Con rod side clearance and piston pin fit

[strictly]

1) Con Rod Side Clearance

One spec i cant find in the books is something on con rod side clearance. In particular im looking at a 71 2.2, but there is no spec for it. The closest i can find from the porsche workshop manual and spec book, is the minimum width of the con rod, but nothing on the maximum width of the corresponding crank journal.

I have a factory 912 manual to hand, which DOES have the maximum spec for con rod side clearance,along with other (VW) books, but i cant find anything in the early work shop manuals.

How important is it? When would it be excessive? 0.012"? 0.016"? 0.008"? 0.020"?

2) Piston Pin Fit

The workshop manuals talk about heating the pistons to both remove and replace the piston pins, with it then goes on to say if it is a push fit at room temperature, then an oversize piston pin must be used and the con rod rebushed.

Bruce andersons book states that the new type pistons the wrist pins are a push fit at room temperature.

So which one is right?

C) Piston Pin to Rod Fit

I read somewhere that the piston pin to rod, should fit like this...

The weight of the piston pin should SLOWLY fall through the small rod bush under its own weight .

Does that sound right?

[356RS]

Con rod side clearance for a 2.2 is .008" to .015"
Piston Pin fitting on original Mahle's sometimes had to heat pistons. On new pistons like JE and others the pins can be taped in.
Piston Pin to Rod Fit; Very light coat of thin oil, Yes should slowly fall out, not drop right out.

[cgarr]

Originally Posted by 356RS

Con rod side clearance for a 2.2 is .008" to .015"

Piston Pin fitting on original Mahle's sometimes had to heat pistons. On new pistons like JE and others the pins can be taped in.

Piston Pin to Rod Fit; Very light coat of thin oil, Yes should slowly fall out, not drop right out.

I think you need to add a zero?


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[Neil Harvey]

Originally Posted by strictly

1) Con Rod Side Clearance

One spec i cant find in the books is something on con rod side clearance. In particular im looking at a 71 2.2, but there is no spec for it. The closest i can find from the porsche workshop manual and spec book, is the minimum width of the con rod, but nothing on the maximum width of the corresponding crank journal.

I have a factory 912 manual to hand, which DOES have the maximum spec for con rod side clearance,along with other (VW) books, but i cant find anything in the early work shop manuals.

How important is it? When would it be excessive? 0.012"? 0.016"? 0.008"? 0.020"?

2) Piston Pin Fit

The workshop manuals talk about heating the pistons to both remove and replace the piston pins, with it then goes on to say if it is a push fit at room temperature, then an oversize piston pin must be used and the con rod rebushed.

Bruce andersons book states that the new type pistons the wrist pins are a push fit at room temperature.

So which one is right?

C) Piston Pin to Rod Fit

I read somewhere that the piston pin to rod, should fit like this...

The weight of the piston pin should SLOWLY fall through the small rod bush under its own weight .

Does that sound right?

NO.

No, nothing sounds right. Engines should never be built by checking if, “parts fall out”.

Rod side clearance is given so the bearing shell does not come into contact with the radius on the crank journal at the web or cheek.
For most OE applications, the bearing shell will have a chamfer on its length and the BE will contact the cheek of the crank before the shell comes close to the journal radius.

Porsche engines have one rod per journal, so the shells can be either top or bottom and there is a chamfer on each side of the shell.

Porsche engines typically have bottom guided rods, so the clearance is typically around 0.004”- 0.005”. You are dealing with production engines here, so the tolerances are high. Do not be surprised if you have less or more. If less, then maybe have the sides of the rod ground to give the mentioned clearance, or have the crank re ground to spec. If you have a lot, then you need to fit the bearing shells and check you have clearance to the journal radius. If the shells come into contact with the radius you will have issues. To “fix” this you need new rods, or the crank needs to be repaired or replaced, and or the shells need to be machined with a larger chamfer. The tang position could limit this.

Pin fit to the Rod SE is critical with press fit pins. If you have these, then this is something I highly advise you give to someone who knows how to install. You have to heat the Rod SE so the pin will slide in. The spacing to the piston pin bosses is critical so the rod is centralized in the pin bosses. Not something you should try at home in my opinion. You need to make sure the SE bore is correct, and the pin OD is correct, so the correct amount of interference is held. The bores in the piston need to be checked as well. They are typically the same for floating pin pistons.

Most Porsche pistons have floating pins, so all three parts still need to be checked and made sure they are correct. Pin bores in the piston, pin bores in the rods and the pin OD are all critical. Floating pins can have anywhere from 0.0008” to 0.001” clearance to the piston and often up to 0.002” to the rod. The builder will choose this based upon use, any issues seen etc. For a street engine, go on the tighter side. The piston to pin, maybe hit the middle at 0.0009”. Pin to pin bore in the rod, go with 0.001” 0.0012”. Pretty sure this spec is common in all Porsche spec book. From memory, I think they give a big tolerance here. A little more clearance can help with a thicker oil film to remove the heat from the SE bushing. Make sure the bores in the rods are 90° to the beam axis. Otherwise the pin will act as a slide hammer and beat the clip out of the piston boss.

[356RS]

Thanks Neil, I see my mistakes. I have the wrong information and I'm glad you caught it. Don't want to give wrong information.

[356RS]

This is where I found the rod radial play measurements. Porsche Service Manual 2.0 to 2.7 engines.

[strictly]

Originally Posted by 356RS

This is where I found the rod radial play measurements. Porsche Service Manual 2.0 to 2.7 engines.

Funny enough i have that sheet in my workshop manual, and must off just missed the play, yes 007 to 015 is what i have seen before in running engines torn down. I have never been lucky enough to see 004 to 005.

Interesting i have seen engines running with 0.020 to 0.030 but then they were worn out.

Thanks that was exactly what i was looking for