin bushing is not just a matter of pressing the new bushing into the rod but it should also be burnished. That procedure involves pressing a tool through the newly installed bushing that is a few thousands of an inch larger the the bushing ID. It expands the bushing into the rod to insure good contact between the rod and bushing. That should leave the bushing about .005" smaller than the final size.

I normally shoot for about .0002 to .0003 clearance between pin and bushing when I fit them. At that clearance the pin will slowly slide through the bushing when oiled and held vertical.

Rock2000 · Jul 14, 2014

Well you're right that I don't like it.

If the rod tolerances were not great than I guess there is no getting around that. But I don't really see why the same problems don't occur with main or rod bearings. They need to have around 3 thousandths clearance, and I can buy a standard 10 thousandths oversize bearing set that I can just install and doesn't require any further custom work. I have to have the crank made to spec, but that is similar to having the wrist pin to spec (which it should be since it's new in this case). I don't need to (generally) worry about the the block or rod holes that house those bearings though, and I don't need to worry about any custom work on the bearings after I install them. So why is this bushing so much different?
Thanks
Rock

Jim Naden · Jul 15, 2014

(quoted from post at 21:01:00 07/14/14) Well you're right that I don't like it.

If the rod tolerances were not great than I guess there is no getting around that. But I don't really see why the same problems don't occur with main or rod bearings. They need to have around 3 thousandths clearance, and I can buy a standard 10 thousandths oversize bearing set that I can just install and doesn't require any further custom work. I have to have the crank made to spec, but that is similar to having the wrist pin to spec (which it should be since it's new in this case). I don't need to (generally) worry about the the block or rod holes that house those bearings though, and I don't need to worry about any custom work on the bearings after I install them. [i:88e8478ccc][b:88e8478ccc]So why is this bushing so much different? [/b:88e8478ccc][/i:88e8478ccc]
Thanks
Rock

[i:88e8478ccc]That's just the way it is.
[/i:88e8478ccc]

Owen Aaland · Jul 15, 2014

The pin bushings are fitted with 1/10 the clearance of the rod bearings. The clearance is 0.002 - 0.003 on the rods versus 0.0002 - 0.0003 on the pins.

wrist pin wont fit in new bushing in rod

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