problem with pistons

[67rcks]

I got my pistons back from a motor shop today. They put new pistons (with new wrist pins) to old connecting rods. About half of them seems pretty tight (wristpin to piston) while the other half seems quite loose (e.g. piston won't stay level if holding it up). One or two connecting rods are furthermore little offset, about .05" ie they are not perfectly in the middle of the wrist pin.

Do connecting rods have to be in the middle of the wrist pin? Is it normal that new pistons would have such big tolerance? These are speed-pro brand I got from summit not the best quality or cheapest ones. Can it be somehow fixed or it's a trash?

 

[D.Hearne]

Sounds like you need to find a better machineshop to do you work. Pins should be centered in the rods, pin to piston clearance should be the same for all. I'd also bet some of the pistons are installed backwards too.

 

[bnickel]

i agree with D, take them to another machine shop and see what they think. i hope they didn't heat the pin end of the rod with a torch like a lot of people seem to do....that always seems to me like they are screwing with the metalurgy of the rod. i've always used a hydraulic press to press them in myself. i've seen a lot of rods come apart at the pin end and you could easily tell that someone used a torch to install the pins because of the buing that's always present from hetaing the rods with a torch. whether or not that was the actual cause of the rod breaking or not i can't really say but it does make me nervous to see that.

 

[D.Hearne]

The older machine shops sometimes used a heater to heat the pin end, that would be preferable to a torch, at least the heating would be evenly done. But I'd prefer a pressed fit without the heat too.

 

[67rcks]

He did not do anything with pistons, just heated rods + pressed new wrist pins (he had some laser thermometer to control heat). And today it turned out that a few of them have pin about 0.030" shorter than the distance between piston clips so pin+conn rod slides inside the piston. well are these pistons trash?

 

[D.Hearne]

As long as the pins are centered, you're fine. The rod & pin need to have lateral movement in the piston to function. Just as long as the pin doesn't slide in the rod.

 

[TOM B]

I agree with "D" but I like full floating pins. I would have had the bushings honed for a full float and not worry about pin centering, less stress on rod to.
On a 302- 351 the rod to piston relationship is,
With the piston notch or arrow facing toward the front of motor the numbers on the rodsand caps should face to the outside of the block. 4 one way and 4 the other way.

 

[bnickel]

I agree with "D" but I like full floating pins. I would have had the bushings honed for a full float and not worry about pin centering, less stress on rod to.
On a 302- 351 the rod to piston relationship is,
With the piston notch or arrow facing toward the front of motor the numbers on the rodsand caps should face to the outside of the block. 4 one way and 4 the other way.

the rods also have a slightly larger chamfer on side of the big end for the fillet on the crank, this side goes to the outside edge of the crank and the smaller chamfer on the rods face each other.

 

[67rcks]

Sounds like you need to find a better machineshop to do you work. Pins should be centered in the rods, pin to piston clearance should be the same for all. I'd also bet some of the pistons are installed backwards too.

Your bet is right. 1-4 are installed backwards. If I had a shop press I would do it by myself. Now I can only hope my crank is machined right... have no plastigage to check it.