Piston Pins Too Tight?

[hulbyw]

I neglected to check the piston pin fitment in the rebuilt stock connecting rods before I sealed up the case. Now when I need to assemble a piston and cylinder to check deck height, the pin will not push through the rod by hand. If I freeze it and heat the rod small end it will go through with some hits with a plastic hammer however when the temps even out it will likely not be a floating piston anymore. Do I need to split the case, disassemble then take the pins and rods to a machinist to get the rods reamed out so the piston can self centre in the barrel?

[oprn]

Why would you have to split the case to remove the rods? You can reach the rod bolts through the cylinder holes.
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[Nitramrebrab72]

You can't split the case without removing the pistons. I would of thought the piston pin needs to be turned down as the con rod has a bearing shell. Having said that the piston pin has a chrome surface treatment. So you need the right parts for the job.

[MuzzcoVW]

Sounds like it's time to consider hone the rod bushings. No, the pins don't get turned. Seen this many times with rebuild rods.

[hulbyw]

[oprn]

On this subject, the machinist I took my engine parts to for balancing told me the pin fit in the pistons was too tight and had to be corrected. He said they should never be an interference fit at room temperature. They should be a thumb press fit, about 1/2 thou clearance. These were not new pistons either.

I told him that all air cooled VW pistons were like that. He insisted that it was still wrong. I let him have at it and he did an amazing job, they were indeed a thumb press fit when he was done! When I think back on all the hassle over the years trying to get used pins out of pistons without bending the rods I think he is on to something. In all the engines I have disassembled over the years I have never seen pin bores in the pistons worn. It's always the rod bushing. They are probably too tight to even turn most of the time!
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We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[MuzzcoVW]

[jbbugs]

A small ball hone works well for this.
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[oprn]

He used a bushing reamer that is long enough to do both pin holes at once on the piston to keep the holes parallel. A brake cylinder hone could taper the hole if the stone pivot point is not centered in the hole. As for ball hones... they don't tend to keeps things true at all!
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We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[Vanapplebomb]

I agree 100% with oprn. The reamer is the way to go. Pistons are soft, and abrasives from homes, like a ball hone embed the bore, and can score the piston pin. Like he says, ball homes don’t necessarily keep things round… especially when there may be different surface areas from one side to the other. The side with less area wears more, and the bore goes egg shaped.
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http://www.thesamba.com/vw/forum/viewtopic.php?t=487021

[RWK]

Check for burrs around the snap ring groove from mfg. always one there on used pistons. Also burrs from driving them in and out if driver touches piston bore, best to have pin driver fit into ID of pin closely and plenty of clearance around OD of driver to clear bore, that's how I make my pin drivers.
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[modok]

And this is why you need to have a rod honing machine to build engines.

Or rather you need that as part of your team, maybe not you personally.
If there is no such service in your city then I would buy a rod honing machine, or move to a different city.

[tcoop1100]

My understanding has always been to have a push fit in the rod and an interference fit in the piston at room temperature. Any "float" of the piston in the cylinder is from the pin in the rod, not the pin in the piston. If you ream the piston for a push fit wouldnʻt you have too much clearance at operating temps and have pistons rocking in the cylinders, increasing wear, scuffing and galling? I briefly heat the pistons at the pin boss and pins slide right in, easy-peasy, no hammer required. Removal, same thing. No hammer, no chance of damaging anything.
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Aloha,
Tim
January '57 Sedan with April '64 40hp big bore

[Rome]

My choice is this small sanding cylinder for a 1/4" electric drill, found at local HW store in a fine grit such as 220. The diameter I chose is slightly larger (e.g., 1" = 25 mm) than the small end bushing (22 mm) so that there is nearly no compression of the flaps. Just a single push-thru and pull-back. Spray the bushing with WD-40 to clean off debris, and do a test fit of the wrist pin with a nice easy and smooth push-thru. I also run this cylinder around the perimeter of the wrist pin holes of the pistons so that any minor standing-up fragment does not hinder the movement of the wrist pin into each side of the piston hole.

Hack? Yes, but effective.

[VWporscheGT3]

[modok]

You probably aren't old enough to be familiar with why it was how it was.
It has a lot to do with the piston alloy, and the piston alloy had to do with ease of casting and machining, before CARBIDE never mind PCD.

LOW silicon cast pistons in our OLD vw they had the pins a zero fit at room temp. So did many engines with similar pistons and floating pins.
This gave better longevity.... keeping in mind in those days it was common to re-groove and knerl the piston and use it again, and in they even had oversize pins too.
Floating pins were necessary to get away with running the pin so tight in the piston.

Today almost nothing is using pistons like that, but there are still a few if you look.
oil and oil filters and piston alloys have changed to the point that the pin wearing out the piston bores is a very rare problem so it isn't necessary to make it as tight as practical to start.
The physics have not changed. If you wanted to run a low silicon racing piston the same way today you could, and it would work, but it would be super rare anybody would want to do that.

Personally, on the pin bushing I would like to have slightly MORE clearance than VW spec, ideally .0008", because loose is happy when your crank is a noodle. And with an oil filter and a good tune there is almost no wear.

[hulbyw]

Thanks for all the help folks. I will have a chat with local VW shop today to see what they can offer to ream out the small ends and maybe the pistons as well. Pistons are NOS Mahle 90.5 (bought about 25 years ago). Seem to remember being told they were forged but honestly have no idea and no skills to tell by just looking at them whether they are cast or forged.

[tcoop1100]

[modok]

25 year old mahle pistons would be a good time capsule for 50 year old piston technology.

They didn't really change much over the years.

And another thing that experience teaches better than words..... is you can hone a very close fit of say .0003" and have the pin slide free, but then leave that part on a shelf for a year or two then it won't slip through anymore. It only gets smaller as oxide forms. Pin bushings and valve guides are this way.

If that's the problem here I might recommend you make a basic scotch bright flap wheel and run that wet with solvent to freshen up the pin bushing bore.

Pistons bored with PCD will do that far LESS than older methods because it's so much smoother, or you might say pre-glazed surface finish.

And yet again another difference in technology is that a HONED surface CAN run tighter than a super smooth PCD bored hole because of the surface texture, but when that texture gets smooshed flat in operation then the true clearance is is slightly larger than measured.

New ways work, old ways work, but mixing them together may be confusing.

[tcoop1100]