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Wrist pin to rod small end clearances

Practical Engine Building

Discussion and questions related to the course Practical Engine Building


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Hey mate,

Something I've come across during my build is that when installing my pistons onto the rods, the wrist pin is very tight my #1 rod. I've run the pin through the other 3 rods and the fit seems to be what I would call a floating design type clearance. On these I can push the pin in quite easily and spin it in the rod as I twist it with my hands.

With the pin in the #1 rod though once its pushed in, I can't spin it with my hands and its impossible to pull it out. I must push it out with a bit of force. Almost requiring the use of a drift.

The rod states to use a 21mm (0.827") pin which it is. Rod paper work states about a 1-1.5 thou clearance.

Obviously in an OEM press fit design, the wrist pin just spins in the piston anyway but if its designed to "float" it should float right?

Is this something I should get my machinist to look into or can I just run some emery through the small end of the rod? Or not worry about it and just send it?

Thanks

This is something that you will need to address. It's not common but I have struck this a hand full of times. You tend to develop a bit of a feel for what is normal here rather than physically making measurements. If it is as tight as you say it is then I'd get your machinist to give the bush a light hone to get the clearance where it should be.