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MG MGB Technical - fitting new pistons

I've just trial fitted one of my new pistons onto a con rod and note that the gudgeon pin fit into the little end is tighter than the gudgeon pin into the piston. This means that any rotation takes place within the piston. Is this ok or do I need to have more clearance in the little end?
Steve Church

As long as the piston and rod are free when hot all will be ok. Try warming piston/rod assembly in hot water and recheck freedom of movement.

Peter
Peter Burgess Tuning

And if they are not free? I'll check them all in hot water as you suggest Peter. Some may be freer than others.
Steve Church

Ok so I've had a chance to check all the pins with all the rods. Three are ok just the one that is tighter. Putting the little end with gudgeon pin fitted into hot water results in the fit tightening up. I also tried the old gudgeon pin in the little end it came from and thats also tight. Looks like just the one little end that is slightly narrower than the other three.

Also when piston and gudgeon pin are in hot water the fit gets looser. So is there going to be a problem with the gudgeon pin turning in the aluminium of the piston rather than the phospour bronze of the little end?

Why the piston (aluminium)/pin joint loosens but the con rod ( steel)/pin joint tightens is a bit beyond my O level physics.

Comments please. Should I just assemble and hope for the best?
Steve Church

Hi Steve

Just check you have no burrs of dinks on the tighter little end. Even so, remember the pistons are happy to run when they are press fit and only move in the piston. You will usually find a lubrication hole to feed the piston/pin.



Peter
Peter Burgess Tuning

I wiondered about the press fit pins. I was under the impression that they were fixed in the piston and moved in the little end. If they move in the piston then it's no different to what mine could be doing. In that case I'll go ahead and assemble. There are two lubrication holes in the piston to feed the piston/pin surface.
Steve Church

This thread was discussed between 24/01/2012 and 26/01/2012

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