The Stovebolt Site Forums The Shop Area Engine Shop Honing Piston pin holes

I found a set of NOS .020 oversize pistons for my GMC 270 project. (ELCO brand made in Austria) They came with rings & piston pins but unfortunately the piston pins are a little tight in the pistons. Measuring with a bore gauge they're either zero clearance or .0001" interference fit. The manual calls for them to be hand selected for a "hand press fit" which I would estimate to be .0001 clearance.

I could take them to a shop and have them honed, but is there a good way of honing them at home? I hate to use a flex hone.

I do have an old AMMCO small bore rigid hone that has 3 holders that hold replaceable emery paper inserts. The directions for the tool tell how to hone pistons and rod ends so that's what it was intended for.
Anyone ever use one of these hones? If I try it I'm guessing it would be a good idea to practice on a used piston first.

The problem is keeping both holes in alignment. Unless you use a rigid hone long enough to bridge both holes at once, it's almost impossible to keep them aligned. The rule of thumb I've used for many years is about .0002" clearance for wrist pins smaller than 1" in diameter. My Sunnen hone does that easily, but honing one side at a time and getting the right clearance will be very difficult.
Jerry

I think the AMMCO hone is just long enough to reach through both sides.
The instructions say to clamp a power drill in a vise and chuck the hone. There is a button on the end of the hone you press on to expand the stones. Sort of operates like a Sunnen honing bench.

OK, that might work- - - -use wet/dry sandpaper, maybe 600 or 1000 grit, and keep it wet with lightweight oil or maybe kerosene, charcoal lighter fluid, etc. Sneak up on the clearance a little at a time, and measure frequently by flushing the piston with brake cleaner and try a slip fit with a lightly lubricated pin. You can always make the hole a little bigger, but it you oversize it there's no way to shrink it! Instead of using a drill motor in a vise, how about using a drill press?
Jerry

When you say "a little tight" do you mean they will not go in to the pistons at all?

Before you go through the trouble of honing them, try thoroughly cleaning the pistons and pins, then coat the piston pin and piston pin hole with a light film of engine oil.

Next, heat the piston with a blow dryer (like for hair) and recheck the fit. It's likely that once the piston is warmed up, the pin will slide right in.

Have done this many times on those and it has always worked.

Best Regards...

They're tight enough that you can just get it to barely start and would have to tap them in with a hammer. Since they're a floating pin I'd think heating the piston to install would make them too tight to ever move....unless that's desirable. Not sure if they were intended to work but the pistons removed from this engine were floating in both the rod and piston.

From what i've seen at machine shops is the hones run under a lot of oil, can you rig up a pump of some kind to pump a light oil or cutting fluid into a bucket and hone in it or just over it to keep everything will oiled? Sounds like your tool will do the job once you get some lubrication figured out.

Maybe like jerry said, a drill press with the hone and piston down in a bucket of light oil or cutting fluid. The more you can flush, the longer the cutters will last and the finish will be better.

What type of clips hold the wrist pin in the piston? Not that it really matters, just curious.

I’ve used purpose built jig and reamers for motorcycle pistons before, not sure if these things are made for the size you need. I would guess it would be less expensive to take to a machine shop than buy the jig and several different sized reamers though.

They have spring wire clips.

I sourced these pistons from a military surplus dealer in France. The pistons are ELKO brand that seem to be from Austria. Looking at the box they were probably made back in the 1960's or earlier.
They have cast iron rings with the oil ring being one piece cast iron. If I can find more modern rings that will fit I'll probably change them out.

If those pistons are German-made, "floating" is a relative term. Air cooled Volkswagen wrist pins were the same way- - - -the pistons had to be heated to around 250 degrees before the pins could be installed, but they had spring wire clips at the ends. I rebuilt a 40 HP 63 VW engine a few times in the 200K miles I drove that car. I believe the German concept of "floating" is at operating temperature.
Jerry

That's a good point. By the time the engine is warm these would probably be a slip fit.

Did the math on the expansion differential of aluminum vs. steel. If 1" pin and piston hole are the same size cold, an increase in temp from 68-250°F will give you about .0012 clearance when hot.

Sounds like I need to leave the pistons alone and assemble them heated like was suggested.

You beat me to the punch on the reply.

As you point out, the thermal expansion coefficient for aluminum is approximately twice that of steel, so the clearance is only going to increase when the engine is operating.

As long as the pin to rod bushing clearance is good, I think you will be fine.

Best Regads...

Quick follow on, to the above from an "official" source...

TM 9-1802A (June 1943) Par.60, Sec. C (1) Fit Piston Pins in Pistons says:

"At normal room temperature, piston pins should be tight in pistons, but at normal operating temperature, piston pins will be floating fit in pistons and connecting rod bushings. When piston pin boss is accurately reamed, piston pin cannot be installed in piston by hand at room temperature of 70 degrees. Pin should be a smooth "hand press" fit in piston after piston is heated in water to approximately 30 degrees above room temperature.* Pin should fit into piston freely after piston is heated to approximately 170 degrees."

* I guess 4th echelon maintenance units weren't equipped with hair driers back then.

Best Regards...

This is why forums are good. I hadn't thought about the differential in expansion. Wouldn't have been the first time I "fixed" something that didn't need it.