Making wristpin pads...

[Paul Gibeault]

There is nothing like a perfectly good engine minus 1 tiny bad part urges me to attempt an engine fix. Whilst changing out a piston /cylinder assembly on my O.S. 25LA motor, I noticed one of the wrist pin pads was coming apart. It seems the top of the pad is separating from the rest. This has happened to others also. I'm not aware if these tiny pads are available. This disintegrating pad issue was also a problem on my very high performance K&B 6.5cc speed engines. I was able to get replacements direct from the factory back in the day & I ended up using most of my stock over the years. These days, out of production parts are extremely difficult to come by hence the DIY method is needed.
So, with a scrap of 3/8" Teflon rod, I attempted to make one. In 20 minutes on the lathe it was a success, I think... Only running with tell the true story. I made my replacement pad thicker (close to the wristpin I.D.) so that it needed a tiny squeeze to press in. I also made the brim a bit thicker also (without being too thick to rub on the cylinder wall). The final shape of the 'top hat' was finished by hand with a fine file & sandpaper. This being said, I was wondering if anybody here has any thoughts as to the right way in which to make these tiny pads & any details that need to be kept in mind. Also would nylon work as well for a material??

 

[James Barker]

I am not sure about this, but, I would think that Nylon might be a bit too abrasive and cause wear on the cylinder walls, Teflon is a bit soft but is not abrasive in nature. The material needs to be able to stand up to high temperatures though and not cause excessive wear. I am not too certain about Delrin. I would have to dig out the material specs book. Of course whatever is used is there only as a spacer really and is not subject to heavy loads directly.

BC1
Jim

 

[Scotty]

Lou Ross had a nice trick. He used alu rivets in the ends of the rod pin. Quick and easy, assuming the rod hole is .125.

 

[petertha]

Paul, I think you got it looking pretty right. There was probably some manufacturer engineering as to how much wristpin float was desirable for wear, temperature expansion, contact area within the piston... so I would think mimicking that length would be the best primary strategy. For example a flat faced pad with corner chamfer will contact the bore profile on the 2 chamfers, whereas a dome profile will contact on its single tangent. So even though the nominal pad thickness may be the same in cross section, the resultant float length may be slightly different. But its not like you can wander too far off because you are constrained by both the bore & wristpin length.

An important thing when machining the pad is the corner where it meets the wristpin hole. If your tool radius is larger than the wristpin hole chamfer, the pad will hang up on this & stick out further than the nominal pad thickness even though it looks flush to the naked eye. To check you could just position your new assembly in the bore with one end contacting the ID. Then measure other end gap with a ?numbered drill or tiny wire? to compare to the stock assembly. I would think a tighter length would be undesirable.

I've seen some plastic pads with holes or dimples even though the wristpin was solid. Not sure if this is an oil retention thing or somehow assists seating or retention? On my 4S radial (cast iron liners) I went with aluminum pads. The hole was necessary to allow Loctite escape path, otherwise the pads got hydraulically locked & didn't sit flush.

Material wise I cant state which would be better but check the heat distortion temp. I bought some teflon for my exhaust washers which I've seen employed in other methanol engines &that woudl have to be a lot hotter than the pads would ever see. It turned reasonably well with a razor sharp tool but examine under magnification because it has a habit of leaving fuzz edge & doesn't take kindly to sanding like metals. As we used to say working with Kevlar composites, its like trying tosand your floor carpet LOL.