Wrist Pin ???'s

[patkeefe]

I am assembling the 2.7 track engine. P&C are good used Mahle, evidently 10.5:1 CR. Wrist pins were all mismatched, weight-wise, so I picked up a matching set of 22x58 pins. The wrist pins I had were tapered bore (RSR according to Andial). New ones are the standard non tapered bore, at 112.5 grams each.

My question is: can I assume these are full floating pins (floating in the rod and piston bores)? And, what sort of clearance should there be in the piston bores? I am guessing just enough to let them slide through the bores, lubricated, without any noticeable radial play. I would further guess that would put running clearance at around 0.0015 0.002".
TIA
Pat

[patkeefe]

The bores themselves were OK. Where the retaining clips go into the bores, the bores were a bit smaller, such that the piston pins don't want to slide into the bores. I think the dynamic is that these two areas probably saw a lot of stress fron the pins banging on the clips, and pushed them up a bit. Or, someone tried to drive the pins out with the clips still in.

Anyway, I ended up cleaning the high spots with a jewelers file, and trying not to nick up the actual pin bearing surfaces. I manually cleaned the bores with some 400 grit wet paper in the parts washer. They're pretty nice now, but don't have that dull grey bore surface any longer. What made the bore surface that shiny grey, like on new pistons? Some sort of treatment? BTW, we are talking about hours of work here, and I have only finished three pistons. If my spare time were money, I would have bought Nickies and JE pistons.

Pat

[patkeefe]

Well, OK, they are full floating. I just need to know what the running clearance tolerances should be, if anyone knows. For some odd reason, I cannot find this information anywhere.

[Eagledriver]

They are not what I would call full floating. They are a tight fit but not really an interference fit either. I've had sets that could be pushed in by hand cold and other sets that had to have the pistons heated and taped in with a dowel. So basically there should be minimal running clearance or none at all. The bearing surface is the brass rod insert. If you can push them in by hand but have no noticable play in the piston then that should be ok. The running clearance in the rod I don't know off hand but I imagine a couple thousands of an inch like the main and rod bearings.

-Andy

-Andy

[patkeefe]

I found a very interesting article on pin/bore design:

http://www.paper.edu.cn/downloadpaper.php?serial_number=200703-52