Gudgeon Pin Fit Inside Piston

[mikedsilva]

Question for the experienced.
I find that when disassembling a motor, the pin in the piston is always a tight fit and sometimes I need to heat the piston in order to get the pin out.

When having rods resized and parts prepared, what is the norm for how tight the pin should fit in the piston itself?
My thoughts are that the pin should be quite snug as when running and hot, the piston would likely expand more than the steel pin and hence the fit would become a little more 'sloppy'... and you would want the pin rotating in the bushing, rather than the pin rotating in the piston.

Reason I ask, is I just tried a different shop, and they "cleaned up" the pins, then rebushed the rods, and sized the bushes to the "cleaned" pins.
Problem is, now the pins are a very easy slide in and out of the piston. I've not experienced this before and looking for advice.
When I quizzed them they said they used 240 grit to clean up any irregularities.

Thank you.

(I'll try to upload a short vid to demonstrate shortly)

[jess p]

I just had new bushings installed in all my rods. I have new Mahle piston sets from our host. I took my rods to a reputable machine shop that sees a lot of Porsches. I gave them all new piston pins and I made sure each pin was returned with its matching rod along with the piston it came with. I notice that yes,they do slide in and out easily. I’m wondering that perhaps with the pin being steel, the bushings bronze and so forth there are different coefficients of expansion.

[Jeff Alton]

Mike,

Are you finding this more on SC and older motors? The pin fit on the SC often required heat. Where the 74.4/76.4 stroke motors (all with same rods 3.2/3.3/3.6 964) don't seem to have this issue.

[mikedsilva]

Originally Posted by Jeff Alton

Mike,

Are you finding this more on SC and older motors? The pin fit on the SC often required heat. Where the 74.4/76.4 stroke motors (all with same rods 3.2/3.3/3.6 964) don't seem to have this issue.

Jeff, now that you mention it, yes. In fact, 3.0 engines with Alusil pistons (not in this particular case) seem to have the tightest. I normally use a threaded rod with a nut at the end, and a small weight, and use it like a slide hammer to gently pull the rods out of each piston, but with the tight ones, I end up heating up the piston to help.
This 3.2 motor with Mahle pistons, had a little bit of resistance removing. But did not require heat to get them out.

[icarp]

Mike , all of your wrist pin fits are TOO LOOSE !!!!!!

The pins should not have any side play "rock" in the piston.

What are the pistons? old 911 pistons can run as little as .0001"-.0002" and still not be a press fit .
Modern Honda engines have an interference fit with the rod , the rod must be heated to install or remove the wrist pins

The modern "CP" - "Mahle" and the like pistons, usually have .0008" as new . "8 tenths"

A Sharpie marker makes an ink line .0004"- .0006" thick

The fit should be a "steady constant push" through the piston and rod bushing.
Maybe with oil, light thumb to 8-12 oz of pressure , thumb press fit @90f degrees temp
edit I just tested a new set of CP

The rod and pistons should have the same clearance , as a floating wrist pin system.

I think your rod bushings are out of spec.
I think your wrist pins are out of spec.

Good on you for calling this to the fore, I think it would ended up with piston knock or worse.

.020mm = .00074" and .037mm = .00014" These are the 911 sc pin clearance specs

Call the piston maker and ask them

Ian

[gled49]

Remember when disassembling P&C’s that the pins hammer the wire locks and create ridges that make the pins hard to remove. If you are reusing the pistons it’s your choice weather to hone the piston bore a little to remove the ridge.

[PeteKz]

One too many zeros, Ian. .037mm=.0014"

[Dpmulvan]

Some shops are lazy or ignorant when it comes to honing wrist pin bushings.

[Jeff Alton]

Ian, is this one of those threads we chatted on the phone about???? You know what I mean.

Hope you are well.

Cheers