How centered does a piston pin need to be?

[modok]

How far off center is acceptable in your opinion for a press fit piston pin?
I've just had a customer want me to re-do another shops work. The worst one was maybe .050 off center.
You can guess what I said.
But what would you say?

He also thought you could heat the rod with the piston on it, but, hey, maybe it is possible.

[Tom68]

Rods aren't centered in pistons anyway.

[BCjohnny]

Is it 0.025/0.025" or 0.050/0.050" ?

Either way, too much IMO ...... and can only get worse depending on how the rod is then centred on the piston when built up onto the big end

Admittedly 'sweating' pins on can be a crap shoot, ideally you need fixturing to tweak the final position ......... or at least I often do

[modok]

I meant it as I said it, the pin needed to be moved about .050
Late model big block chevy.
As far as I know, most people just eyeball it, and could be off .025, and you'd never know.

But if there are people out there getting super accurate about it, I guess that's fine too.

Tho I have concerns. Rod alignment, end play, ect, piston, bore locaton, pin bosses as-cast, that's a big stack. the side play of the rod in the piston is there to accommodate all that tolerance stack. So, even if you think it's centered......it probably isnt in the end.

[turbo camino]

So this dude is going to take his pressed-pin big block and go enter it in this year's 24H Le Mans or something?

[BCjohnny]

In some respects it's ultimately as much to do with customer perception/expectation as engineering ...... pretty much what started this thread

So if the obvious things haven't been done with reasonable care, how about the rest of it ?

I tend to get them visually centred on the pins, within reason, and they fall wherever they are when assembled into the block ..... failing specific instructions to the contrary

[modok]

Lets put it like this, old american v8s, when you take the old ones apart, on the pin you see the two rings on either side, you can see if it was centered, and quite often they are visually not equal, but rarely enough to cause trouble.
But as for why.. if that was the piston, or the rod, or the block, or the crank, you don't know. So I'd say that's where I developed my idea of the tolerance.

[Tom68]

Another way of looking at is moving it could do more harm than good.

[Dragsinger]

If it were mine to do, I would proceed in this way. Install a piston/rod assembly, without rings, as a mock up test. "IF" the pin completely covers the piston pin boss then the assemble is acceptable. Even if one side of the pin extends beyond the piston pin boss, there is nothing to gain if the opposite side is at least flush in the piston pin boss.

Other issues are part of the situation in ref to what is the expected power output, what is the potential "abuse factor" from the driver, will the owner be satisfied regardless? It is a tedious package because of have several hands working on the deal. I "think" if I were to take it on, it would be only if I built the complete engine so I could recheck everything.

[Dragsinger]

Also consider this. If the piston and rod assembly were to be separated [as in reusing rods on new pistons] a press with proper tooling could be used to do so that would not harm the piston nor rod. Thus, the same tooling setup could be used to slightly tweak the pin centering.

But, this may not be the case, the original shop may not have the tooling to use with a press. They may simply press the assembly apart without regard to piston damage because the old piston is scrap. Then they heat the rod and install the pin with an alignment tool. Are all pins off center the same amount? If so, the alignment tool could be improper.

[turbo camino]

I have never in my life seen a problem caused by a pressed pin not being centered in the piston - as long as it hasn't lost the press and walked out and chewed up the cylinder wall. I have seen them all over the place. It is just not something to worry about.

Yes, sure, if you are the one installing them, and you have the free time to test fit everything and determine where the rod will sit between the bosses, and then the free time to build a fixture so that all the pins end up centered in the piston when the rod is wherever it's going to be, then doing all that won't hurt anything. Won't help, either, but you know. If it matters that much go to a floating pin.

[BillK]

I certainly would not worry about it for .050" Like a couple of others have said the rods usually are not exactly centered anyway. I have seen pistons with pins a lot shorter than the pin bore so what then ?

I would be more concerned about damaging things trying to move them. Most new pistons are made in such away that you simply cannot move a pin in the rod press without hurting something. I have the OTC setup that slides inside the piston and supports the rod and even that wont work.

I had a set of 1998 454 rods in the shop today and wanted to take one off without hurting it if possible to try and measure pin height. Wasn't going to happen. I got them off without hurting them too bad and they were not being reused anyway but there is no way I could have moved the pin at all without hurting something.

By the way, I normally just eyeball them

[fordified]

I would tell him not to sweat it over .050, especially since the rod’s gonna be off center anyway. If it doesn’t hit the bore wall then don’t worry about it. I’d also tell him that you always run the risk of damaging the piston on the pin press. You could also ask why he didn’t get pistons for floating pins in the first place if he was that concerned about it.

Pass on the job.

[Tom68]

I made my own little end supporting tool just cause I couldn't bring my self to push on the pin whilst pushing the rod against the angled edge of the piston pin bore.

You can also easily shuffle a pin over after install if you need to.

[fordified]

I have a Kent Moore fixture. It supports the rod similar to what you’re doing.