Retaining a steel wrist pin in an aluminum piston

[digiex-chris]

I did a small glow two stroke rebuild a few months ago, and things went well till I realized it was losing power as it heated up. I tore it down, and found that my press fit of the wrist pin in the aluminum piston was no longer a press fit at running temps and was wearing a groove in the cylinder liner! It's a .125" diameter pin, and with my current skills I'm going to have a bit of a problem making a circlip groove that small. Any ideas on pin retention? Will loctite survive a situation like that where there's thermal expansion of one part more than the other?

 

[Jasonb]

Make the pin a bit shorter and then fit a soft end pads from something like bronze.

J

 

[vcutajar]

Although I haven't done them yet to my Kiwi piston, Westbury suggests end pads of either aluminium or brass so that if the wrist pin moves sideways the aluminium or brass will not score the cylinder liner.

Vince

p.s. I see that Jason has explained it better.

 

[Brian Rupnow]

Pictures worth a thousand word maybe??--There are no ring grooves in that piston.--the .jpg doesn't show it that well but the two round holes are tapped for set screws to hold the wrist pin from moving.---Brian

 

[sssfox]

I've owned small R/C engines that had pads made of either Nylon or Teflon, not sure which. It was just a white plastic.

 

[Jasonb]

As Brian says the screw method also works if you have the room! A grub screw should do on a small IC

 

[kvom]

Green loctite (620) is good to 500F.

 

[beuford]

A small teflon cap in the ends of the pin work well and are lighter than bronse or aluminum. They use them in performance auto engines.

 

[hi speed scrap]

quote
....A small teflon cap in the ends of the pin work well and are lighter than bronse or aluminum.
....They use them in performance auto engines.
....beuford.

And Caterpillars

Dave

 

[keith5700]

I did the ptfe cap thing in my pins. Not fired it up yet but I'm not anticipating any problems.

 

[jpeter]

Used Teflon in the v8. Got it from MC.

 

[jeroairwolf]

I did the ptfe cap thing in my pins. Not fired it up yet but I'm not anticipating any problems.

Mr. KEITH HARLOW certainly is the best designer on the planet ...

deserve all my admiration

If There Were more people in the world as the master Keith would be the world perfect

cheers to you Keith

 

[digiex-chris]

wow thanks for the responses. I tore apart my other engines to see what they did, and look at that, all but one use little ptfe pucks stuck in the hollow wrist pins. The other uses impossibly small circlip grooves.

I made little plastic ends like you all said, and ran it for an hour (lots of hot/cold cycles, but not too gently either, 12,000 rpm on a 10x6 prop!) and took it apart to take a look, not a spec of wear on the plastic ends or the cylinder walls where they ride. Thanks for the advice! Major problem solved!

I also ran out of tool steel to make a hardened pin, but one turned out of a high tensile bolt seems to have worked out ok. It should out last the bronze bushing at least.

Of course, I got excited, assembled it, and turned it over before I got pictures.